100% Complete Notes
Paper I & Paper II

1.1 Basis of Animal Classification

1.2 Protozoa — Detailed

General Characters

• Unicellular eukaryotes; all life functions in one cell
• Locomotion: pseudopodia (Sarcodina), flagella (Mastigophora), cilia (Ciliophora), none (Sporozoa)
• Nutrition: Holozoic (ingestion), Saprozoic (absorption), Holophytic (photosynthesis), Mixotrophic
• Excretion: Contractile vacuole (osmoregulation in freshwater forms)
• Reproduction: Asexual (binary fission, multiple fission, budding, sporulation); Sexual (conjugation, syngamy)
• Encystment: protective cyst during adverse conditions

Class Sarcodina

• Amoeba proteus: pseudopodia (lobose); cytoplasmic streaming; binary fission; contractile vacuole for osmoregulation
• Entamoeba histolytica: causes amoebiasis/amoebic dysentery; cysts in contaminated water; metronidazole treatment; no contractile vacuole (parasitic)
• Entamoeba gingivalis: non-pathogenic; oral cavity
• Radiolaria & Foraminifera: marine; siliceous/calcareous shells; form geological deposits (chalk, ooze)

Class Mastigophora (Flagellata)

• Trypanosoma gambiense: African sleeping sickness; Tsetse fly (Glossina) vector; kinetoplast; undulating membrane; CNS involvement (somnolence, coma)
• Trypanosoma cruzi: Chagas disease; Triatomid bug; South America; invades cardiac muscle
• Leishmania donovani: Kala-azar (visceral leishmaniasis); Phlebotomus sandfly; amastigotes in macrophages; splenomegaly; Sb-based treatment
• Leishmania tropica: Cutaneous leishmaniasis; Oriental sore; self-healing ulcer
• Giardia lamblia: Giardiasis; two nuclei; sucking disc; contaminated water; 'beaver fever'
• Trichomonas vaginalis: STI; no cyst stage; transmitted sexually; flagellated trophozoite

Class Ciliophora

• Paramecium caudatum: slipper-shaped; two nuclei (macronucleus — metabolic; micronucleus — reproductive); oral groove → buccal cavity → cytostome; trichocysts for defence
• Conjugation: sexual reproduction; exchange of micronuclei through cytoplasmic bridge; genetic recombination without increase in numbers
• Vorticella: sessile; stalked; retractile; colonial forms
• Balantidium coli: only ciliate pathogenic to humans; causes balantidiasis (large intestine)

Class Sporozoa (Apicomplexa)

• No locomotory organelles; all parasitic; apical complex for host cell invasion
• Monocystis lumbrici: parasite in seminal vesicles of Pheretima; no intermediate host; pseudonavicella spores
• Eimeria: causes coccidiosis in poultry; sporozoites infect intestinal epithelium

Plasmodium — Complete Life Cycle

1.3 Porifera (Sponges)

• Cellular grade of organization — no tissues or organs; cells show division of labour
• Totipotency: any cell can regenerate entire organism
• Choanocytes (collar cells): flagellated; create water current; phagocytose food particles
• Archaeocytes: totipotent amoeboid cells; can differentiate into all cell types
• Pinacocytes: flat cells lining outer surface; epithelium-like
• Porocytes: tubular cells forming ostia (pores) through which water enters

Canal Systems

• Skeleton: calcareous spicules (Class Calcarea), siliceous spicules (Class Hexactinellida), spongin fibres (Class Demospongia), or combination
• Reproduction: Asexual — budding, fragmentation, gemmules (freshwater sponges survive drought)
• Sexual: hermaphrodite; cross-fertilisation; free-swimming amphiblastula or parenchymula larva
• Economic importance: Euspongia (bath sponge); Hippospongia (horse sponge)

1.4 Coelenterata / Cnidaria

• Diploblastic; radial symmetry; tissue grade; gastrovascular cavity (coelenteron)
• Nematocysts (cnidoblasts/cnidocytes): unique stinging cells — penetrant (inject toxin), volvent (entangle), glutinant (sticky) types
• Polymorphism: polyp (asexual, sessile — hydranth + gonozooid) + medusa (sexual, free-swimming)
• Metagenesis: alternation of polyp and medusa generations
• Nerve net: diffuse nervous system without brain; no centralization

Important Examples

1.5 Platyhelminthes (Flatworms)

• Acoelomate; triploblastic; dorsoventrally flattened; bilateral symmetry
• No circulatory or respiratory system; exchange by diffusion (flattened body ensures short diffusion distance)
• Protonephridia with flame cells (solenocytes) — excretion and osmoregulation
• Hermaphroditic (except Schistosoma which is dioecious)
• Tegument: syncytial; metabolically active; absorbs nutrients in endoparasites

Class Turbellaria

• Free-living; Planaria maculata — used in regeneration experiments
• Auricles (chemosensory); eyespots (photoreceptors); cilia for locomotion
• Remarkable regeneration: 1/279th piece can regenerate whole organism

Class Trematoda (Flukes)

Class Cestoda (Tapeworms)

• No digestive system; absorb predigested food through tegument
• Scolex (head) with suckers ± hooks; proglottids (segments); strobila (chain of proglottids)
• Each mature proglottid is hermaphroditic; gravid proglottid packed with eggs

1.6 Aschelminthes (Nematoda / Roundworms)

• Pseudocoelomate; cylindrical; unsegmented; bilaterally symmetrical
• Cuticle: trilaminar collagen; shed 4 times during larval development (molting)
• Separate sexes (dioecious); sexual dimorphism — female larger
• Complete digestive system (mouth to anus)
• No circulatory or respiratory system; excretion by renette cells or H-shaped excretory canals
• C. elegans: model organism; 959 somatic cells; complete cell lineage mapped; first animal to have genome sequenced

1.7 Annelida

• True (eucoelomate) coelom — schizocoelous origin; lined by coelomic epithelium (peritoneum)
• Metamerism (true segmentation): both external (septa) and internal segmental organs
• Closed circulatory system; haemoglobin dissolved in plasma (Pheretima); chlorocruorin (Sabella)
• Nephridia: segmental excretory organs (metanephridia in Pheretima)
• Ventral nerve cord with ganglia per segment; cerebral ganglia (brain)

Class Polychaeta

• Marine; parapodia (paddle-like lateral appendages) for locomotion and gas exchange
• Separate sexes; trochophore larva (connects to Mollusca evolutionarily)
• Nereis: ragworm; errant (free-moving); epitoky — posterior proglottids (epitokes) detach for reproduction
• Aphrodite: sea mouse; dorsal surface covered with felt-like setae
• Arenicola: lugworm; tube-dweller; used as fish bait

Class Oligochaeta — Pheretima posthuma (Earthworm)

• ~108 segments; clitellum segments 14–16 (secretes cocoon)
• 4 pairs of setae per segment (except 1, clitellum, last)
• Hermaphrodite; cross-fertilisation; cocoon formation
• Typhlosole: dorsal fold of intestine that increases absorptive surface area
• Calciferous glands (segments 10–12): neutralise excess Ca²⁺; eliminate CO₂ as CaCO₃
• Spermathecae (6–9): store sperm received from partner
• Hearts: 4 pairs of lateral hearts in segments 7–11
• Ecological role: burrowing aerates soil; organic matter decomposition; vermicomposting

Class Hirudinea — Hirudo medicinalis (Leech)

• 33 segments but externally appear more due to annulations
• No setae; anterior + posterior suckers; coelom reduced (filled with connective tissue)
• Hirudin: potent anticoagulant; prevents thrombin action; used in medicine (hirudotherapy)
• Medical uses: microsurgery — reattachment of severed digits/ears; anticoagulant drug development

1.8 Arthropoda

• Largest animal phylum (~80% of all animal species)
• Chitinous exoskeleton: protection, prevents desiccation, provides attachment for muscles
• Moulting (ecdysis): controlled by ecdysone hormone; new cuticle secreted before old shed
• Jointed appendages: modified for walking, swimming, feeding, reproduction, sensory functions
• Open circulatory system: haemocoel; haemolymph bathes organs directly
• Compound eyes: many ommatidia; wide field of view; good motion detection; poor resolution

Classes and Examples

Insect Metamorphosis

Palaemon (Prawn) — Structure

• 19 pairs of appendages: 2 pairs antennae, 1 pair mandibles, 2 pairs maxillae, 3 pairs maxillipeds, 5 pairs pereiopods, 5 pairs pleopods, 1 pair uropods + telson
• Nauplius → zoea → mysis → postlarva development
• Hepatopancreas (digestive gland): secretes digestive enzymes; nutrient absorption

1.9 Mollusca

• Soft unsegmented body; mantle secretes shell (CaCO₃); haemocoel; open circulation (except Cephalopoda)
• Radula: rasping organ with teeth; absent in Bivalvia (filter feeders)
• Ctenidia: gills in mantle cavity; modified in different classes
• Trochophore larva → veliger larva (connects to Annelida)
• Statocysts: balance organs; osphradia: chemoreceptors

1.10 Echinodermata

• Pentamerous radial symmetry (adults); bilateral larva → radial adult (secondary radial symmetry)
• Water Vascular System (WVS): unique; tube feet (podia) for locomotion, feeding, gas exchange, osmoregulation
• WVS: madreporite → stone canal → ring canal → radial canals → ampullae → tube feet
• Endoskeleton: ossicles (CaCO₃) with spines; mesodermal origin
• No excretory organs; nitrogenous waste diffuses through body wall
• Haemal system (water-vascular derived): rudimentary circulatory function
• Excellent regeneration: Asterias can regenerate from a single arm + part of central disc
• Autotomy: deliberate self-amputation of body part (Holothuria eviscerates viscera)
• Deuterostome; enterocoelous coelom; bipinnaria larva similar to tornaria of Hemichordata

1.11 Hemichordata

• Marine; soft-bodied; divided into proboscis, collar, and trunk
• Stomochord: short diverticulum of buccal cavity — NOT homologous to true notochord
• Gill slits: pharyngeal gill slits in collar — link to chordates
• Dorsal hollow nerve cord in collar region (comparable to chordate neural tube)
• Tornaria larva: resembles echinoderm bipinnaria — evidence of evolutionary link
• Balanoglossus: acorn worm; burrows in sand/mud; mucus secretion for food collection
• Two classes: Enteropneusta (acorn worms) and Pterobranchia (colonial, tentaculated)

1.12 Protochordata

Urochordata (Tunicata)

• Free-swimming tadpole larva has ALL four chordate characters: notochord (in tail), dorsal hollow nerve cord, pharyngeal gill slits, post-anal tail
• Metamorphosis: larva settles, notochord and tail resorbed — adult retains only gill slits → RETROGRESSIVE metamorphosis
• Adult secretes tunic (cellulose-like tunicin) — only animals with cellulose
• Ascidia: solitary; sessile adult; colonial forms (Botryllus)
• Salpa: pelagic; barrel-shaped; barrel-shaped; alternation of generations
• Doliolum: pelagic; free-swimming adult; complex polymorphism

Cephalochordata

• Branchiostoma (Amphioxus/Lancelet): all four chordate characters persist throughout life
• No brain, no eyes, no paired fins, no distinct head — primitive chordate
• Notochord extends to anterior tip of body (hence Cephalochordata)
• Wheel organ (Hatschek's pit): rostral; secretes mucus; creates feeding current
• Metapleural folds: ventral; homologous to vertebrate paired fins?
• Filter feeding: water → mouth → pharynx (gill slits) → atrium → atriopore
• Asymmetrical in young (left side → right side during development)

1.13 Vertebrata — Detailed Classification

Superclass Agnatha (Jawless)

• No jaws, no paired fins; cartilaginous skeleton; no scales
• Petromyzon (Lamprey): ectoparasite; rasping disc; anadromous; ammocoetes larva (resembles Amphioxus); Pronephros functional
• Myxine (Hagfish): deep sea scavenger; slime glands; 4 pairs of tentacles; rudimentary eyes; osmoconformer

Class Chondrichthyes (Cartilaginous Fish)

• Cartilaginous endoskeleton; placoid scales (dermal denticles — similar to teeth); no operculum
• Spiracle: vestigial gill slit; used for water intake in some
• Spiral valve intestine: increases surface area for absorption
• Viviparous or oviparous; internal fertilisation via claspers (males)
• No air bladder; maintain buoyancy via large oily liver (squalene)
• Scoliodon: dogfish shark; urea retained for osmoregulation in seawater; rectal gland for NaCl excretion
• Torpedo: electric ray; electric organs modified muscle; can generate up to 200V
• Pristis: sawfish; rostral teeth; cartilaginous
• Chimaera: subclass Holocephali; operculum-like flap; no spiracle

Class Osteichthyes (Bony Fish)

• Ossified endoskeleton; operculum covers gills; cycloid/ctenoid/ganoid scales
• Air bladder (swim bladder): buoyancy organ; physostomous (connected to gut) or physoclistous (closed)
• Lateral line system: detects water pressure changes (mechanoreception)
• Subclass Actinopterygii (ray-finned): most bony fish — Labeo, Catla, Clarias
• Subclass Sarcopterygii (lobe-finned): Lungfish (Dipnoi), Coelacanth (Latimeria) — link to tetrapods
• Lungfish (Protopterus, Lepidosiren, Neoceratodus): both gills AND lungs; aestivation in mucus cocoon; bimodal breathing

Class Amphibia

• First tetrapods; ectothermic; dual life; moist glandular skin for cutaneous respiration
• 3-chambered heart (2 auricles + 1 ventricle); incomplete separation; partial double circulation
• External fertilisation; larvae aquatic with gills (tadpole); metamorphosis to adult
• Neoteny: retention of larval characters in adult — Necturus (mudpuppy) — permanent larval gills; Axolotl (Ambystoma mexicanum) — neotenic normally but can metamorphose

Class Reptilia

• Dry keratinized scales; amniotic egg (reptiles/birds/mammals = amniotes)
• 3-chambered heart (incomplete ventricular septum) — except Crocodilia (4-chambered, but with foramen of Panizza)
• Jacobson's organ (vomeronasal organ): chemoreception; snakes flick tongue to sample air
• Ectothermic; poikilothermic (cold-blooded)
• Uricotelic: excrete uric acid (water conservation in terrestrial life)

Class Aves (Birds)

• Feathers: unique to birds; contour (vane) feathers, down feathers, filoplumes, bristles
• Modifications for flight: hollow pneumatic bones, fused clavicles (furcula/wishbone), keeled sternum (carina) for muscle attachment, uncinate processes on ribs, air sacs (9), high metabolic rate
• 4-chambered heart; complete double circulation; endothermic (warm-blooded)
• Syrinx: vocal organ at junction of trachea and bronchi
• Crop: food storage; Gizzard (proventriculus): mechanical digestion; Cloaca: common exit
• Hallux: opposable toe in perching birds
• Endothermy: requires high metabolic rate; 10× higher BMR than reptiles

Class Mammalia

• Mammary glands: modified sweat glands; produce milk for young
• Hair/fur: insulation; endothermic
• 3 middle ear ossicles: malleus + incus (from articular + quadrate bones of reptile jaw) + stapes
• Heterodont teeth: incisors, canines, premolars, molars; diphyodont (two sets — milk + permanent)
• Diaphragm: muscular partition separating thorax from abdomen; unique to mammals
• 4-chambered heart; left aortic arch (vs right in birds); complete double circulation
• Cerebral cortex (neocortex): highly developed; intelligence, behaviour

2.1 Cell Theory and Prokaryote vs Eukaryote

• Classical Cell Theory: Schleiden (1838 — plants) + Schwann (1839 — animals); Virchow (1858) added "Omnis cellula e cellula"
• Modern additions: cells carry hereditary info (DNA); all metabolism occurs in cells; energy flows through cells

2.2 Cell Membrane — Structure and Transport

Fluid Mosaic Model (Singer & Nicolson, 1972)

• Phospholipid bilayer: hydrophilic heads (outward), hydrophobic tails (inward)
• Integral (transmembrane) proteins: span full bilayer; channels, pumps, receptors
• Peripheral proteins: loosely associated with surface; cytoskeletal anchoring
• Glycocalyx: glycoproteins + glycolipids on outer face; cell recognition, adhesion, antigenicity
• Membrane fluidity: increases with unsaturated FA and temperature; cholesterol buffers fluidity (prevents crystallization at low temp, prevents excess fluidity at high temp)
• Lipid rafts: ordered microdomains enriched in cholesterol + sphingolipids; signaling platforms

Membrane Transport

2.3 Cell Organelles — Structure and Function

Mitochondria

• Double membrane: outer (smooth, porins); inner (folded into cristae — increases surface area for oxidative phosphorylation)
• Intermembrane space: H⁺ accumulates during ETC → drives ATP synthase
• Matrix: Krebs cycle enzymes; own circular DNA (37 genes); 70S ribosomes; fatty acid oxidation
• Endosymbiont theory (Margulis): mitochondria evolved from engulfed α-proteobacteria; evidence — own DNA/ribosomes, divide by binary fission, double membrane, antibiotic sensitivity
• ATP synthase (F₀F₁-ATPase): F₀ in inner membrane (H⁺ channel); F₁ in matrix (catalytic head); ~3 H⁺ per ATP
• Permeability transition pore (mPTP): opens during apoptosis → cytochrome c release → caspase activation

Endoplasmic Reticulum

• Rough ER: ribosomes on cytoplasmic face; synthesizes secretory and membrane proteins; signal peptide targets proteins to RER; N-glycosylation begins here
• Smooth ER: lipid synthesis (phospholipids, cholesterol); steroid hormone synthesis in adrenal cortex/gonads; drug/toxin detoxification (CYP450 enzymes); Ca²⁺ storage and release (muscle: sarcoplasmic reticulum)
• ERAD (ER-associated degradation): misfolded proteins retrotranslocated and degraded by proteasome; ER stress → Unfolded Protein Response (UPR)

Golgi Apparatus

• Stack of cisternae (saccules): Cis-Golgi network (CGN, receiving) → medial → Trans-Golgi network (TGN, dispatching)
• Functions: further glycosylation (O-glycosylation, trimming); sulfation; sorting of proteins to lysosomes, secretory vesicles, or plasma membrane
• Mannose-6-phosphate (M6P) tag: targets enzymes to lysosomes
• I-cell disease (Mucolipidosis): M6P missing → lysosomal enzymes secreted outside instead of entering lysosomes

Lysosomes

• Membrane-bound; pH ~4.5–5 (maintained by V-type H⁺-ATPase proton pump)
• ~50 hydrolytic enzymes: proteases, lipases, nucleases, glycosidases
• Primary lysosomes: inactive; Secondary lysosomes: fused with phagosome/endosome; Residual bodies: undigested material
• Autophagy: recycling of damaged organelles (mitophagy = mitochondria; ER-phagy); important in starvation
• Autolysis: lysosome rupture → cell self-digestion; occurs during metamorphosis (tadpole tail resorption), programmed tissue regression
• Lysosomal storage diseases: Gaucher (glucocerebrosidase deficiency), Tay-Sachs (hexosaminidase A deficiency — accumulation of GM2 ganglioside)

Peroxisomes

• Single membrane; contain oxidative enzymes: catalase, peroxidase, urate oxidase
• β-oxidation of very long chain fatty acids (VLCFAs) — unlike mitochondria which handles shorter chains
• Catalase: converts H₂O₂ → H₂O + O₂ (prevents oxidative damage)
• Zellweger syndrome: absent peroxisomes → accumulation of VLCFAs → CNS damage

Nucleus

• Nuclear envelope: double membrane (outer = rough ER); nuclear pores (~3,000/nucleus) for RNA and protein transport
• Nuclear pore complex (NPC): ~120 MDa; selective barrier; importins/exportins shuttle cargo; GTP-dependent
• Nucleolus: dense fibrillar component (rDNA transcription) + granular component (rRNA processing); rRNA synthesis and ribosome assembly; disappears during mitosis
• Chromatin: DNA + histones; nucleosome = 147 bp DNA + histone octamer (H2A, H2B, H3, H4)₂; 30nm fibre
• Euchromatin: loosely packed; transcriptionally active (light-staining); Heterochromatin: densely packed; mostly inactive (dark-staining)
• Constitutive heterochromatin: always condensed (centromeres, telomeres); Facultative heterochromatin: sometimes condensed (Barr body = inactivated X chromosome)

Cytoskeleton

2.4 Chromosomes

• Centromere: primary constriction; attachment site for kinetochore (spindle fibres bind here)
• Telomere: TTAGGG repeats (human); protect chromosome ends; shortened with each replication → cellular senescence; telomerase adds repeats (active in germ cells, stem cells, cancer)
• Chromosome types by centromere: Metacentric (arms equal, V-shape), Submetacentric (arms unequal, L-shape), Acrocentric (one arm very short, J-shape), Telocentric (terminal centromere, rod-shape)

Special Chromosomes

Nucleosome and Chromatin Organisation

• Nucleosome: 147 bp DNA wound ~1.65 turns around histone octamer (2 each of H2A, H2B, H3, H4); linker DNA + H1 connects nucleosomes (beads-on-a-string = 10nm fibre)
• 30nm fibre: solenoid model — 6 nucleosomes per turn; stabilised by H1
• Higher order packing: radial loop domains → scaffold → metaphase chromosome (10,000-fold compaction)
• Histone modifications: acetylation (activation — neutralises positive charge → looser chromatin); methylation (activation or repression depending on site); phosphorylation (mitosis); ubiquitination

2.5 Cell Cycle and Division

Cell Cycle Phases

Checkpoints

• G₁/S checkpoint: checks DNA integrity and cell size (p53, Rb tumour suppressors)
• G₂/M checkpoint: checks DNA replication completion and DNA damage
• Spindle assembly checkpoint (SAC): ensures all chromosomes attached to spindle before anaphase (Mad1, Mad2, BubR1)

Mitosis — Detailed

• Prophase: chromatin condenses; centrosomes separate; spindle begins forming; nucleolus disappears
• Prometaphase: nuclear envelope breaks down; spindle fibres attach to kinetochores; chromosomes begin moving
• Metaphase: chromosomes align at metaphase plate; kinetochore microtubules under tension; best stage for karyotyping
• Anaphase: centromeres split (APC/C → separase cleaves cohesin); sister chromatids pulled to poles; kinetochore microtubules shorten; polar microtubules elongate
• Telophase: nuclear envelopes reform around each set of chromosomes; chromosomes decondense; nucleolus reappears
• Cytokinesis: cleavage furrow (animal — actin-myosin contractile ring); cell plate (plant — vesicles from Golgi fuse)

Meiosis — Detailed

• Two successive divisions (Meiosis I + II); one S phase; results in 4 haploid cells
• Meiosis I (Reductional): homologous chromosomes separate

• Metaphase I: bivalents at plate; independent assortment — random alignment of homologs
• Anaphase I: homologs separate (not sister chromatids); chiasma resolution
• Telophase I: two haploid cells formed (each chromosome still has 2 chromatids)
• Meiosis II (Equational): similar to mitosis; sister chromatids separate → 4 haploid cells

2.6 DNA Structure and Replication

DNA Double Helix (Watson & Crick, 1953)

• Antiparallel strands: one 5'→3', other 3'→5'
• Base pairing: A=T (2 hydrogen bonds), G≡C (3 hydrogen bonds) — Chargaff's rules: [A]=[T], [G]=[C]
• B-DNA: right-handed; 10 bp/turn; 3.4 nm pitch; major groove (protein binding) + minor groove
• A-DNA: right-handed; wider, shorter; found in low humidity; RNA:DNA hybrids adopt A-form
• Z-DNA: left-handed; alternating purines-pyrimidines (GC)ₙ; may regulate transcription

DNA Replication — Semi-Conservative (Meselson & Stahl, 1958)

• Origin of replication (oriC): specific sequences where replication begins; multiple origins in eukaryotes
• Helicase (DnaB): unwinds double helix; creates replication fork
• Topoisomerase I: relaxes supercoiling ahead of fork (single-strand nick); Topoisomerase II: removes knots and tangles (double-strand break)
• SSB proteins (single-strand binding): prevent reannealing; stabilize ssDNA template
• Primase: synthesizes short RNA primer (5–10 nt) — needed because DNA polymerase cannot start de novo
• DNA Pol III: main replicative enzyme (prokaryote); 5'→3' synthesis; proofreading (3'→5' exonuclease)
• Leading strand: continuous synthesis in direction of fork movement
• Lagging strand: discontinuous synthesis — Okazaki fragments (100–200 nt in eukaryotes; 1000–2000 in prokaryotes)
• DNA Pol I: removes RNA primers; fills gaps (5'→3' exonuclease + polymerase)
• DNA Ligase: joins Okazaki fragments; seals nicks; requires NAD⁺ (prokaryote) or ATP (eukaryote)
• Eukaryotic: DNA Pol α (primer synthesis), δ (lagging strand), ε (leading strand), β (repair); PCNA (sliding clamp)
• Telomere replication: telomerase (RNA-dependent DNA polymerase + RNA template); extends 3' end; prevents chromosome shortening

2.7 RNA Types and Gene Expression

Transcription

• Template (antisense) strand read 3'→5'; RNA synthesized 5'→3'
• Prokaryotes: one RNA polymerase (σ factor recognises –10/–35 promoter elements); no post-transcriptional modification; coupled transcription-translation
• Eukaryotes: RNA Pol I (28S, 18S, 5.8S rRNA — nucleolus); RNA Pol II (mRNA + snRNA); RNA Pol III (tRNA, 5S rRNA)
• General transcription factors (TFs) + enhancers + mediator complex → RNA Pol II recruitment

Post-transcriptional Processing (Eukaryotes)

• 5' capping: 7-methylguanosine (m⁷G) cap; protects from degradation; ribosome recognition; added co-transcriptionally
• 3' polyadenylation: poly-A tail (200–250 A residues); protects from 3' exonucleases; aids export and translation
• Splicing: introns removed; exons joined; by spliceosome (snRNPs: U1, U2, U4, U5, U6); two transesterification reactions; branch point A → lariat intermediate
• Alternative splicing: different exon combinations → multiple proteins from one gene; ~95% of human multi-exon genes alternatively spliced

Translation

• Genetic code: triplet codon; 64 codons for 20 amino acids + 3 stop codons; degenerate (synonymous codons); universal; non-overlapping; comma-free
• Start codon: AUG = methionine (fMet in prokaryotes); Stop codons: UAA, UAG, UGA
• Single codons: Methionine (AUG), Tryptophan (UGG); most amino acids have 2–6 codons
• Wobble hypothesis (Crick): 3rd position of codon (wobble position) less specific; I (inosine) in anticodon can pair with U, C, or A

tRNA Structure

• Cloverleaf secondary structure; L-shaped tertiary structure
• Anticodon loop (position 34–36): base-pairs with mRNA codon
• Acceptor stem: CCA-3' end; amino acid attachment (aminoacyl-tRNA synthetase — "second genetic code")
• DHU loop: binds EF-Tu; T-loop: ribosome binding

Translation Steps

• Initiation: small subunit binds mRNA (Shine-Dalgarno sequence in prokaryotes; 5' cap scanning in eukaryotes); initiator tRNA binds P site; large subunit joins; GTP hydrolysis
• Elongation: Aminoacyl-tRNA enters A site (EF-Tu, GTP); Peptidyl transferase (23S rRNA — ribozyme) forms peptide bond; Translocation (EF-G, GTP) — peptidyl-tRNA moves P→E, A→P; empty tRNA exits E site
• Termination: Release factor (RF1/RF2) recognises stop codon; water attacks peptidyl-tRNA bond; polypeptide released; ribosome dissociates
• Polyribosomes (polysomes): multiple ribosomes simultaneously translate one mRNA; visible by EM

3.1 Mendelian Genetics

• Law of Segregation (1st Law): two alleles for each trait separate during gamete formation; each gamete receives one allele
• Law of Independent Assortment (2nd Law): genes on different chromosomes assort independently during meiosis
• Test cross: cross unknown genotype with homozygous recessive → phenotype ratio = genotype ratio of unknown parent
• Back cross: cross F₁ with either parent

Modified Mendelian Ratios — Epistasis

3.2 Linkage and Recombination

• Linkage: genes on same chromosome tend to be inherited together (violates independent assortment)
• Complete linkage: no recombination; genes always inherited together (rare)
• Incomplete linkage: crossing over separates some linked genes
• Recombination frequency (RF) = (recombinant class / total offspring) × 100 = map distance in cM (centimorgans)
• 1 cM = 1% recombination = approximately 1 Mb in humans
• Maximum RF = 50% (genes appear unlinked even on same chromosome if far apart)
• Morgan's work on Drosophila: Y-chromosome; sex-linkage; first chromosomal linkage maps

Three-Point Cross — Gene Mapping

• Determines order of 3 genes on chromosome
• Double crossovers: rarest class → identify middle gene (whose combination differs from parental)
• Interference (I) = 1 – Coefficient of Coincidence (COC); COC = observed DCO / expected DCO
• Positive interference (I>0): one crossover reduces probability of nearby crossover

3.3 Mutation — Types and Mechanisms

Gene (Point) Mutations

Chromosomal Structural Mutations

Chromosomal Numerical Mutations

• Aneuploidy: change in individual chromosome number
• Polyploidy: multiples of haploid set (3n, 4n) — common in plants, rare in animals
• Mechanism: non-disjunction during meiosis I or II; most commonly at Meiosis I

3.4 Sex-Linked Inheritance

• X-linked recessive: haemophilia A (Factor VIII), haemophilia B (Christmas disease, Factor IX), red-green colour blindness (OPN1LW/OPN1MW genes), Duchenne muscular dystrophy (dystrophin), G6PD deficiency, Lesch-Nyhan syndrome (HGPRT)
• Carrier female (X^A X^a): 50% sons affected; 50% daughters carrier
• X-linked dominant: hypophosphataemia (Vitamin D-resistant rickets); incontinentia pigmenti; affected father → all daughters affected, no sons
• Y-linked (holandric): hypertrichosis of ear pinnae; only males affected; father → all sons

X-Inactivation (Lyon Hypothesis, 1961)

• One X chromosome randomly inactivated in each somatic cell of females (early embryo, ~day 16)
• Inactivation is random but permanent (clonal); mosaic expression in heterozygous females (e.g., calico/tortoiseshell cats)
• Barr body: condensed inactive X (heterochromatin); number of Barr bodies = n–1 (n = X chromosomes)
• XIST gene: on inactive X; produces Xist RNA that coats and silences the chromosome
• Pseudoautosomal regions (PAR): tip of X and Y short arms; not inactivated; maintain pairing during meiosis

3.5 Quantitative Genetics and Heritability

• Polygenic traits: controlled by multiple genes each with small additive effect + environment; continuous variation; normal distribution
• Skin colour in humans: estimated 4–6 gene pairs; melanin production genes (MC1R, SLC45A2, TYR, OCA2 etc.)
• Phenotypic variance: VP = VG + VE + VGE; VG = VA (additive) + VD (dominance) + VI (interaction/epistatic)
• Broad-sense heritability: H² = VG/VP (proportion of total phenotypic variance due to genetic variance)
• Narrow-sense heritability: h² = VA/VP (proportion due to additive genetic variance — predicts response to selection)
• Breeder's equation: R = h² × S (R = response to selection; S = selection differential)
• QTL (Quantitative Trait Loci) mapping: identifies chromosomal regions containing genes contributing to quantitative traits

3.6 Gene Regulation

Lac Operon (Jacob & Monod, 1961) — Inducible

• Structural genes: lacZ (β-galactosidase — lactose → glucose + galactose), lacY (permease — lactose uptake), lacA (transacetylase)
• Negative regulation: Lac repressor (lacI gene) binds operator → blocks RNA pol → genes OFF by default
• Induction: Allolactose (from lactose) binds repressor → conformational change → repressor leaves operator → transcription ON
• Positive regulation (Catabolite repression): Glucose present → low cAMP → no CAP activation → reduced transcription even without repressor; Glucose absent → high cAMP → cAMP-CAP complex binds → activates transcription (full induction)

Trp Operon — Repressible

• trpEDCBA genes: enzymes for tryptophan synthesis
• Apo-repressor (inactive repressor) + Tryptophan (co-repressor) → active repressor → binds operator → genes OFF
• Attenuation: secondary control; ribosome stalling at Trp codons in leader sequence → transcriptional termination hairpin forms → mRNA release (if Trp abundant)

Eukaryotic Gene Regulation

• Chromatin remodelling: SWI/SNF complex repositions nucleosomes; opens or closes chromatin
• Histone acetylation (HATs): opens chromatin → activation; HDACs (deacetylases) → repression
• DNA methylation: CpG islands in promoters; methylation → gene silencing; established by DNMTs; maintained through replication
• Transcription factors: general TFs (TFIIA, TFIIB, TFIID/TBP, TFIIE, TFIIF, TFIIH) + specific TFs (activators, repressors)
• Enhancers: cis-acting; distance-independent; orientation-independent; tissue-specific; bound by activators
• Silencers: repress transcription; bound by repressors
• Insulators/boundary elements: prevent enhancers from activating wrong genes; CTCF protein; organise topologically associating domains (TADs)
• miRNA: 22 nt; RISC complex; target mRNA → cleavage or translational repression; ~60% human genes regulated by miRNAs
• siRNA: 21–23 nt; exogenous; highly complementary to target → mRNA cleavage; basis of RNAi (Fire & Mello, Nobel 2006)

3.7 Evolution — Theories and Evidence

Pre-Darwinian Theories

Darwinism and Neo-Darwinism

• Darwin (On Origin of Species, 1859): natural selection; variation within populations; overproduction → competition → survival of fittest → differential reproduction
• Limitations of Darwinism: no mechanism for heredity; no explanation of origin of variation
• Neo-Darwinism (Modern Synthetic Theory, 1930s–40s — Fisher, Haldane, Wright, Dobzhansky, Mayr, Simpson): natural selection + Mendelian genetics + population genetics + palaeontology + systematics

Evidence for Evolution

Population Genetics — Hardy-Weinberg Equilibrium

• For 2 alleles (A, a) with frequencies p and q (p + q = 1): genotype frequencies p²(AA) + 2pq(Aa) + q²(aa) = 1
• Conditions for HWE: (1) infinitely large population, (2) random mating (panmixis), (3) no mutation, (4) no migration/gene flow, (5) no natural selection
• Real populations deviate from HWE → evolution occurs

Evolutionary Forces

Speciation

• Biological species concept requires reproductive isolation; pre-zygotic and post-zygotic barriers
• Allopatric speciation: geographic isolation → separate evolution → reproductive isolation (mountains, rivers, oceans as barriers); most common mode
• Sympatric speciation: within same geographic area; polyploidy (allopolyploidy — interspecific hybrid that chromosome doubles → fertile; most common in plants; e.g., wheat Triticum aestivum = hexaploid 6n=42); ecological divergence; assortative mating
• Parapatric speciation: adjacent populations with narrow contact zone; selection drives divergence despite some gene flow
• Reproductive isolating mechanisms (Pre-zygotic): ecological, temporal, behavioural (ethological), mechanical, gametic incompatibility; (Post-zygotic): hybrid inviability, hybrid sterility (mule), hybrid breakdown
• Adaptive radiation: one ancestral species → many ecologically diverse species in short time; Darwin's finches (Galápagos), Cichlid fish (African Rift Valley lakes), Hawaiian honeycreepers

4.1 Species Concepts

4.2 Taxonomic Hierarchy and Nomenclature

• Obligatory categories: Kingdom → Phylum → Class → Order → Family → Genus → Species
• Optional categories: Superphylum, Subphylum, Superorder, Suborder, Superfamily, Subfamily, Subspecies
• Binomial nomenclature: Carolus Linnaeus (Systema Naturae, 10th edition, 1758 — baseline for zoological nomenclature)
• Rules: Genus capitalised; specific epithet lowercase; italicised when printed (underlined when written); author's name may follow; principle of priority — first validly published name is the correct name
• ICZN (International Code of Zoological Nomenclature): governs animal names
• Type specimen: Holotype (single specimen on which species description is based); Paratype (additional specimens in original description); Lectotype (selected from syntypes if no holotype designated); Neotype (replaces lost holotype)

4.3 Schools of Taxonomy

Types of Characters in Cladistics

Types of Groups

5.1 Carbohydrates

• Empirical formula: (CH₂O)ₙ; polyhydroxy aldehydes (aldoses) or ketones (ketoses)
• Monosaccharides: glucose (C₆H₁₂O₆); D/L configuration (based on -OH on penultimate carbon); α/β anomers (glucose in ring form)
• Disaccharides: Maltose (α-1,4-glycosidic bond); Sucrose (α-1,β-2 bond, non-reducing); Lactose (β-1,4 bond)

5.2 Lipids and Membranes

• Fatty acids: RCOOH; saturated (no double bonds, solid at RT, e.g., palmitic acid C16:0), monounsaturated (1 double bond, e.g., oleic acid C18:1), polyunsaturated (PUFA, e.g., linoleic C18:2, arachidonic C20:4)
• Cis vs Trans fatty acids: cis (natural, kinked) unsaturated FAs; trans fats from partial hydrogenation → raise LDL, lower HDL
• Essential fatty acids: linoleic (ω-6) and α-linolenic (ω-3) — cannot be synthesized; dietary requirement
• Phospholipids: glycerophospholipids (phosphatidylcholine, phosphatidylserine, phosphatidylinositol — PIP₂ → IP₃ signalling); sphingomyelin (sphingolipid)
• Steroids: 4 fused rings (cyclopentanoperhydrophenanthrene); cholesterol (membrane fluidity, steroid hormone precursor, bile acid precursor)
• Eicosanoids: derived from arachidonic acid (C20:4); prostaglandins (inflammation, fever, pain, smooth muscle contraction), thromboxanes, leukotrienes (asthma)
• Waxes: long-chain fatty acid + long-chain alcohol; plant cuticle, insect exoskeleton, beeswax (C₃₀H₆₁COOC₃₂H₆₅ — myricyl palmitate)

5.3 Proteins — Structure and Function

• 20 standard amino acids; central carbon (α-carbon) bonded to: NH₂ (amino group), COOH (carboxyl group), H, and R (side chain)
• Essential amino acids (9): Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine (His I Like Lying Men Perfectly To Wear Vests)
• Peptide bond: amide bond between α-amino and α-carboxyl groups; condensation reaction; planar (partial double bond character); trans configuration preferred

Protein Denaturation

• Unfolding of 3D structure without breaking peptide bonds (1° structure intact)
• Agents: heat, pH extremes, urea (disrupts H-bonds), guanidinium chloride, detergents (SDS), organic solvents, heavy metals
• Often irreversible; some small proteins can refold (Anfinsen's experiment — RNase renaturation)
• Anfinsen's dogma: primary sequence determines tertiary structure (all information for folding is in the sequence)
• Chaperones (Hsp70, GroEL/GroES): prevent misfolding and aggregation; do not determine final structure

5.4 Enzymes

• Catalysts: lower activation energy (Ea) without being consumed; do not change reaction equilibrium, only rate
• Active site: 3D cleft/pocket; induced fit model (Koshland) — substrate induces conformational change; lock-and-key (Fischer) — rigid complementarity
• Cofactors: inorganic metal ions (Fe²⁺, Zn²⁺, Mg²⁺, Cu²⁺, Mn²⁺); tightly bound
• Coenzymes: organic; often vitamin-derived; loosely bound (NAD⁺/NADH from Vit B₃; FAD from B₂; CoA from B₅; Thiamine PPi from B₁; PLP from B₆; Biotin from B₇; THF from folate)
• Prosthetic group: tightly bound non-protein component (e.g., haem in haemoglobin and cytochromes)

Enzyme Kinetics

• Michaelis-Menten equation: v = (Vmax × [S]) / (Km + [S])
• When [S] = Km: v = Vmax/2 (Km is substrate concentration giving half-maximum velocity)
• High Km → low affinity (enzyme requires more substrate for half-saturation); Low Km → high affinity
• Kcat (turnover number): maximum number of substrate molecules converted per enzyme molecule per second; Catalytic efficiency = Kcat/Km
• Lineweaver-Burk plot (double reciprocal): 1/v vs 1/[S]; y-intercept = 1/Vmax; x-intercept = -1/Km; used to determine type of inhibition

Allosteric Enzymes

• Multiple subunits; sigmoidal (not hyperbolic) kinetics; cooperative binding
• Activators and inhibitors bind allosteric sites → conformational change → change enzyme activity
• Feedback inhibition: end product inhibits first committed step (e.g., threonine → isoleucine pathway; CTP inhibits ATCase)
• Concerted model (MWC): all subunits simultaneously switch T (tense, inactive) ↔ R (relaxed, active) state
• Sequential model (KNF): binding induces conformational change in one subunit at a time; propagates to neighbours

5.5 Cellular Respiration — Complete

Glycolysis (Embden-Meyerhof-Parnas Pathway) — Cytoplasm

• 10 enzymatic steps; glucose (6C) → 2 pyruvate (3C)
• Investment phase (steps 1–5): uses 2 ATP; glucose → fructose-1,6-bisphosphate → 2 glyceraldehyde-3-phosphate
• Payoff phase (steps 6–10): generates 4 ATP + 2 NADH
• Net yield per glucose: 2 ATP (substrate-level phosphorylation) + 2 NADH
• Key regulated enzymes: Hexokinase (step 1; inhibited by glucose-6-phosphate); Phosphofructokinase-1 (step 3 — rate-limiting; inhibited by ATP, citrate; activated by AMP, ADP, fructose-2,6-bisphosphate); Pyruvate kinase (step 10; inhibited by ATP, alanine; activated by fructose-1,6-bisphosphate)

Pyruvate Oxidation and Krebs Cycle

• Pyruvate → Acetyl-CoA: pyruvate dehydrogenase complex (PDH); CO₂ + NADH produced; in mitochondrial matrix
• Krebs/TCA cycle: Acetyl-CoA (2C) + OAA (4C) → Citrate (6C) → … → OAA (4C) + 2CO₂
• Per turn: 3 NADH + 1 FADH₂ + 1 GTP (substrate-level phosphorylation) + 2 CO₂
• Per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 GTP
• Key enzymes: Citrate synthase (entry point; inhibited by ATP, NADH, citrate); Isocitrate dehydrogenase (rate-limiting; activated by ADP, Ca²⁺); α-Ketoglutarate dehydrogenase; Succinate dehydrogenase (FAD-linked; also Complex II of ETC)
• Anaplerotic reactions: replenish OAA (pyruvate carboxylase → OAA) when cycle intermediates are diverted for biosynthesis

Oxidative Phosphorylation — Electron Transport Chain

• Complex I (NADH:ubiquinone oxidoreductase): NADH → NAD⁺; 2e⁻ → ubiquinone (CoQ); pumps 4 H⁺ across inner membrane; inhibited by rotenone
• Complex II (Succinate dehydrogenase): FADH₂ → FAD; 2e⁻ → CoQ; does NOT pump H⁺; also in Krebs cycle
• CoQ (Ubiquinone): lipid-soluble carrier; mobile; collects electrons from Complex I and II → Complex III
• Complex III (Cytochrome bc₁): CoQH₂ → CoQ; electrons → cytochrome c; pumps 4 H⁺ (Q cycle); inhibited by antimycin A
• Cytochrome c: peripheral protein on inner membrane surface; mobile electron carrier
• Complex IV (Cytochrome c oxidase): 4 cyt c → 4 electrons → O₂ + 4H⁺ → 2H₂O; pumps 2 H⁺; inhibited by cyanide, CO, azide
• Chemiosmosis (Mitchell hypothesis): H⁺ gradient (proton-motive force) drives ATP synthesis through F₀F₁-ATPase
• P/O ratio: NADH = 2.5 ATP; FADH₂ = 1.5 ATP
• Total ATP per glucose: Glycolysis (2 ATP + 2 NADH→5 ATP) + Pyruvate oxidation (2 NADH→5 ATP) + Krebs (6 NADH→15 ATP + 2 FADH₂→3 ATP + 2 GTP) = ~30–32 ATP
• Uncouplers (e.g., DNP, thermogenin in brown fat): dissipate H⁺ gradient without ATP synthesis → energy released as heat; basis of non-shivering thermogenesis in neonates and hibernating mammals

5.6 Muscle Physiology — Sliding Filament Theory

• Skeletal muscle: striated; voluntary; fast-twitch (type II, white, glycolytic) and slow-twitch (type I, red, oxidative)
• Sarcomere: functional unit; Z-disc to Z-disc; contains A-band (dark, myosin) and I-band (light, actin only)
• Filaments: Thick (myosin II: head, neck, tail); Thin (actin + tropomyosin + troponin complex: TnT, TnI, TnC)

Cross-Bridge Cycle (Sliding Filament — Huxley, 1954)

• 1. Action potential → T-tubules → sarcoplasmic reticulum → Ca²⁺ released into cytoplasm
• 2. Ca²⁺ binds troponin C → conformational change → tropomyosin shifts → actin binding sites exposed
• 3. Myosin head (with ADP+Pi) binds actin → cross-bridge formed
• 4. Power stroke: Pi released → myosin head pivots → actin filament slides toward M-line (~10 nm displacement)
• 5. ADP released; rigor state (myosin tightly bound — rigor mortis without ATP)
• 6. ATP binds myosin head → cross-bridge detaches
• 7. ATP hydrolysis (ATPase) → ADP + Pi; myosin head re-cocks to original position
• 8. Cycle repeats if Ca²⁺ still elevated and ATP available
• Relaxation: Ca²⁺ pumped back to SR by SERCA (Ca²⁺-ATPase); tropomyosin re-covers actin sites; muscle elongates passively

Neuromuscular Junction

• Motor end plate: specialised postsynaptic region of muscle fibre
• AP reaches axon terminal → voltage-gated Ca²⁺ channels open → Ca²⁺ influx → ACh vesicle exocytosis
• ACh binds nicotinic receptors (ionotropic) → Na⁺/K⁺ channels open → depolarisation → end-plate potential (EPP)
• EPP triggers AP in muscle fibre → spreads via T-tubules
• Acetylcholinesterase (AChE): breaks ACh → choline + acetate in synaptic cleft; terminates signal
• Myasthenia gravis: autoantibodies against nicotinic ACh receptors → muscle weakness; treated with AChE inhibitors
• Curare: competitive antagonist at nAChR → paralysis (used in surgery)
• Organophosphates (nerve agents): irreversible AChE inhibitors → ACh accumulation → continuous muscle stimulation → spastic paralysis

5.7 Digestion and Absorption

Digestive Enzymes

Absorption Mechanisms

• Glucose and galactose: secondary active transport — Na⁺-glucose cotransporter (SGLT1) apically; GLUT2 basolaterally
• Fructose: facilitated diffusion via GLUT5 apically; GLUT2 basolaterally
• Amino acids: Na⁺-dependent cotransporters; multiple families for different amino acid classes
• Fat digestion: bile salts emulsify fats → lipase acts → fatty acids + 2-monoglycerides → form mixed micelles → diffuse into enterocyte → re-esterified to triglycerides → chylomicrons (ApoB-48) → lacteals → lymph → thoracic duct → blood
• Bile acids: enterohepatic circulation — 95% reabsorbed in terminal ileum; recycled 2–5 times per meal
• Vitamin B₁₂: intrinsic factor (IF, from gastric parietal cells) required; IF-B₁₂ complex absorbed in terminal ileum
• Iron: Fe³⁺ (dietary) → Fe²⁺ (stomach acid + vitamin C → soluble); Fe²⁺ absorbed via DMT1 (divalent metal transporter)

Gut Hormones

5.8 Blood, Circulation and Haemoglobin

Blood Composition

Haemoglobin — Structure and Function

• HbA: α₂β₂ (adult); HbA₂: α₂δ₂; HbF (foetal): α₂γ₂ — higher O₂ affinity than HbA (γ chains bind 2,3-BPG less tightly)
• Each subunit has one haem group; haem = protoporphyrin IX + Fe²⁺ (ferrous); Fe²⁺ binds O₂; Fe³⁺ (methaemoglobin) cannot bind O₂
• O₂-Haemoglobin dissociation curve: sigmoidal (cooperative binding); T (tense, deoxy-Hb) ↔ R (relaxed, oxy-Hb) states
• Bohr effect: CO₂ + H⁺ → shifts curve right → ↓ O₂ affinity → promotes O₂ release at metabolically active tissues; reverse in lungs
• 2,3-BPG (Bisphosphoglycerate): binds β-chains of deoxyHb; stabilises T-state; shifts curve right; ↑ at high altitude → releases more O₂
• CO poisoning: CO binds Hb with 200× higher affinity than O₂ → carboxyhaemoglobin → left shift of remaining curve (Haldane effect) → tissue hypoxia

Blood Coagulation

• Extrinsic pathway: tissue factor (TF) + Factor VII → TF-VIIa → activates Factor X
• Intrinsic pathway: Factor XII → XI → IX → VIIIa-IXa → activates Factor X
• Common pathway: Xa + Va (prothrombinase complex) → Prothrombin (II) → Thrombin (IIa) → Fibrinogen (I) → Fibrin (Ia) → cross-linked clot (Factor XIIIa + Ca²⁺)
• Vitamin K required for: II, VII, IX, X, Protein C, Protein S (γ-carboxylation of glutamate residues)
• Heparin: activates Antithrombin III → inhibits thrombin and Xa; given IV/SC
• Warfarin: Vitamin K antagonist; inhibits clotting factors II, VII, IX, X; oral anticoagulant

5.9 Nervous System Physiology

Resting Membrane Potential

• ~–70 mV (neuron); inside negative relative to outside
• K⁺ more permeable at rest (K⁺ leak channels); K⁺ moves out down concentration gradient (established by Na⁺-K⁺ ATPase: 3 Na⁺ out, 2 K⁺ in)
• Nernst equation: E_ion = (RT/zF) × ln([ion]outside/[ion]inside)
• Goldman equation: incorporates permeability and concentration of all major ions

Action Potential

• All-or-none principle; threshold: ~–55 mV; depolarisation → repolarisation → hyperpolarisation
• Rising phase: voltage-gated Na⁺ channels open → Na⁺ influx → inside becomes positive (+30 mV)
• Falling phase: Na⁺ channels inactivate; voltage-gated K⁺ channels open → K⁺ efflux → repolarisation
• Undershoot (afterhyperpolarisation): K⁺ channels slow to close → briefly more negative than resting potential
• Absolute refractory period: Na⁺ channels inactivated; no AP possible regardless of stimulus
• Relative refractory period: K⁺ channels still open; AP possible only with stronger stimulus
• Conduction velocity: increases with axon diameter and myelination
• Saltatory conduction: AP jumps from node of Ranvier to node (myelinated); faster and more energy-efficient

Synaptic Transmission

5.10 Osmoregulation and Excretion

Strategies in Different Animals

Kidney — Nephron in Detail

• Glomerular filtration: GFR = 125 ml/min; 180 L/day filtered; filtration pressure = hydrostatic (55) – oncotic (30) – Bowman's capsule pressure (15) = 10 mmHg net
• PCT: reabsorbs 65% NaCl, 65% H₂O, 100% glucose, 100% amino acids, 90% HCO₃⁻; isosmotic reabsorption
• Descending Loop of Henle (thin): highly permeable to H₂O; water leaves; filtrate becomes concentrated
• Ascending Loop of Henle (thick): impermeable to H₂O; NaCl actively transported out; dilutes filtrate; establishes medullary gradient
• Countercurrent multiplier: descending (concentrated filtrate) + ascending (NaCl pumped out) → hyperosmotic medullary interstitium → up to 1200 mOsm/kg in human medulla (vs 300 mOsm in cortex)
• DCT: aldosterone-sensitive NaCl reabsorption; Ca²⁺ reabsorption (PTH-stimulated)
• Collecting duct: final regulation; ADH (vasopressin) → inserts aquaporin-2 (AQP2) → water reabsorption; Aldosterone → Na⁺ reabsorption, K⁺ excretion
• RAAS: Low BP → Juxtaglomerular cells release Renin → cleaves Angiotensinogen → Angiotensin I → ACE (lung) → Angiotensin II → Aldosterone (adrenal cortex) + vasoconstriction + ADH release

6.1 Gametogenesis — Complete

Spermatogenesis (Testis — Seminiferous Tubules)

• Spermatogonia (2n, mitosis) → Primary spermatocyte (2n, enters meiosis I) → 2 Secondary spermatocytes (n) → 4 Spermatids (n) → 4 Spermatozoa (spermiogenesis)
• Spermiogenesis: spermatid → spermatozoon; acrosome forms from Golgi; flagellum from centriole; mitochondria form sheath around midpiece; nucleus condenses; cytoplasm shed as residual body
• Sertoli cells: blood-testis barrier (tight junctions); nourishment (lactate to spermatids); phagocytose residual bodies; Inhibin (→ inhibits FSH); Androgen-Binding Protein (ABP) — concentrates testosterone in tubule lumen; Anti-Müllerian Hormone (AMH) in development
• Leydig (interstitial) cells: testosterone synthesis (stimulated by LH); testosterone → spermatogenesis, secondary sex characters, muscle growth
• Spermatozoon structure: Head (nucleus + acrosome — contains hyaluronidase, acrosin, neuraminidase), Midpiece (mitochondrial sheath — ATP for movement), Tail (flagellum: 9+2 microtubule axoneme)
• Sperm capacitation: functional maturation in female reproductive tract (6–8h); hyperactivation; acrosome reaction readiness; Ca²⁺ influx

Oogenesis (Ovary)

• Oogonia → Primary oocyte (2n) → arrested at Prophase I (Dictyotene) — from foetal life through puberty
• At puberty: LH surge → primary oocyte completes Meiosis I → Secondary oocyte (n) + first polar body
• Meiosis II begins but arrested again at Metaphase II (secondary oocyte is ovulated)
• Meiosis II completes ONLY if fertilisation occurs → Ovum (n) + second polar body
• First polar body: may divide → 2 polar bodies; non-functional; allow equitable division of chromosomes without dividing cytoplasm
• Zona pellucida: extracellular glycoprotein coat (ZP1, ZP2, ZP3); ZP3 = primary sperm receptor; zona reaction (cortical reaction) hardens zona to prevent polyspermy
• Cumulus oophorus: granulosa cells surrounding oocyte; hyaluronic acid matrix

6.2 Fertilisation

Steps of Fertilisation

• 1. Sperm penetration of cumulus: hyaluronidase disperses cumulus cells
• 2. Zona binding: sperm surface protein (SED1/ADAM3) binds ZP3 → triggers acrosome reaction
• 3. Acrosome reaction: exocytosis of acrosomal contents (hyaluronidase, acrosin) → digests zona; inner acrosomal membrane exposes new proteins (Izumo) for egg membrane fusion
• 4. Sperm-egg plasma membrane fusion: Juno (egg) + Izumo (sperm) — essential receptor pair (discovered 2014)
• 5. Cortical reaction: sperm entry → Ca²⁺ wave (IP₃-mediated from ER) → cortical granule exocytosis → modified zona (zona hardening) → blocks polyspermy (slow block)
• 6. Fast block to polyspermy: membrane depolarisation (Na⁺ influx, –70 → +20 mV) — prevents further sperm fusion instantly
• 7. Egg activation: completion of Meiosis II; second polar body extruded; male and female pronuclei form; DNA replication begins
• 8. Pronuclei fusion (amphimixis): male + female pronuclei migrate toward each other → nuclear envelopes dissolve → chromosomes intermingle → first cleavage division

6.3 Cleavage

• Mitotic divisions without growth: cells (blastomeres) get progressively smaller; total cytoplasmic volume unchanged
• Nucleo-cytoplasmic ratio increases with each cleavage
• Yolk distribution determines cleavage pattern: Isolecithal (little yolk, equal) → holoblastic equal; Mesolecithal (moderate yolk, vegetal pole) → holoblastic unequal; Telolecithal (abundant yolk) → meroblastic discoidal; Centrolecithal (yolk in centre) → meroblastic superficial

Blastocyst (Mammalian Blastula)

• Morula (16–32 cells) → compaction (E-cadherin-mediated) → fluid accumulates (blastocoel) → Blastocyst
• Blastocyst: Inner Cell Mass (ICM/embryoblast) → embryo proper + amnion + yolk sac
• Trophoblast → placenta (Cytotrophoblast → Syncytiotrophoblast → invades endometrium)
• Implantation: day 6–7 post-fertilisation; syncytiotrophoblast secretes hCG → maintains corpus luteum → progesterone continues

Fate Maps and Determination

• Fate map: chart showing what each blastomere region normally becomes; determined by Vogt using vital dyes in amphibians
• Mosaic development (Protostomes): early determination by cytoplasmic determinants (autonomous specification); isolated blastomere develops into partial embryo
• Regulative development (Deuterostomes): early blastomeres totipotent; specification by induction (conditional specification); isolated blastomere → complete embryo (basis of identical twinning)

6.4 Gastrulation

Gastrulation in Frog (Rana)

• Blastula: ~10,000 cells; blastocoel shifted to animal pole (yolk heavy vegetal pole)
• Dorsal lip of blastopore (DLBP): first site of invagination; key organizer region
• Bottle cells: flask-shaped cells at DLBP; constrict apically → initiate involution
• Involution: surface cells roll over DLBP inward → endoderm and mesoderm formed inside
• Epiboly: animal cap cells spread to cover vegetal yolk mass
• Blastopore: circular opening; eventually closed by yolk plug; becomes anus (deuterostome)
• Archenteron (gastrocoel): primitive gut; new internal cavity formed by invagination; displaces blastocoel

Gastrulation in Chick (Gallus)

• Discoblastula: two layers — epiblast (upper) + hypoblast (lower)
• Primitive streak: thickening at posterior midline; Hensen's node at anterior end (= organizer)
• Ingression: epiblast cells migrate through primitive streak → endoderm (displaces hypoblast) and mesoderm (between epiblast and endoderm)
• Remaining epiblast = ectoderm
• Intraembryonic vs extraembryonic regions: body folds separate embryo from extraembryonic membranes

Germ Layer Derivatives — Complete

6.5 Neurulation and Organogenesis

Neurulation

• Neural plate: thickening of dorsal ectoderm overlying notochord; induced by notochord (primary induction)
• Neural folds: lateral edges of neural plate rise up
• Neural groove: midline depression between neural folds
• Neural tube: neural folds fuse dorsally (anterior first, then posteriorly); hollow tube → brain + spinal cord
• Anterior neuropore: closes first (day 24 in humans); failure → anencephaly
• Posterior neuropore: closes last (day 26); failure → spina bifida
• Neural crest cells: at the lips of the neural folds; delaminate and migrate extensively; epithelial-mesenchymal transition (EMT)

Embryonic Induction

• Induction: one tissue (inducer) directs developmental fate of adjacent tissue (responder) — requires competence
• Competence: ability of tissue to respond to inducer signal; time-limited window
• Primary induction: notochord induces overlying ectoderm → neural plate (Spemann & Mangold, 1924 — Nobel Prize)
• Spemann organiser = dorsal lip of blastopore in amphibians; signals: Chordin, Noggin, Follistatin (BMP antagonists) → neural fate; Wnt signalling → dorsal axis
• Secondary induction: lens induction — optic vesicle (brain outgrowth) induces overlying ectoderm → lens placode
• Tertiary induction: lens induces cornea formation
• Inductive molecules: FGFs, BMPs, Wnt, Shh (Sonic hedgehog), Nodal

Somitogenesis and Limb Development

• Somites: segmental blocks of paraxial mesoderm; form in anterior to posterior sequence (one pair every 90 min in chick)
• Segmentation clock: oscillating Notch/Wnt/FGF signalling; wavefront of FGF determines somite boundary
• Limb bud: lateral plate mesoderm + ectoderm; AER (Apical Ectodermal Ridge) drives proximal→distal growth
• ZPA (Zone of Polarising Activity): posterior mesenchyme; Shh signal → anterior-posterior axis (thumb to little finger)
• Hox genes: collinear expression — position along anterior-posterior axis; homeodomain TFs; mutations → homeotic transformations
• Programmed cell death (apoptosis): interdigital webbing removal; controlled by Caspases; mitochondrial pathway (cytochrome c → Apaf-1 → Caspase 9 → 3)

Extraembryonic Membranes

6.6 Placentation

Placenta Structure and Function

• Formed by fetal chorion + maternal decidua basalis (modified endometrium)
• Functions: respiratory gas exchange (O₂ in, CO₂ out), nutrient transfer (glucose, amino acids, FAs, vitamins), waste removal (urea, CO₂), hormone production, immunological protection (IgG transfer)
• Chorionic villi: tree-like projections increase surface area; intervillous space (maternal blood bathes villi in haemochorial type)

Placental Hormones

6.7 Growth, Regeneration and Ageing

Types of Growth

Regeneration

• Reparative regeneration: replacement of lost parts; differs from wound healing
• Epimorphosis: wound epithelium covers stump → blastema forms (dedifferentiation of stump cells) → redifferentiation into missing structures; salamander limb, newt lens (from iris), Axolotl
• Morphallaxis: reorganization of existing material without major cell proliferation; Hydra — if cut into pieces, each piece reorganizes with reduced size but complete organism
• Compensatory hypertrophy: remaining organ increases size/function after partial removal; liver (up to 75% removed → regrows in mammals); kidney (contralateral hypertrophy)
• Planaria: most remarkable regeneration; any piece can reform whole organism; neoblasts (pluripotent stem cells) in mesenchyme
• Starfish: arm + part of central disc → complete organism; each arm alone (in some species)
• Lizard tail regeneration: cartilage replaces bone; incomplete but functional
• Invertebrates generally have greater regenerative capacity than vertebrates

Theories of Ageing

7.1 Ecological Concepts and Niche

• Levels: Individual → Population → Community → Ecosystem → Landscape → Biome → Biosphere
• Habitat: physical address (where an organism lives); characterized by abiotic factors
• Ecological niche: n-dimensional hypervolume (Hutchinson, 1957) — all abiotic + biotic conditions an organism can tolerate
• Fundamental niche: full potential range without competition; Realized niche: actual niche with competition and predation constraints
• Niche breadth: specialist (narrow, e.g., panda) vs generalist (broad, e.g., raccoon, cockroach)
• Competitive exclusion principle (Gause's principle): two species cannot coexist indefinitely in the same niche; one outcompetes the other
• Character displacement: competing species evolve to differ more when coexisting (e.g., beak sizes of Darwin's finches on same island)

7.2 Population Ecology

Population Growth Models

• Exponential growth: dN/dt = rN; N(t) = N₀eʳᵗ; J-shaped curve; r = intrinsic rate of natural increase (rmax in ideal conditions)
• r = b – d (birth rate – death rate) + immigration – emigration
• Logistic growth: dN/dt = rN(K–N)/K; S-shaped (sigmoidal); K = carrying capacity; inflection point at N = K/2 (maximum growth rate)
• At N = K: growth rate = 0; population stabilizes
• Allee effect: population growth rate decreases at very low population density (difficulty finding mates, cooperative behaviour lost)

Life History Strategies

Age Structure and Life Tables

• Age pyramid: expanding (triangular — high birth rate, young population); stable (bell-shaped); declining (narrow base — low birth rate)
• Life table: lx (survivorship = probability of surviving to age x); mx (fecundity at age x); Ro = Σlx·mx (net reproductive rate); Generation time T = Σ(x·lx·mx)/Ro
• Survivorship curves: Type I (convex — good survival early, death in old age — humans, elephants); Type II (linear — constant mortality rate — birds, squirrels); Type III (concave — high early mortality, survivors long-lived — fish, oysters, most invertebrates)

Population Regulation

7.3 Community Ecology

Species Diversity Indices

• Species richness (S): simply number of species
• Shannon-Wiener index (H'): H' = –Σ(pᵢ × ln pᵢ); where pᵢ = proportion of species i; higher H' = greater diversity
• Simpson's index (D): D = Σ(nᵢ(nᵢ–1)) / (N(N–1)); D = probability that two randomly chosen individuals belong to different species; higher D = higher diversity
• Evenness (Pielou's J): J = H'/H'max; ranges 0–1; 1 = perfectly even distribution
• Alpha diversity: within-habitat diversity; Beta diversity: between-habitat diversity (species turnover); Gamma diversity: total regional diversity
• Dominance: most abundant/biomass species; Keystone species: disproportionately large ecological impact relative to biomass (sea otter — controls sea urchin → protects kelp forests; Pisaster seastar — controls mussels)

Ecological Succession

• Primary succession: begins on bare substrate with no soil; slow; lichens → mosses → herbaceous plants → shrubs → trees
• Secondary succession: begins after disturbance in area with soil; faster than primary; abandoned farmland → grassland → shrubland → woodland → forest
• Pioneer species: first to colonize; tolerant of harsh conditions; modify environment for later species; lichens (in rock); Eichhornia (in aquatic)
• Climax community: stable end state; in equilibrium with climate; self-reproducing; composition varies with climate (tropical rainforest, temperate deciduous forest, boreal forest, grassland, desert)
• Facilitation model: early species modify environment → make it suitable for later species (classical model)
• Tolerance model: later species tolerate what earlier species cannot; competition determines succession
• Inhibition model: first occupants inhibit later colonizers; succession only when disturbance removes current occupants

Hydrosere (Aquatic to Terrestrial Succession)

• Open water → Phytoplankton/submerged plants → Emergent plants (reeds, sedges) → Swamp/marsh → Fen/wet meadow → Alder/willow woodland → Oak/ash climax woodland

Lithosere (Bare Rock)

• Bare rock → Crustose lichens → Foliose lichens → Mosses → Herbaceous plants → Shrubs → Woodland climax

Species Interactions — Detailed

Predator-Prey Dynamics (Lotka-Volterra)

• Prey: dN/dt = rN – aNP (r = growth rate; a = predation rate; P = predator density)
• Predator: dP/dt = baNP – mP (b = efficiency converting prey to predator offspring; m = predator mortality)
• Oscillating cycles: prey increases → predator increases → prey decreases → predator decreases → cycle repeats
• Classic example: Canadian lynx (Lynx canadensis) and snowshoe hare (Lepus americanus) — 10-year cycles from Hudson Bay Company fur records

7.4 Ecosystem Ecology

Energy Flow

• Producers (autotrophs, P) → Primary consumers (herbivores, C1) → Secondary consumers (C2) → Tertiary consumers (C3)
• Lindemann's 10% law (1942): only ~10% of energy passes from one trophic level to next; rest lost as heat/respiration
• Ecological efficiency = production at level n+1 / production at level n
• Gross Primary Production (GPP): total energy fixed by photosynthesis
• Net Primary Production (NPP): GPP – plant respiration; available for consumers; highest in tropical forests and estuaries
• Secondary production: energy incorporated by consumers; conversion efficiency varies by organism

Ecological Pyramids

Biogeochemical Cycles — Complete

• Carbon cycle: Photosynthesis (CO₂ → organic C) and Respiration (organic C → CO₂); Decomposition; Combustion; Ocean dissolution (CO₂ + H₂O → H₂CO₃ → HCO₃⁻); Geological sequestration (fossil fuels); Net flux: ~8 Gt C/year added to atmosphere by fossil fuels; forests and oceans as carbon sinks
• Nitrogen cycle: N₂ fixation (biological: Rhizobium, Azotobacter, Anabaena, Clostridium; abiotic: lightning; industrial: Haber-Bosch process) → Ammonification (organic N → NH₄⁺ by decomposers) → Nitrification (NH₄⁺ → NO₂⁻ by Nitrosomonas; NO₂⁻ → NO₃⁻ by Nitrobacter) → Assimilation (plants absorb NH₄⁺ or NO₃⁻) → Denitrification (NO₃⁻ → N₂ by Pseudomonas, Thiobacillus; anaerobic conditions)
• Phosphorus cycle: Sedimentary cycle (no gaseous phase); rock weathering releases PO₄³⁻; plant uptake; animal consumption; decomposition returns P to soil; ocean sedimentation → geological uplift; limiting nutrient in many ecosystems (eutrophication)
• Sulphur cycle: Rock weathering; volcanic activity (SO₂); oxidation (SO₂ → H₂SO₄ → acid rain); sulphur bacteria (desulfovibrio — reduces SO₄²⁻ → H₂S; Thiobacillus — oxidises H₂S → SO₄²⁻); assimilation in proteins (cysteine, methionine)
• Water cycle (Hydrological): Evaporation, Transpiration (evapotranspiration), Condensation, Precipitation, Runoff, Infiltration, Groundwater flow; forests regulate local water cycles

Biomes of the World

7.5 Environmental Pollution — Detailed

Air Pollution

• Primary pollutants (directly emitted): SO₂ (burning sulphur-containing fuels → acid rain), NOₓ (combustion, vehicle exhaust), CO (incomplete combustion), PM (particulate matter — PM₂.₅ causes lung disease), VOCs (volatile organic compounds — solvents, paints)
• Secondary pollutants (formed in atmosphere): Ground-level O₃ (VOC + NOₓ + sunlight → photochemical smog), H₂SO₄ (SO₂ + OH → → acid rain), PAN (peroxyacetyl nitrate — photochemical smog component, eye irritant)
• Acid rain: pH < 5.6; dissolves nutrients, leaches Al³⁺ (toxic to fish); damages forests; corrodes buildings (Taj Mahal affected)
• Greenhouse gases (GHGs): CO₂ (fossil fuels, deforestation — most important), CH₄ (cattle, paddy fields, landfills — 25× GWP of CO₂), N₂O (fertilizers, livestock — 298× GWP), CFCs (refrigerants — also deplete ozone; 5000–10,000× GWP), Tropospheric O₃
• Ozone depletion: CFCs → Cl· radicals in stratosphere → catalytically destroy O₃ (one Cl· destroys 100,000 O₃ molecules); Antarctic ozone hole; Montreal Protocol (1987) → phase-out of CFCs; ozone slowly recovering
• AQI (Air Quality Index): India: Good (0–50), Satisfactory, Moderate, Poor, Very Poor, Severe (401–500)

Water Pollution and Eutrophication

• BOD (Biological Oxygen Demand): O₂ consumed by microbes to decompose organic matter in water; indicator of organic pollution; clean water < 1 mg/L; polluted water > 5 mg/L
• DO (Dissolved Oxygen): healthy water > 6 ppm; fish die below 4 ppm; pollution reduces DO
• Eutrophication: enrichment with nutrients (N, P from fertilizers, sewage) → algal bloom → decomposition consumes O₂ → hypoxia/anoxia → fish kill; dead zones (e.g., Gulf of Mexico)
• Biomagnification: pollutants (DDT, Hg, Pb, PCBs) increase in concentration up food chain; lipophilic compounds accumulate in fat
• Classic cases: DDT in pelicans/eagles (eggshell thinning, reproductive failure); Minamata disease (organic Hg, Japan 1956); Itai-itai disease (Cd pollution, Japan); Arsenic contamination (Bangladesh/West Bengal — groundwater)
• Thermal pollution: hot water discharge from power plants → reduces DO, disrupts aquatic organisms

Biodiversity and Conservation

• Biodiversity levels: Genetic (within species); Species (number of species); Ecosystem/Habitat (variety of habitats)
• Latitudinal diversity gradient: biodiversity highest at equator, decreases towards poles
• Island Biogeography Theory (MacArthur & Wilson, 1967): species richness on island = equilibrium between immigration and extinction; S ~ A^z (species-area relationship; z ≈ 0.25 for islands, 0.15 for mainland)

7.6 Ethology — Animal Behaviour

Innate vs Learned Behaviour

Communication in Animals

• Visual: Firefly (species-specific bioluminescent flash patterns — mating); peacock display; facial expressions; body postures; territorial displays
• Chemical (Pheromones): Species-specific; sex attractants (Bombykol — Bombyx mori female; identified by Butenandt 1959; first pheromone isolated); alarm pheromones (ants, bees — isopentyl acetate); trail pheromones (ants); aggregation pheromones; primer pheromones (alter physiology, e.g., queen substance in bees inhibits ovary development in workers)
• Acoustic: Frog calling (species-specific; mate attraction; territorial); bird song (learned; dialects; sexual selection); bat echolocation (biosonar; ultrasound; CF and FM calls); whale song (humpback — complex; long-range; changes annually); cricket chirping (stridulation; temperature-dependent; Dolbear's law)
• Tactile: Grooming (primate social bonding); vibration (spider web sensing); bee dance (kinesthetic communication)
• Bee dance (von Frisch, 1967 Nobel Prize): Round dance → food within 50–100m; Waggle dance → food >100m; Angle of waggle run relative to vertical = angle of food source relative to sun; Duration of waggle = distance; Number of waggle runs = distance; Straight up = towards sun

Social Behaviour

• Altruism: behaviour benefiting others at cost to self; apparently paradoxical from evolutionary standpoint
• Kin selection (Hamilton, 1964): altruism towards relatives; rb > c (r = coefficient of relatedness, b = benefit to recipient, c = cost to actor); inclusive fitness = direct (own reproduction) + indirect (through relatives)
• r values: identical twins = 1.0; parent-offspring = 0.5; siblings = 0.5; aunt/nephew = 0.25; first cousins = 0.125
• Eusociality: extreme altruism; in Hymenoptera (bees, ants, wasps) and termites; workers sterile; haplodiploidy in Hymenoptera → workers related by 0.75 (supersisters) → evolutionarily favourable to raise sisters than own offspring (relatedness 0.5)
• Reciprocal altruism (Trivers, 1971): altruism between unrelated individuals; expectation of future reciprocation; iterated Prisoner's Dilemma; examples: vampire bat blood sharing; cleaner fish; grooming in primates
• Dominance hierarchy (pecking order): linear rank; reduces intragroup aggression; top individual gets priority access to food, mates; established by fighting, then maintained by signals
• Territorial behaviour: advertisement (song, scent marking, visual displays) → intimidation/escalation → combat (rarely); benefits (resource defence, mate quality signalling) vs costs (energy, injury risk)
• Sociobiology (E.O. Wilson, 1975): evolutionary basis of all social behaviour; extension of neo-Darwinian synthesis to behaviour

Migration

• Seasonal movement for feeding or breeding; programmed by photoperiod and endocrine system
• Navigation mechanisms: Sun compass (birds, bees); Star compass (birds use stellar rotation patterns); Magnetic field (magnetite crystals in upper beak of birds, Cryptochrome flavoproteins in eyes); Olfaction (salmon imprint on home stream odour); Landmarks
• Examples: Arctic tern (Sterna paradisaea) — longest migration 70,000 km; Monarch butterfly (Danaus plexippus) — Mexico ↔ N America; European eel (Anguilla anguilla) — Atlantic Ocean; Bar-tailed godwit — 11,000 km non-stop flight

8.1 Sericulture — Complete

Silk-Producing Species

Bombyx mori — Life Cycle

• Egg: ~600 eggs/female; 1–2 cm long; hatching in 10–12 days; eggs can be stored at low temp (hibernated eggs for univoltine strains)
• Larva (caterpillar/silkworm): 5 instars; 4 moults; 28–33 days total; feeding on mulberry leaves; body weight increases 10,000-fold; silk glands develop
• Spinning: larva stops feeding; silk glands fill with liquid silk (fibroin); larva moves head in figure-8 → spins cocoon over 3–4 days; ~1.5 km silk filament in single cocoon
• Pupa: 9–12 days; metamorphosis inside cocoon
• Adult moth: wings vestigial; cannot fly; cannot feed; lives 2–3 days; females release sex pheromone (Bombykol) to attract males; mates → lays eggs → dies

Silk Processing

• Stifling: killing pupa before it cuts cocoon (heat, steam, or sunlight)
• Reeling: cocoons softened in hot water (80–90°C) → sericin (gum) dissolves → filament from 5–8 cocoons twisted together → raw silk thread
• Throwing: two or more raw silk threads twisted together for strength
• Weaving → degumming → dyeing → finishing
• Silk composition: Fibroin (75–80%; core protein; crystalline β-sheet structure; gives strength); Sericin (20–25%; outer gum; binds filaments; removed in degumming)
• India: 2nd largest producer and largest consumer of silk; Karnataka (Kolar district) → mulberry silk capital

Diseases of Silkworm

8.2 Apiculture — Complete

Honey Bee Species

Colony Structure and Castes

Honey Production and Products

• Nectar collection: forager bees collect nectar (sucrose solution); carry in crop; pass to house bees
• Honey making: Invertase (sucrase) converts sucrose → glucose + fructose; water evaporated from 80% → 20% by fanning; stored in capped cells
• Honey composition: ~80% sugars (glucose + fructose + sucrose); ~17% water; organic acids (gluconic); enzymes; antioxidants; hydrogen peroxide (antibacterial)
• Beeswax: secreted by 4 pairs of wax glands on worker abdomen; composed of esters of long-chain fatty acids + long-chain alcohols; melting point 62–65°C; used for comb construction
• Royal jelly: mandibular gland secretion of young workers; 65% water; proteins (MRJPs — major royal jelly proteins), sugars, fatty acids (10-HDA — unique); fed to all larvae initially, only to queen larvae throughout
• Propolis: resinous material from plant buds; antimicrobial; used to seal cracks in hive; rich in flavonoids
• Bee venom (apitoxin): from sting gland; Melittin (major component — 50%; lytic peptide); Phospholipase A2 (enzyme; most allergenic); Apamin (neurotoxin); Histamine; used in apitherapy
• Bee pollen: collected by foragers in pollen baskets (corbiculae); rich in proteins, vitamins, minerals; sold as health food supplement

Bee Diseases and Pests

8.3 Lac Culture

• Lac insect: Laccifer lacca (= Kerria lacca); family Kerriidae; scale insect (Hemiptera); female secretes lac resin
• Lac is the only commercial resin of animal origin; India produces ~50% of world's lac
• States: Jharkhand (largest), Chhattisgarh, West Bengal, Odisha, Maharashtra
• Host trees: Palas (Butea monosperma — best), Kusum (Schleichera oleosa — superior quality lac), Ber (Ziziphus mauritiana), Peepal
• Life cycle: female insect settles on twig → secretes lac over body → forms hard encrustation → lays eggs → 300 nymphs → crawlers emerge (infests new twigs)
• Harvesting: Sticklac (broodstic scraped from twigs) → Seedlac (washed, heated, filtered) → Shellac (melted, strained, sheet-formed)
• Crops: Baisakhi crop (inoculation Oct-Nov; harvest June-July); Katki crop (inoculation June-July; harvest Jan-Feb)
• Products and uses: Shellac (varnish, lacquer, nail polish, gramophone records, food glazing E904); Lac dye (red pigment; used in food colouring, cosmetics); Seed lac (base for all lac products)

8.4 Pearl Culture

• Pearl oyster (marine): Pinctada fucata (Japanese/Indian pearl oyster), P. margaritifera (black-lipped), P. maxima (gold/silver-lipped); freshwater pearl mussel: Hyriopsis cumingii (China)
• Pearl formation: foreign particle (grain of sand) enters mantle → mantle cells proliferate → form pearl sac → nacre secreted concentrically around nucleus
• Nacre (mother-of-pearl): alternating layers of aragonite (CaCO₃) + conchiolin (protein); iridescence due to light diffraction through thin layers
• Pearl composition: ~92% CaCO₃ (aragonite) + ~6% conchiolin + ~2% water

Cultured Pearl Production (Mikimoto Method)

• Nucleus: spherical shell bead (from Mississippi freshwater mussel shell) + small piece of mantle epithelium from donor oyster
• Surgical implantation: nucleus + mantle tissue piece inserted into gonad of host oyster
• Rearing: oysters suspended in cages in sea (2–5 years); periodic cleaning; rotation
• Harvesting: oysters opened; pearl extracted; cleaned; graded by size, shape, lustre, surface quality, colour
• Kokichi Mikimoto: first commercial cultured pearls (1893 — Japan); revolutionised pearl industry
• India: Gulf of Mannar (Tamil Nadu), Palk Bay, Gulf of Kutch; Central Marine Fisheries Research Institute (CMFRI, Kochi) develops techniques
• Types: Akoya (saltwater; Japanese); South Sea (large, gold/white); Tahitian (black); Freshwater (China — most produced)

8.5 Poultry Science

Important Breeds

Poultry Management

• Layers vs Broilers: Layers → egg production (begin at 18–20 weeks; peak at 25–30 weeks; 280–320 eggs/year); Broilers → meat (market weight 2–3 kg in 6–7 weeks)
• Incubation: 21 days (chicken); 28 days (turkey, duck, guinea fowl); 18 days (quail); temperature 37.5–38°C; humidity 55–65% (60–70% last 3 days); turning 3 times/day (days 1–18)
• Housing systems: Deep litter (floor housing, 10 birds/m²); Cage (battery cage, individual); Semi-intensive; Free range
• Feed conversion ratio (FCR): kg feed consumed per kg weight gain; Broiler FCR ≈ 1.8–2.0; lower FCR = more efficient
• Common diseases: Ranikhet (Newcastle Disease) — paramyxovirus — most important; Infectious Bursal Disease (Gumboro); Marek's disease (herpesvirus; lymphoma); Fowl cholera (Pasteurella); Coccidiosis; Infectious bronchitis
• Vaccination schedule: Marek's (day 1 at hatchery); Ranikhet (F1 strain day 5–7, R2B strain day 30, booster every 6 months)

8.6 Pisciculture and Aquaculture

Major Carp Culture (Composite Fish Culture)

• Principle: different species occupy different water levels and feed on different food → maximum utilisation of pond resources
• Stocking ratio (typical): Catla (20%): surface feeder, zooplankton; Rohu (30%): column feeder, vegetation; Mrigal (20%): bottom feeder, detritus; Grass carp (10%): column, aquatic weeds; Silver carp (10%): surface, phytoplankton; Common carp (10%): bottom, omnivore

Induced Breeding

• Problem: Indian major carps don't breed in captivity (need flood conditions — flowing water, rise in level, turbidity)
• Hormone injection triggers maturation and spawning
• Pituitary extract: homogenised pituitary gland from carp; first used by Houssay (1931) in Argentina; in India by Dr. K.H. Alikunhi (CIFRI)
• Ovaprim: commercially available; Salmon GnRH analogue + domperidone (blocks dopamine inhibition); single injection; safe; widely used
• WOVA-FH: Whitefish ovulatory variant-Fish hormone; GnRH analogue
• Strip spawning: females injected → eggs stripped manually → mixed with milt → fertilised artificially → incubated in jars

Mariculture (Marine Aquaculture)

• Shrimp culture: Penaeus monodon (giant tiger prawn — Black tiger shrimp) and Litopenaeus vannamei (Pacific white shrimp — now dominant in India); semi-intensive and intensive systems
• Mussel culture: Perna viridis (green mussel); rope/raft culture; Kerala, Goa
• Oyster culture: Crassostrea madrasensis (Indian backwater oyster); rack-and-tray methods
• Seaweed farming: Kappaphycus alvarezii; Gracilaria; produces carrageenan and agar; used in food, pharmaceuticals
• Sea cage farming: Cobia (Rachycentron canadum), Grouper; offshore cages

Ornamental Fish Trade

• India: major exporter of ornamental fish; Kolkata is hub; Kerala (wild collection), Tamil Nadu, West Bengal
• Popular species: Guppy (Poecilia reticulata), Molly, Goldfish (Carassius auratus), Angelfish (Pterophyllum scalare), Discus, Arowana
• Indigenous ornamental fish: Tiger barb (Puntius tetrazona), Clown loach, Koi (variety of common carp)

9.1 Parasitic Diseases — Complete

Protozoan Diseases

Helminthic Diseases — Detailed

9.2 Insect Vectors — Complete

Vector Control Strategies

• Biological control: Gambusia affinis (mosquitofish) — larvae predator; Bacillus thuringiensis israelensis (Bti) — toxin kills mosquito and blackfly larvae; Beauveria bassiana (entomopathogenic fungus); Neem (Azadirachta indica) — natural pesticide
• Sterile Insect Technique (SIT): mass-reared males sterilised (radiation/chemical) → released → mate with wild females → no offspring; used against tsetse fly (IAEA programme); screw-worm fly eradication
• Integrated Vector Management (IVM): WHO recommended approach combining multiple methods; environmental management + biological + chemical + personal protection
• Genetically modified mosquitoes: OX513A (Oxitec) — male mosquitoes with lethal gene; offspring die before reproducing; tested in Brazil, Cayman Islands; reducing Aedes populations

9.3 Biotechnology in Zoology

Recombinant DNA Technology

• Restriction enzymes (molecular scissors): Type II most used (EcoRI cuts GAATTC; BamHI cuts GGATCC; HindIII cuts AAGCTT); produce sticky or blunt ends
• Vectors: pBR322 plasmid (ampicillin + tetracycline resistance for selection); pUC19; lambda phage; cosmid; BAC (bacterial artificial chromosome); YAC (yeast AC) for large inserts
• DNA ligase: joins DNA fragments; T4 DNA ligase in recombinant work
• Transformation: introducing recombinant DNA into host; CaCl₂ method (heat shock); electroporation
• Selection: blue-white screening (lacZ gene disruption) or antibiotic resistance

Key Applications

9.4 Wildlife Conservation — India

10.1 Integumentary System

Feather Structure

• Quill (calamus): base embedded in follicle; shaft (rachis): central axis; Vane: barbs + barbules + hooks (hamuli) — hooks interlock to form smooth surface (Velcro-like)
• Contour feathers: define body shape; wing (remiges) and tail (rectrices) flight feathers; covert feathers
• Down feathers: fluffy; no hooks; excellent insulation; closest to skin
• Filoplumes: hair-like; few barbs; sensory; detect feather position
• Bristles: stiff; around bill; filter food; in insectivores
• Powder down feathers: disintegrate into powder; waterproofing in herons, cockatoos

10.2 Digestive System Comparative

10.3 Circulatory System Comparative

Aortic Arches — Comparative Embryology

• All vertebrates start with 6 pairs of pharyngeal aortic arches (I–VI) in embryo
• I, II: disappear early; III: becomes carotid arteries (internal); IV: becomes systemic arch (right in birds, left in mammals); V: disappears; VI: becomes pulmonary artery (ductus arteriosus connects to systemic — closes at birth)
• Fish retain all 6 as gill arches; Amphibia 3 remain (carotid, systemic, pulmocutaneous)

10.4 Respiratory System Comparative

10.5 Nervous System Comparative

10.6 Reproductive System Comparative

10.7 Skull Types and Dentition

Skull Temporal Fenestrae

Dentition Types

Dental Formulae (Mammals)

• Human: I 2/2 C 1/1 PM 2/2 M 3/3 = 32 permanent teeth
• Dog: I 3/3 C 1/1 PM 4/4 M 2/3 = 42 teeth
• Rabbit: I 2/1 C 0/0 PM 3/2 M 3/3 = 28 teeth (gap = diastema)
• Elephant: I 1/0 C 0/0 PM 0/0 M 3/3; tusks are modified incisors; teeth replaced 6 times horizontally (last at age 70)

11.1 Hormone Classification and Action Mechanisms

Second Messenger Pathways

• cAMP pathway: Hormone → Gs-coupled GPCR → Adenylyl cyclase → cAMP → PKA (protein kinase A) → phosphorylation of target proteins; terminated by PDE (phosphodiesterase)
• IP₃/DAG pathway: Hormone → Gq-coupled GPCR → Phospholipase C → PIP₂ → IP₃ (→ Ca²⁺ release from ER) + DAG (→ PKC activation)
• Tyrosine kinase pathway (RTK): Insulin → Insulin receptor (RTK) → autophosphorylation → IRS-1 → PI3K → Akt → GLUT4 translocation; Grb2 → Ras → MAPK cascade (growth effects)
• Nuclear receptor pathway: Steroid → enters cell → binds receptor → ligand-receptor complex → DNA binding (HRE — Hormone Response Element) → transcription activation/repression
• Feedback control: negative feedback (most hormone systems); positive feedback (LH surge triggered by oestrogen in ovulatory cycle)

11.2 Hypothalamus — Pituitary Axis

• Hypothalamus: master regulator; produces releasing (RH) and inhibiting hormones (IH) → portal circulation → anterior pituitary
• Portal system: hypophyseal portal blood supply; hypothalamic hormones reach pituitary in high concentrations without dilution

Anterior Pituitary Hormones — Complete

Posterior Pituitary Hormones

• Synthesized in hypothalamus (supraoptic and paraventricular nuclei) → transported along axons → stored and released from posterior pituitary
• ADH (Arginine Vasopressin, AVP): stimulated by high plasma osmolarity (osmoreceptors) and low blood volume/pressure; V2 receptors in collecting duct → cAMP → AQP2 insertion → water reabsorption; V1a receptors in blood vessels → vasoconstriction; Diabetes insipidus: cranial (no ADH) or nephrogenic (no ADH response); treated with desmopressin (DDAVP) for cranial DI
• Oxytocin: stimulated by uterine stretching (Ferguson reflex) and nipple stimulation; promotes uterine contractions (positive feedback during labour); milk ejection (let-down reflex); social bonding (pair bonding, maternal behaviour, trust); used clinically to induce/augment labour (Syntocinon)

11.3 Thyroid, Parathyroid and Adrenal — Complete

Thyroid Gland

• Two lobes connected by isthmus; follicular cells synthesize T₃/T₄; C cells (parafollicular) synthesize Calcitonin
• Thyroid hormone synthesis: dietary iodine → I⁻ → oxidised to I₂ (thyroid peroxidase, TPO) → iodination of tyrosine residues on thyroglobulin → MIT (monoiodotyrosine) + DIT (diiodotyrosine) → coupling (MIT+DIT=T₃; DIT+DIT=T₄) → stored in colloid
• Release: TSH → proteolysis of thyroglobulin → T₃ + T₄ released; T₄ converted to T₃ by 5'-deiodinase (active form) in peripheral tissues
• Transport: 99%+ bound to TBG (Thyroxine-binding globulin), prealbumin, albumin; only free hormone is active
• Actions: Calorigenic (↑ BMR — all tissues except brain, spleen, testis); Permissive for GH; Brain development (critical in foetal/neonatal period); Cardiac (↑ heart rate, contractility); Metabolism (↑ glucose absorption, lipolysis, cholesterol conversion)

Parathyroid Gland

• 4 small glands on posterior thyroid; chief cells secrete PTH; oxyphil cells (unknown function)
• PTH (Parathyroid Hormone): stimulated by low Ca²⁺ (Ca²⁺-sensing receptor, CaSR); raises blood Ca²⁺ via: (1) Osteoclast activation (bone resorption → Ca²⁺ + PO₄³⁻ released), (2) Renal Ca²⁺ reabsorption ↑ (DCT), (3) Renal PO₄³⁻ excretion ↑ (phosphaturic), (4) ↑ 1α-hydroxylase → calcitriol (1,25-OH₂D₃ = active Vitamin D) → intestinal Ca²⁺ absorption ↑
• Calcitonin (from C cells): opposes PTH; lowers blood Ca²⁺; inhibits osteoclasts; used therapeutically in Paget's disease and hypercalcaemia
• Vitamin D metabolism: Diet/Skin (UV light) → Cholecalciferol (D₃) → Liver (25-hydroxylase) → 25-OH-D₃ → Kidney (1α-hydroxylase, stimulated by PTH) → 1,25-(OH)₂-D₃ (Calcitriol — active)

Adrenal Gland — Zones

Adrenal Disorders

11.4 Pancreatic Endocrinology and Diabetes

• Islets of Langerhans: 1–2 million; 1–2% of pancreas; α cells (20% — glucagon), β cells (70% — insulin), δ cells (5% — somatostatin), PP cells (pancreatic polypeptide), ε cells (ghrelin)

Insulin Synthesis and Secretion

• Pre-proinsulin → Proinsulin (in RER) → Insulin + C-peptide (in Golgi, by prohormone convertases PC1/PC2)
• C-peptide: equimolar with insulin; longer half-life; marker of endogenous insulin secretion (distinguishes exogenous insulin from endogenous in diabetics)
• Insulin secretion (KATP channel model): Glucose enters β cell (GLUT2) → Glucokinase (hexokinase IV) phosphorylates → Glycolysis → ↑ ATP:ADP ratio → KATP channel closes → Membrane depolarises → Voltage-gated Ca²⁺ channel opens → Ca²⁺ influx → Insulin vesicle exocytosis
• Biphasic secretion: 1st phase (rapid, 2–5 min) — stored insulin; 2nd phase (slow, 30+ min) — newly synthesized
• Incretin effect: GLP-1 and GIP enhance glucose-stimulated insulin secretion; basis for GLP-1 agonist drugs (liraglutide)

11.5 Reproductive Endocrinology

Menstrual Cycle

• 28-day cycle; Day 1 = first day of menstruation; Follicular phase (days 1–14) + Ovulatory phase (day 14) + Luteal phase (days 14–28)
• Follicular phase: FSH → follicle development → oestrogen ↑ (from granulosa cells under LH stimulation via theca); oestrogen → proliferative endometrium; rising oestrogen → positive feedback → LH surge (day 13–14)
• Ovulation: LH surge (36–40h before ovulation) → oocyte completes Meiosis I; dominant follicle ruptures → secondary oocyte released
• Luteal phase: ruptured follicle → corpus luteum (CL) under LH; CL secretes Progesterone (dominant) + Oestrogen; Progesterone → secretory endometrium (glands, decidualisation); negative feedback → low FSH/LH; if no fertilisation → CL involutes (progesterone ↓) → menstruation
• If fertilisation: hCG from trophoblast → maintains CL → progesterone continues until placenta takes over (~8–10 weeks)

11.6 Other Endocrine Glands

12.1 Innate Immunity — Complete

Physical and Chemical Barriers (First Line)

• Skin: keratinized epidermis; sebum (fatty acids — antibacterial); low pH (~5); normal flora competition
• Mucous membranes: mucus traps pathogens; cilia sweep upward (mucociliary escalator); IgA in secretions
• Lysozyme: in tears, saliva, nasal secretions; cleaves peptidoglycan of bacterial cell wall (especially Gram+)
• Stomach acid (pH 1–2): kills most ingested pathogens
• Normal microbiota: competitive exclusion; produce bacteriocins; train immune system

Cellular Innate Immunity (Second Line)

• Pattern Recognition Receptors (PRRs): recognize PAMPs (Pathogen-Associated Molecular Patterns) — conserved microbial structures; and DAMPs (Damage-Associated Molecular Patterns — from injured cells)
• Toll-Like Receptors (TLRs): transmembrane PRRs; TLR4 recognizes LPS (Gram-negative bacteria); TLR3 recognizes dsRNA (viruses); signal via MyD88 → NF-κB → inflammatory cytokines
• NOD-like receptors (NLRs): intracellular PRRs; NLRP3 inflammasome → Caspase-1 → IL-1β, IL-18 processing and secretion
• RIG-I-like receptors (RLRs): detect cytoplasmic viral RNA → type I interferons (IFN-α, IFN-β)
• cGAS-STING: detects cytoplasmic dsDNA → type I interferons

Phagocytosis

• Neutrophils: most numerous WBC; first to arrive at infection; short-lived (hours); oxidative burst (NADPH oxidase → O₂⁻ → H₂O₂ + HOCl via myeloperoxidase — kill bacteria); form NETs (Neutrophil Extracellular Traps)
• Macrophages: tissue-resident (Kupffer cells in liver, microglia in brain, alveolar macrophages in lung, Langerhans cells in skin); long-lived; professional phagocytes; present antigens (MHC II); secrete cytokines (IL-1, IL-6, IL-12, TNF-α)
• Opsonisation: coating of pathogen with antibody (IgG Fc portion) or complement (C3b) → enhanced phagocytosis (Fc receptors + CR3 on phagocytes)
• Phagolysosome formation: phagosome fuses with lysosome → acid hydrolases, defensins, ROS kill pathogen; successful pathogens evade this (Mycobacterium — prevents phagolysosome fusion; Listeria — escapes into cytoplasm)

NK Cells (Natural Killer Cells)

• Innate lymphoid cells; no antigen-specific receptor; kill virus-infected and tumour cells
• Missing-self recognition: normal cells express MHC I → inhibitory KIR (Killer-cell Immunoglobulin-like Receptor) on NK → no killing; virus-infected/tumour cells downregulate MHC I → missing-self → NK activated → killing
• Activated by IL-2, IL-12, IL-15, type I interferons; NKG2D activating receptor binds stress ligands (MICA, MICB)
• Killing: perforin (pore-forming), granzyme B (serine protease → caspase activation → apoptosis); ADCC (antibody-dependent cellular cytotoxicity via CD16/FcγRIII)

Complement System — Complete

• ~30 serum proteins; three activation pathways converge on C3 convertase → C3b → C5 convertase → MAC
• Classical pathway: Antibody (IgM or IgG) binds antigen → C1q binding → C1r, C1s activation → C4 + C2 → C3 convertase (C4b2a)
• Lectin pathway: MBL (Mannose-Binding Lectin) or Ficolins bind microbial carbohydrates → MASP-1,2 → same C4+C2 pathway
• Alternative pathway: Spontaneous C3 hydrolysis on microbial surfaces; amplification loop; C3bBb convertase (properdin stabilises)
• C3b: opsonin (coats bacteria → CR3 on phagocytes); also C3a: anaphylatoxin (mast cell degranulation)
• C5a: most potent anaphylatoxin; chemotaxin (attracts neutrophils)
• MAC (C5b-6-7-8-9): pore in membrane → osmotic lysis; effective against Gram-negative bacteria and enveloped viruses
• Complement regulation: CD55 (DAF), CD59 (protectin) on host cells prevent self-attack; C1-INH (hereditary angioedema when deficient); Factor I + H (degrade C3b)

Inflammation

• Cardinal signs: Redness (rubor), Heat (calor), Swelling (tumour), Pain (dolor), Loss of function (functio laesa)
• Acute inflammation: vasodilation (histamine, bradykinin, PGE₂) → ↑blood flow → redness + heat; ↑vascular permeability → plasma proteins + fluid leak → oedema; neutrophil margination → rolling (selectins) → firm adhesion (integrins, ICAM-1) → transmigration (diapedesis)
• Mediators: Histamine (mast cells/basophils, preformed — immediate); Bradykinin (plasma kinin system); Prostaglandins + Leukotrienes (arachidonic acid → COX → PGs; LOX → LTs); IL-1, TNF-α, IL-6 (macrophage cytokines); IL-8/CXCL8 (chemokine — recruits neutrophils)
• Systemic inflammation: fever (IL-1 + TNF-α + IL-6 → hypothalamus → PGE₂ → thermostat reset; COX inhibitors = antipyretics); acute phase response (CRP, fibrinogen, SAA ↑ from liver); neutrophilia
• Chronic inflammation: macrophages + lymphocytes + plasma cells + fibroblasts; granuloma formation (TB, sarcoidosis, Crohn's); tissue destruction and fibrosis

12.2 Adaptive Immunity — Complete

Lymphocyte Development

• All lymphocytes from pluripotent HSC (haematopoietic stem cell) in bone marrow
• T lymphocytes: progenitors migrate to thymus → positive selection (must recognise self-MHC) + negative selection (must NOT react to self-antigens — central tolerance) → naïve T cells released
• B lymphocytes: mature in bone marrow; negative selection (self-reactive B cells eliminated or made anergic)
• Primary lymphoid organs: Bone marrow (B cells, all haematopoiesis) + Thymus (T cell maturation)
• Secondary lymphoid organs: Spleen, lymph nodes, MALT (Mucosa-Associated Lymphoid Tissue — tonsils, Peyer's patches in ileum, appendix); sites of adaptive immune responses

Antigen Presentation

• MHC I (HLA-A, B, C in humans): on all nucleated cells; presents endogenous peptides (8–10 aa) from cytoplasm → CD8+ cytotoxic T cells; β₂-microglobulin non-covalently associated
• MHC II (HLA-DR, DP, DQ): only on APCs (macrophages, dendritic cells, B cells); presents exogenous peptides (13–25 aa) from phagosome → CD4+ helper T cells
• Dendritic cells: most potent APCs; reside in tissues; capture antigen → migrate to lymph node → activate naïve T cells (prime response)
• Cross-presentation: dendritic cells can present exogenous antigen on MHC I → activate CD8+ T cells (important for anti-tumour immunity and some vaccines)

T Cell Activation and Subsets

• Two signals required: Signal 1 (TCR + MHC:peptide) + Signal 2 (CD28 + B7/CD80/86 costimulation); without Signal 2 → anergy (unresponsiveness)
• Clonal expansion: IL-2 (autocrine) drives rapid proliferation → effector T cells + memory T cells

B Cell Activation and Antibody Production

• T-dependent activation: B cell recognises antigen → BCR internalises and presents peptide on MHC II → Tfh cell provides CD40L:CD40 + IL-4/IL-21 → B cell activation → germinal centre reaction
• Germinal centre: somatic hypermutation (AID enzyme introduces point mutations in V regions → higher or lower affinity variants); affinity maturation (higher affinity B cells selected by FDCs); class switch recombination (IgM → IgG, IgA, IgE)
• Plasma cells: antibody factories; short-lived (days) or long-lived (bone marrow niches); secrete 2000 antibodies/second
• T-independent antigens: polysaccharides, LPS; activate B cells via BCR crosslinking; only IgM produced (no Tfh help); no memory; important for pneumococcus, meningococcus vaccines (conjugated vaccines add T-dependence)

Antibody Structure and Function — Complete

• Basic structure: 2 heavy chains + 2 light chains; connected by disulfide bonds; Y-shaped
• Fab (Fragment antigen-binding): N-terminal; contains VH + VL (variable domains); CDRs (Complementarity-Determining Regions, hypervariable loops) actually contact antigen
• Fc (Fragment crystallisable): C-terminal; effector functions; binds Fc receptors on effector cells; activates complement (classical pathway)
• Hinge region: between Fab and Fc; flexible; allows arm movement; rich in Pro and Cys
• Affinity: strength of single Fab arm binding antigen; Avidity: overall binding strength of whole antibody (IgM pentamer = very high avidity)

12.3 Hypersensitivity — Complete (Gell and Coombs Classification)

Anaphylaxis Mechanism (Type I in Detail)

• Sensitisation: first allergen exposure → Th2 response → IgE production → IgE binds FcεRI on mast cells/basophils (armed)
• Elicitation (2nd exposure): allergen crosslinks IgE on mast cell → immediate degranulation (histamine, tryptase, heparin — preformed granules) → lipid mediators synthesised (PGD₂, LTC₄/D₄/E₄ — slow-reacting substances of anaphylaxis) → cytokines (IL-4, IL-13, TNF-α)
• Systemic anaphylaxis: bronchospasm (LTs), vasodilation + capillary leak (histamine → hypotension → shock), laryngeal oedema, urticaria; treat with IM Adrenaline (reverses all effects), antihistamine, corticosteroids, O₂

Autoimmune Diseases

12.4 Vaccines and Immunology of Infection

13.1 Data Types, Scales and Collection

Sampling Methods

13.2 Descriptive Statistics

Measures of Central Tendency

• Mean (x̄): sum of all values / n; sensitive to outliers; best for normally distributed data; arithmetic, geometric (nth root of product — for growth rates), harmonic (reciprocals — for rates/speeds)
• Median: middle value when arranged in order; resistant to outliers; better for skewed data; for even n = average of two middle values
• Mode: most frequent value; nominal data; can be bimodal or multimodal; not unique
• Relationship in normal distribution: Mean = Median = Mode; in positively skewed (right tail): Mode < Median < Mean; in negatively skewed (left tail): Mean < Median < Mode

Measures of Dispersion

• Range: maximum – minimum; simple; sensitive to outliers
• Variance (s²): s² = Σ(xᵢ – x̄)² / (n–1); population variance uses N; sample variance uses n–1 (Bessel's correction for unbiased estimate)
• Standard Deviation (s): √variance; same units as data; most used; 68-95-99.7 rule
• Standard Error of Mean (SEM): SE = s/√n; measures precision of sample mean estimate; smaller n → larger SE; used in error bars in graphs
• Coefficient of Variation (CV): (SD/Mean) × 100%; dimensionless; allows comparison of variability between different units/scales; useful in biology for comparing trait variability
• Interquartile Range (IQR): Q3 – Q1; 50% of data; resistant to outliers; used in boxplots

13.3 Probability Distributions

13.4 Hypothesis Testing — Framework

• Null hypothesis (H₀): no effect/difference; what we test against; stated specifically (H₀: μ₁ = μ₂)
• Alternative hypothesis (H₁ or Hₐ): what we hope to show; directional (one-tailed) or non-directional (two-tailed)
• Test statistic: calculated from data (t, F, χ², z); compared to critical value from theoretical distribution
• p-value: probability of obtaining results as extreme as observed IF H₀ were true; NOT probability that H₀ is true
• Significance level (α): threshold for rejecting H₀; conventionally α = 0.05 (5%); if p < α → reject H₀
• Type I error (α error, false positive): reject true H₀; probability = α; e.g., concluding drug works when it doesn't
• Type II error (β error, false negative): fail to reject false H₀; probability = β; e.g., concluding drug doesn't work when it does
• Power (1–β): probability of correctly rejecting false H₀; increases with larger sample size and larger effect size
• Effect size: magnitude of difference (Cohen's d for means, Pearson r for correlation, odds ratio for proportions)
• Degrees of freedom (df): number of independent values in calculation; for one-sample t-test: df = n–1; for two-sample: df = n₁+n₂–2

13.5 Statistical Tests — Complete

Chi-Square Applied to Genetics (UPSC Key)

• Cross: Aa × Aa → 1/4 AA : 2/4 Aa : 1/4 aa; phenotypically 3:1
• If observed: 290 tall : 110 short (total 400); Expected: 300 tall : 100 short
• χ² = (290–300)²/300 + (110–100)²/100 = 100/300 + 100/100 = 0.33 + 1.0 = 1.33
• df = 2–1 = 1; Critical value at α=0.05 = 3.84; 1.33 < 3.84 → fail to reject H₀ → data fits 3:1 ratio
• Null hypothesis in genetics: observed data fits expected Mendelian ratio

13.6 Correlation vs Causation, and Research Design

• Correlation ≠ causation; confounding variables can create spurious correlations
• Experimental design: Control group; Randomisation; Blinding (single/double); Replication; Matching
• Randomised Controlled Trial (RCT): gold standard for causation; animals randomly allocated to treatment/control
• Observational studies: cohort, case-control, cross-sectional; cannot establish causation; association only
• Sample size determination: power analysis; larger n → more power → smaller detectable effect; Cohen's formula
• Outliers: >2 SD from mean; investigate cause; Grubbs test for outlier detection; don't remove without reason

14.1 Microscopy — Complete

14.2 Centrifugation

• Principle: sedimentation rate depends on particle size, shape, density, centrifugal force
• RCF (Relative Centrifugal Force) = 1.12 × r × (RPM/1000)²; in units of 'g' (gravity)
• Sedimentation coefficient (S or Svedberg): 1S = 10⁻¹³ sec; ribosome: 70S (prokaryote), 80S (eukaryote)

14.3 Chromatography — Complete

• Principle: differential distribution of components between stationary phase and mobile phase
• Rf value (paper/TLC): distance moved by spot / distance moved by solvent front; unique for each compound in given system; 0 to 1

14.4 Electrophoresis — Complete

• Principle: charged molecules migrate in electric field; rate depends on charge, size, shape, gel pore size
• DNA/RNA: uniform negative charge (phosphate backbone) → migration solely by size (smaller runs faster toward anode +)

Blotting Techniques

14.5 Spectrophotometry and Spectrometry

• Beer-Lambert Law: A = ε × c × l; Absorbance (A) is dimensionless; ε = molar absorptivity (M⁻¹cm⁻¹); c = concentration (M); l = path length (cm); A = log₁₀(I₀/I) = log₁₀(1/T)
• Transmittance (T) = I/I₀; %T = T × 100; A = 2 – log₁₀(%T)
• Linear relationship between A and c only at low concentrations (deviation at high A due to stray light, concentration quenching)
• Useful wavelengths: 260 nm (nucleic acids — aromatic bases absorb); 280 nm (proteins — Trp, Tyr, Phe absorb); 595 nm (Bradford assay — protein-dye complex); 450 nm (ELISA OPD substrate); 405 nm (ELISA pNPP substrate)
• A₂₆₀/A₂₈₀ ratio: pure DNA = 1.8–2.0; pure RNA = ~2.0; ratio <1.8 indicates protein contamination (use phenol-chloroform extraction); ratio >2.0 indicates RNA contamination in DNA prep

Mass Spectrometry (MS)

• Ionises molecules → separates by mass-to-charge ratio (m/z) → detects
• Ionisation methods: ESI (Electrospray Ionisation — proteins/peptides; soft; multiply charged); MALDI (Matrix-Assisted Laser Desorption Ionisation — singly charged; large molecules)
• Analysers: Time-of-Flight (TOF); Quadrupole; Ion trap; Orbitrap (highest resolution)
• Applications: protein identification (bottom-up proteomics: trypsin digest → peptides → MS/MS → database search); metabolomics; drug metabolism; forensic toxicology; GC-MS and LC-MS/MS

14.6 Molecular Biology Techniques

PCR — Complete

• Invented by Kary Mullis (1983, Nobel Prize 1993); exponential amplification of specific DNA sequence
• Components: Template DNA; Forward + reverse primers (flanking target, ~20 nt, complementary to each strand); Taq DNA polymerase (thermostable, from Thermus aquaticus; lacks proofreading); dNTPs (dATP, dCTP, dGTP, dTTP); MgCl₂ (cofactor for Taq); Buffer
• Three steps (cycle repeated 25–40 times): Denaturation (94–95°C, 30s–1min; strands separate); Annealing (50–65°C, 30s; primers bind template; Tm-based — usually Tm–5°C); Extension (72°C, 1 min/kb; Taq synthesises new strand 5'→3')
• Amplification: n cycles → 2ⁿ copies; from 1 molecule → 10⁹ copies in 30 cycles
• Variants: RT-PCR (Reverse Transcriptase-PCR): RNA → cDNA (reverse transcriptase) → PCR; detects RNA viruses (HIV, SARS-CoV-2); gene expression; qPCR (real-time): fluorescent detection during PCR (SYBR Green or TaqMan probes); quantifies initial template; gene expression analysis; Multiplex PCR: multiple primer pairs in one reaction; Nested PCR: 2 rounds with nested primers → specificity; Digital PCR: partition sample into thousands of droplets → absolute quantification without standard curve

DNA Sequencing

• Sanger sequencing (chain-termination, 1977 — Nobel Prize): ddNTPs (dideoxy-NTPs, labelled with 4 different fluorescent dyes) terminate extension at specific base → fragments of different lengths → capillary electrophoresis → read sequence from fluorescence signal; gold standard; reads ~1000 bp
• Next-Generation Sequencing (NGS): massively parallel sequencing; sequence billions of fragments simultaneously; 100–300 bp reads; whole genome sequencing, exome sequencing, RNA-seq, ChIP-seq, ATAC-seq
• Third Generation Sequencing: Pacific Biosciences (SMRT) and Oxford Nanopore — long reads (10–100 kb); single molecule; real-time; ideal for structural variants and repeat regions

ELISA (Enzyme-Linked Immunosorbent Assay)

• Detects and quantifies antigen or antibody; uses enzyme-conjugated antibody + colourimetric substrate
• Direct ELISA: Ag coated on plate → enzyme-labelled primary Ab → substrate → colour (simple but less sensitive)
• Indirect ELISA: Ag coated → primary Ab → enzyme-labelled secondary Ab → substrate (amplified signal; detects Ab in serum)
• Sandwich ELISA: Capture Ab coated → Ag added → detection Ab + enzyme conjugate → substrate (most specific; quantitative; 2 Abs bind different epitopes)
• Competitive ELISA: Ag in sample competes with labelled Ag for limited Ab → inverse relationship between colour and Ag concentration; for small molecules/haptens
• HIV diagnosis: ELISA for antibody screening → Western blot for confirmation (p24, gp41, gp120 bands)
• Sensitivity: typically 1–10 pg/ml; absorbance read at 450–550 nm (HRP substrate TMB)

Flow Cytometry

• Cells in suspension pass single-file through laser beam → forward scatter (FSC, cell size) + side scatter (SSC, granularity) + fluorescence channels
• Up to 20+ parameters per cell simultaneously; >10,000 cells/second
• Applications: cell counting and sorting (FACS — Fluorescence-Activated Cell Sorting); immunophenotyping (CD4/CD8 counts in HIV); cell cycle analysis (propidium iodide DNA staining); apoptosis (Annexin V + PI); intracellular cytokine staining
• Key clinical uses: HIV monitoring (CD4 count); leukaemia/lymphoma diagnosis (surface marker immunophenotyping); bone marrow analysis

Radioimmunassay (RIA) and Related

• RIA (Yalow & Berson, 1960 — Nobel Prize): competitive binding; labelled antigen (¹²⁵I) competes with unlabelled antigen from sample for limited antibody; radioactivity inversely proportional to antigen concentration in sample; extremely sensitive (pg range)
• Uses: first used for insulin measurement; still used for PTH, cortisol, aldosterone, steroid hormones; being replaced by ELISA (no radioactive waste)
• IRMA (Immunoradiometric Assay): labelled antibody (not antigen); sandwich format; for larger molecules

Most Frequently Asked Topics (15-mark Questions)

Most Frequently Asked Topics (10-mark Questions)

Unit-wise PYQ Weight

Key Scientists and Discoveries

Larval Forms — Complete Table

Vitamins — Complete Reference

Excretory Organs Across Phyla

Important National Parks and Wildlife Sanctuaries — India

Hormones — Quick Reference Card

Must-Draw Diagrams for UPSC Zoology

Diagram Drawing Tips

Understanding UPSC Directive Words

Answer Structure Templates

For 15-Mark Questions (Target: 300–350 words + 1 diagram)

For 10-Mark Questions (Target: 150–200 words + 1 small diagram)

Golden Rules for Zoology Optional

Sample 15-Mark Answer Framework

Q: Describe the mechanism of muscle contraction with reference to the sliding filament theory.

Standard Textbooks for Zoology Optional