Interactive E-Content · UGC Four Quadrant Approach
Cell Biology
3Units
45Theory Hrs
4Quadrants
30+MCQ Practice
Course Overview
Cell Biology — The Study of Life's Fundamental Unit
A comprehensive guide to cell structure, function, division, and communication.
🎯 Course Objectives
- Distinguish between prokaryotic and eukaryotic cell organisation
- Understand the ultrastructure and functional significance of plasma membrane
- Explain the role of endomembrane system and cytoskeleton
- Describe mitochondrial structure and bioenergetics
- Analyse chromosome structure, DNA packaging, and cell division
- Interpret signal transduction pathways and cell death mechanisms
Prokaryotic vs Eukaryotic
Plasma Membrane Models
Cell Junctions
ER & Golgi
Mitochondria
Cytoskeleton
Nuclear Structure
Chromosomes
Mitosis & Meiosis
Signal Transduction
Apoptosis
Peroxisomes
UGC Four Quadrant Approach
Each topic in this e-content is presented across 4 quadrants: e-Text (conceptual reading), Video (curated resources), Self-Assessment (MCQ quizzes), and Activities (practicals & discussions).
Recommended Video Resources
Cell Biology Overview – MIT OpenCourseWare
NPTEL – Cell Biology (IIT Kharagpur)
CrashCourse Biology – Cell Structure
Khan Academy – Cell Biology
💡 Note
Search these titles on YouTube/NPTEL. All resources are freely available online.Course Entry Assessment
Course Activities
Discussion
Why is the cell the basic unit of life?
Discuss with peers: What features make the cell the minimum structural and functional unit of all living organisms? Compare cell to non-cellular agents like viruses.
Assignment
Timeline of Cell Biology
Prepare a 2-page timeline of major discoveries in cell biology from Robert Hooke (1665) to the present CRISPR era, highlighting Indian scientists' contributions.
Unit 1 · Topic 1
Overview of Cells
🔬 Prokaryotic Cells
Prokaryotic cells (Gk: pro = before; karyon = nucleus) lack a membrane-bound nucleus and other membrane-bound organelles. They are typically 1–10 µm in size.
Key characteristics:- Nucleoid: Region containing circular DNA, not enclosed by membrane
- 70S ribosomes: Smaller than eukaryotic ribosomes (30S + 50S subunits)
- Cell wall: Peptidoglycan in Bacteria; pseudopeptidoglycan in Archaea
- Plasma membrane: No sterols (except Mycoplasma)
- Flagella: Simple protein filaments (flagellin), lack 9+2 microtubule arrangement
- Plasmids: Extrachromosomal circular DNA elements
- Mesosome: Infolding of plasma membrane; involved in respiration and DNA replication
🧫 Eukaryotic Cells
Eukaryotic cells have a true membrane-bound nucleus and a complex endomembrane system. They range from 10–100 µm in size.
Key characteristics:- Nucleus: DNA enclosed by nuclear envelope (double membrane)
- 80S ribosomes: Larger ribosomes (40S + 60S subunits)
- Membrane-bound organelles: ER, Golgi, mitochondria, lysosomes
- Cytoskeleton: Complex network of microtubules, microfilaments, intermediate filaments
- Cilia & Flagella: Complex 9+2 microtubule arrangement
- Cell wall: Cellulose (plants), chitin (fungi), absent in animal cells
Fig. 1 — Structural comparison of Prokaryotic vs Eukaryotic cells
🧫 Virus, Viroids, Mycoplasma & Prions
🦠 Virus ▼
Non-cellular obligate intracellular parasites. Composed of nucleic acid (DNA or RNA, never both) enclosed in a protein coat (capsid). May have a lipid envelope. Sizes range from 20–300 nm. Examples: HIV, Influenza, Bacteriophage. Key features: no cellular structure, no ribosomes, replicate using host machinery. Classified by nucleic acid type, capsid symmetry (icosahedral, helical, complex), and host range.
🌿 Viroids ▼
Smallest known plant pathogens — naked, single-stranded circular RNA molecules (246–375 nucleotides). No protein coat at all. Discovered by T.O. Diener (1971). Cause diseases like Potato Spindle Tuber disease. Replicate in host cell nucleus using host RNA polymerase II. Resistant to heat and organic solvents.
🔵 Mycoplasma ▼
Smallest free-living organisms (0.1–0.8 µm). Prokaryotes that permanently lack a cell wall. Only bacteria that incorporate cholesterol into their plasma membrane (hence pleomorphic — no fixed shape). Resistant to penicillin. Examples: Mycoplasma pneumoniae (walking pneumonia), M. genitalium. Have the smallest known genome (~580 kb).
⚠️ Prions ▼
Proteinaceous infectious particles — misfolded isoforms of the normal cellular prion protein (PrPC). Contain no nucleic acid. Discovered by Stanley Prusiner (Nobel Prize 1997). PrPSC (scrapie form) is protease-resistant and causes normal PrPC to misfold. Cause transmissible spongiform encephalopathies (TSEs): Creutzfeldt-Jakob disease (humans), BSE (cattle), Scrapie (sheep), Kuru.
| Feature | Prokaryote | Eukaryote | Virus | Mycoplasma |
|---|---|---|---|---|
| Cell wall | Peptidoglycan | Cellulose/Chitin (if present) | Absent | Absent (permanently) |
| Nucleus | Nucleoid | True nucleus | Absent | Nucleoid |
| Ribosomes | 70S | 80S (cytoplasm) | Absent | 70S |
| Size | 1–10 µm | 10–100 µm | 20–300 nm | 0.1–0.8 µm |
| Replication | Binary fission | Mitosis/Meiosis | Host-dependent | Binary fission |
Video Resources – Cell Types
Prokaryotic vs Eukaryotic – Amoeba Sisters
Viruses – How Viruses Work
Prions & Misfolded Proteins – Kurzgesagt
NCBI – Mycoplasma Review
Self-Assessment – Cell Types
Activities – Cell Types
Lab Activity
Observation of Bacteria Under Microscope
Prepare a temporary mount of yoghurt bacteria. Perform Gram staining. Observe under oil immersion lens (100×). Record morphology: cocci, bacilli, spirilli. Draw labelled diagrams.
Discussion
Debate: Are Viruses Alive?
Divide into two groups. Group A argues viruses ARE living; Group B argues they are NOT. Discuss criteria for life: metabolism, reproduction, cellular organisation, response to stimuli. Present findings.
Assignment
Prion Disease Case Study
Research the BSE (Mad Cow Disease) epidemic in the UK (1986–1999). Prepare a 3-page report on: origin, transmission, molecular mechanism, public health impact, and lessons learned.
Unit 1 · Topic 2
Plasma Membrane: Structure & Transport
📚 Models of Plasma Membrane
1
Lipid Bilayer Model (Gorter & Grendel, 1925)
Proposed that the membrane consists of a bimolecular lipid leaflet. Showed that lipids extracted from red blood cell membranes covered twice the surface area of the cells.
2
Davson-Danielli Model / Sandwich Model (1935)
Proposed a trilaminar structure: protein–lipid bilayer–protein ("pauci-molecular" model). Proteins were thought to lie flat on both surfaces. Electron microscopy later showed the 3-layered (railroad track) appearance.
3
Unit Membrane Model (Robertson, 1959)
Extended the Davson-Danielli model. Proposed that all biological membranes have the same basic structure — asymmetric trilaminar structure. The inner and outer protein layers are chemically different.
4
Fluid Mosaic Model (Singer & Nicolson, 1972)
Currently accepted model. The membrane is a fluid phospholipid bilayer in which proteins are embedded like mosaic tiles. Key features: (a) Phospholipid bilayer is fluid (lipid molecules can move laterally). (b) Proteins are either integral (transmembrane) or peripheral. (c) Membrane is asymmetric — outer and inner leaflets differ. (d) Cholesterol stabilises membrane fluidity. (e) Supported by freeze-fracture electron microscopy.
Lipid Rafts (Refinement of Fluid Mosaic Model)
Specialized microdomains enriched in cholesterol and sphingolipids. Function as platforms for signaling molecules and membrane proteins. Important for receptor clustering and endocytosis.
Fig. 2 — Fluid Mosaic Model of Plasma Membrane (Singer & Nicolson, 1972)
🚌 Transport Across Membranes
| Type | Energy | Direction | Proteins | Examples |
|---|---|---|---|---|
| Simple Diffusion | None (passive) | High → Low conc. | None | O₂, CO₂, ethanol |
| Osmosis | None (passive) | High H₂O → Low H₂O | Aquaporins | Water movement |
| Facilitated Diffusion | None (passive) | High → Low conc. | Channels/Carriers | Glucose (GLUT), ions |
| Primary Active Transport | ATP directly | Against gradient | Pumps (ATPases) | Na⁺/K⁺ ATPase |
| Secondary Active Transport | Electrochemical gradient | Cotransport | Symporters/Antiporters | Na⁺-glucose cotransporter |
Na⁺/K⁺ ATPase – The Classic Pump
Moves 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed. Maintains resting membrane potential. Accounts for ~30% of total cellular ATP consumption. Critical for neuronal signalling, muscle contraction, and cell volume regulation.
Types of Transporters
- Uniporters: Transport one molecule in one direction (e.g., GLUT1 – glucose)
- Symporters: Transport two molecules in the same direction (e.g., Na⁺-glucose cotransporter in intestine)
- Antiporters: Transport molecules in opposite directions (e.g., Na⁺/H⁺ exchanger, Na⁺/K⁺ ATPase)
- Ion channels: Gated (voltage, ligand, mechanically) or non-gated (leak channels)
Video Resources – Plasma Membrane
Fluid Mosaic Model – Khan Academy
Membrane Transport – Bozeman Science
Na+/K+ ATPase – Osmosis.org
Active vs Passive Transport – CrashCourse
Self-Assessment – Plasma Membrane
Activities – Plasma Membrane
Practical
Osmosis Demonstration with Potato Pieces
Weigh three potato cylinders. Place in: distilled water, 0.9% NaCl (isotonic), and 20% NaCl (hypertonic). After 30 min, re-weigh and record changes. Calculate % change in mass. Relate results to osmotic principles of plasma membrane.
Discussion
Model Comparison Exercise
Draw all four models of membrane (Gorter-Grendel, Davson-Danielli, Unit Membrane, Fluid Mosaic) side by side. Identify the limitations overcome by each successive model. Present in class.
Unit 1 · Topic 3
Cell Junctions
🔒 Tight Junctions (Zonula Occludens)
Structure: Formed by transmembrane proteins claudins and occludins that interact with corresponding proteins on adjacent cells, creating a "zipper-like" seal. A protein scaffolding including ZO-1, ZO-2, and ZO-3 links them to the actin cytoskeleton.
Functions:
- Paracellular barrier: Prevent passage of ions and molecules between cells (e.g., intestinal epithelium prevents uncontrolled ion flow)
- Fence function: Restrict lateral diffusion of membrane lipids and proteins between apical and basolateral domains — maintain cell polarity
- Critical for blood-brain barrier integrity
⚓ Desmosomes (Zonula/Macula Adherens)
Structure: Disc-shaped plaques on opposing cell membranes, connected by cadherins (desmoglein and desmocollin) that traverse the intercellular space (~30 nm). Cytoplasmic plaques contain desmoplakin and plakophilin, which anchor intermediate filaments (keratin or desmin).
Functions:
- Provide mechanical strength to tissues subjected to physical stress (skin, cardiac muscle)
- Spot welds between cells — resist shearing forces
- Pemphigus vulgaris: autoimmune disease where antibodies attack desmoglein → blistering
Hemidesmosomes: Connect cell to extracellular matrix (laminin) via integrins — not cell-to-cell
📡 Gap Junctions (Nexus)
Structure: Formed by connexins — 6 connexin molecules form a connexon (hemichannel). Two connexons from adjacent cells align to form a gap junction channel (~1.5–2 nm pore). Connexin 43 (Cx43) is the most abundant type in the heart.
Functions:
- Allow passage of ions, small metabolites, and second messengers (cAMP, IP₃, Ca²⁺) up to 1 kDa
- Electrical coupling: synchronises cardiac and smooth muscle contraction
- Metabolic coupling: coordinate tissue responses — e.g., liver glucose metabolism
- Regulated by pH, Ca²⁺, voltage — channels close during cell injury
| Feature | Tight Junction | Desmosome | Gap Junction |
|---|---|---|---|
| Proteins | Claudins, Occludins | Desmoglein, Desmocollin | Connexins |
| Cytoskeleton link | Actin | Intermediate filaments | None directly |
| Function | Sealing, polarity | Mechanical adhesion | Communication |
| Intercellular space | None (~0 nm) | ~30 nm | ~2–4 nm gap |
| Permeability | Impermeable | Impermeable | Selective (< 1 kDa) |
Video Resources – Cell Junctions
Cell Junctions – Armando Hasudungan
Tight Junctions & Barrier Function – iBiology
Gap Junctions in the Heart – HHMI
Self-Assessment – Cell Junctions
Activities – Cell Junctions
Discussion
Disease Connections to Cell Junctions
Research and present: (a) Crohn's disease and tight junction disruption; (b) Pemphigus vulgaris and desmosome; (c) Charcot-Marie-Tooth disease and connexin mutations. Focus on molecular mechanisms.
Assignment
Diagram-based assignment
Draw neat labelled diagrams of all three junctions. For each: (i) label all molecular components, (ii) state the associated disease if any, (iii) identify the cytoskeletal element involved.
Unit 1 · Topic 4
Endomembrane System
🌐 Endoplasmic Reticulum (ER)
A network of interconnected membranous tubules and flattened sacs (cisternae) continuous with the nuclear envelope. Constitutes ~50% of total membrane in eukaryotic cells.
| Feature | Rough ER (RER) | Smooth ER (SER) |
|---|---|---|
| Ribosomes | Present (studded) | Absent |
| Shape | Flattened cisternae | Tubular vesicles |
| Function | Protein synthesis, folding, glycosylation, quality control (ERAD) | Lipid synthesis, detoxification, Ca²⁺ storage |
| Abundant in | Secretory cells (pancreatic acini, plasma cells) | Liver cells, steroid-producing cells, muscle cells |
📦 Golgi Apparatus (Golgi Complex)
Discovered by Camillo Golgi (1898). Stack of flattened membrane-bound cisternae (5–8 cisternae per stack), each functionally distinct. The cis face receives vesicles from the ER; the trans face dispatches vesicles to destinations.
Functional compartments:- cis-Golgi network (CGN): Receives COP II vesicles from ER; receives COPI-coated retrograde vesicles
- Medial cisternae: O-linked glycosylation, sulphation
- trans-Golgi network (TGN): Sorting hub — dispatches vesicles to lysosomes (M-6-P signal), plasma membrane, or secretory pathway
- Post-translational modification (glycosylation, sulphation, phosphorylation)
- Proteolytic processing of proproteins
- Lipid and sphingomyelin synthesis
- Sorting and packaging into vesicles — molecular zip codes
♻️ Lysosomes
Membrane-bound organelles containing ~60 hydrolytic enzymes (acid hydrolases — proteases, lipases, nucleases, glycosidases) optimally active at pH 4.5–5. Membrane H⁺-ATPase maintains the acidic interior. Lysosomal membrane proteins (LAMPs) protect it from self-digestion.
Types and functions:- Primary lysosomes: Newly formed, inactive enzyme stores
- Secondary lysosomes: Fused with material to be digested
- Autolysosomes (Autophagosomes): Degrade the cell's own components — autophagy
- Residual bodies: Undigested material remaining after lysosomal digestion
Lysosomal Storage Diseases
Tay-Sachs (absence of hexosaminidase A → GM₂ ganglioside accumulation), Gaucher's disease (glucocerebrosidase deficiency), Hurler syndrome — all result from enzyme deficiency causing accumulation of undigested substrates.Video Resources – Endomembrane System
Endomembrane System – Khan Academy
Golgi Apparatus – HHMI BioInteractive
Lysosomes & Autophagy – Nobel Lecture 2016
Self-Assessment – Endomembrane System
Activities – Endomembrane System
Practical
Tracing the Secretory Pathway
Using a diagram/model, trace the journey of a secretory protein from synthesis on ribosomes → RER lumen → COP II vesicle → Golgi cis → trans → secretory vesicle → plasma membrane exocytosis. Label all steps and explain modifications at each station.
Discussion
Lysosomal Disease Research
Choose any one lysosomal storage disease. Prepare a patient-case presentation: the missing enzyme, the accumulated substrate, clinical features, and current treatment options (enzyme replacement therapy, gene therapy).
Unit 2 · Topic 1
Mitochondria — The Powerhouse of the Cell
🔬 Structure of Mitochondria
Mitochondria are double membrane-bound organelles (1–10 µm), varying in number from 1 (some yeasts) to 2000+ (hepatocytes). They are dynamic — constantly undergo fusion and fission.
- Outer mitochondrial membrane (OMM): Contains porins (VDAC — voltage-dependent anion channels) permeable to small molecules (≤5 kDa). Contains TOM complex for protein import.
- Intermembrane space (IMS): Contains cytochrome c (key apoptotic signal), adenylate kinase, creatine kinase.
- Inner mitochondrial membrane (IMM): Highly folded into cristae (greatly increases surface area). Contains the electron transport chain complexes (I, II, III, IV), ATP synthase (Complex V), and TIM complex. Impermeable — specific transporters required.
- Matrix: Contains mtDNA (circular, 16.5 kb in humans), 70S-like mitoribosomal RNA, tRNAs, enzymes of the TCA cycle and fatty acid oxidation.
🧬 Semi-autonomous Nature & Endosymbiotic Hypothesis
Semi-autonomous: Mitochondria have their own circular DNA (mtDNA), ribosomes (mitoribosomes), and can synthesise some of their own proteins (~13 proteins encoded by human mtDNA). However, ~1500 mitochondrial proteins are nuclear-encoded, translated in cytosol, and imported. Thus they cannot survive independently.
Endosymbiotic Hypothesis (Lynn Margulis, 1967)
Mitochondria evolved from free-living α-proteobacteria that were engulfed by a host cell ~1.5 billion years ago. Evidence: (1) double membrane (inner = bacterial, outer = derived from phagocytosis); (2) 70S-like ribosomes; (3) circular DNA; (4) binary fission-like division; (5) rRNA sequences closely related to α-proteobacteria; (6) sensitivity to antibiotics (chloramphenicol, rifampicin) that affect bacterial ribosomes.
⚡ Mitochondrial Respiratory Chain & Chemiosmosis
The electron transport chain (ETC) consists of four large protein complexes embedded in the inner membrane:
- Complex I (NADH dehydrogenase): Accepts electrons from NADH; pumps 4H⁺ into IMS
- Complex II (Succinate dehydrogenase): Accepts electrons from FADH₂; does NOT pump protons
- Complex III (Cytochrome bc₁): Electrons via ubiquinol; pumps 4H⁺ per pair; Q-cycle
- Complex IV (Cytochrome c oxidase): Final electron acceptor is O₂ → H₂O; pumps 2H⁺ per electron pair
- Complex V (ATP synthase): Rotor-stator mechanism; 3 H⁺ per ATP synthesised
Chemiosmotic Hypothesis (Peter Mitchell, 1961 — Nobel 1978)
Electron transport creates a proton gradient (ΔpH + ΔΨ = proton motive force) across the inner membrane. Protons flow back through ATP synthase (F₀F₁-ATPase), driving rotation of the γ subunit and conformational changes in β subunits (Boyer's binding change mechanism) that synthesise ATP. ~2.5 ATPs per NADH; ~1.5 ATPs per FADH₂.
Video Resources – Mitochondria
Electron Transport Chain – Khan Academy
ATP Synthase Motor – HHMI Animation
Endosymbiosis – Biology LibreTexts
Chemiosmosis – CrashCourse Biology
Self-Assessment – Mitochondria
Activities – Mitochondria
Practical
Mitochondrial Isolation (if facility available)
Differential centrifugation of liver tissue. Homogenize in cold sucrose buffer. Steps: 600×g (10 min) → discard pellet; 10,000×g (10 min) → mitochondrial pellet. Stain with Janus Green B to visualise. Calculate yield and purity.
Assignment
Mitochondrial Diseases Essay
Write a 500-word essay on mitochondrial diseases (e.g., MELAS, Leber's hereditary optic neuropathy). Include: mtDNA inheritance patterns, affected tissues, and why tissues with high energy demand are most affected.
Unit 2 · Topic 2
Peroxisomes
🧪 Structure & Functions
Peroxisomes are single-membrane-bound organelles (0.1–1.0 µm diameter). Named for their role in generating and degrading hydrogen peroxide (H₂O₂). Originally called microbodies. Discovered by Christian de Duve (Nobel 1974). Biogenesis via peroxins (PEX genes) — proliferate by fission or de novo from the ER.
Key enzymes and functions:- Oxidases: Generate H₂O₂ by oxidising substrates (fatty acids, amino acids, uric acid)
- Catalase: Destroys H₂O₂ → H₂O + O₂ (prevents oxidative damage); most abundant peroxisomal enzyme
- β-Oxidation of very long chain fatty acids (VLCFA): Differs from mitochondrial β-oxidation; does not produce ATP directly; generates acetyl-CoA
- Plasmalogen synthesis: Ether phospholipid synthesis — critical for myelin sheaths of neurons
- Bile acid synthesis: In liver peroxisomes
- Glyoxylate metabolism: Conversion of glyoxylate to glycine (defect → primary hyperoxaluria type 1)
Peroxisomal Disorders
Zellweger syndrome (cerebrohepatorenal syndrome): absence of functional peroxisomes due to PEX gene mutations. Features: neurological dysfunction, liver disease, adrenal insufficiency, death in infancy. Adrenoleukodystrophy (ALD): defect in ABCD1 transporter → VLCFA accumulation → demyelination. Made famous by the film "Lorenzo's Oil".Self-Assessment – Peroxisomes
Activities – Peroxisomes
Discussion
Peroxisomes vs Lysosomes vs Mitochondria
Prepare a comparative chart of peroxisomes, lysosomes, and mitochondria — covering: number of membranes, origin (ER/autonomous), key enzymes, major function, and associated diseases.
Assignment
Case Study — Adrenoleukodystrophy
Analyse the molecular basis, clinical presentation, and treatment approaches for ALD. Include the role of Lorenzo Odone and his family in advancing treatment research.
Unit 2 · Topic 3
Cytoskeleton
🕸️ Overview of Cytoskeletal Elements
| Feature | Microtubules | Microfilaments | Intermediate Filaments |
|---|---|---|---|
| Diameter | ~25 nm | ~7 nm | ~10 nm |
| Protein subunit | α/β-tubulin heterodimer | G-actin (globular) | Various (cell-type specific) |
| Polarity | +/– ends | Barbed(+)/Pointed(–) ends | Non-polar |
| Key functions | Cell shape, chromosome segregation, intracellular transport, cilia/flagella | Cell motility, cytokinesis, muscle contraction | Mechanical support, nuclear lamina |
| Associated proteins | MAPs, dynein, kinesin | Myosin, ARP2/3, cofilin | Plectins |
| Drug sensitivity | Colchicine, taxol | Cytochalasin, phalloidin | Resistant to most drugs |
🔩 Microtubules
Hollow cylinders made of 13 protofilaments, each composed of α/β-tubulin heterodimers arranged in a head-to-tail manner. Dynamic instability: rapid growth (polymerisation) and shrinkage (catastrophe) driven by GTP hydrolysis. Nucleated at the MTOC (microtubule organising centre)/centrosome from γ-tubulin ring complexes (γ-TuRC).
Motor proteins: Kinesins move toward the + end (anterograde); Dyneins move toward the – end (retrograde). Used for vesicle/organelle transport and chromosome movement during mitosis.
Cilia and Flagella: Axoneme has 9 doublet microtubules + 2 central singlets (9+2 arrangement). Dynein arm generates sliding force between adjacent doublets → bending motion. Cilia: short, numerous (respiratory epithelium). Flagella: long, few (sperm). Primary (non-motile) cilia: single (9+0) → signalling antenna (Hedgehog, Wnt pathways).
💪 Microfilaments (Actin Filaments)
Two-stranded helical polymers of actin. Treadmilling: addition at barbed (+) end, loss at pointed (–) end — net movement without change in filament length. Regulated by many proteins:
- ARP2/3 complex: Nucleates branched actin networks (important in lamellipodium formation)
- Cofilin/ADF: Severs and depolymerises actin
- Tropomyosin: Stabilises filaments in muscle
- Formins (mDia): Nucleate unbranched actin filaments
Functions: Cortical actin network (cell shape), filopodia and lamellipodia (cell migration), cytokinetic ring (cell division), microvilli (intestinal brush border), muscle contraction (with myosin II).
🧵 Intermediate Filaments
Most mechanically stable cytoskeletal element. Not polar — no motor protein binding. Made of rod-shaped proteins that form coiled-coil dimers → tetramers → protofibrils → mature filament. Types include: keratins (epithelial cells), vimentin (mesenchymal cells), desmin (muscle), neurofilaments (neurons), lamins (nuclear envelope).
Nuclear Lamina
Lamins A, B, and C form the nuclear lamina just inside the inner nuclear membrane. Mutations in Lamin A cause Progeria (Hutchinson-Gilford syndrome — premature ageing) and Emery-Dreifuss muscular dystrophy.Video Resources – Cytoskeleton
The Cytoskeleton – HHMI BioInteractive
Microtubule Dynamic Instability – iBiology
Actin & Myosin – Khan Academy
Cilia Structure 9+2 – YouTube Animation
Self-Assessment – Cytoskeleton
Activities – Cytoskeleton
Practical
Observation of Ciliated Epithelium
Prepare a smear of frog palatal epithelium or tracheal epithelium. Fix, stain with carmine-acetic acid. Observe under high power. Note the 9+2 microtubular arrangement from electron micrograph reference. Draw and label.
Discussion
Cytoskeleton in Disease
Research: (a) Taxol as cancer drug (stabilises microtubules → arrests cell division); (b) Kartagener syndrome (primary ciliary dyskinesia — dynein arm defect); (c) Progeria and nuclear lamins. Discuss how understanding cytoskeleton leads to treatments.
Unit 2 · Topic 4
The Nucleus
⚫ Nuclear Envelope
The nucleus is bounded by two concentric lipid bilayers — the outer nuclear membrane (continuous with RER, studded with ribosomes) and the inner nuclear membrane (lined by nuclear lamina). The space between is the perinuclear space (~50 nm).
The nuclear lamina (lamins A/B/C) provides structural support and anchors chromatin. The inner membrane contains specific proteins (LAP1, LAP2, emerin) that connect to the lamina and chromatin.
🚪 Nuclear Pore Complex (NPC)
Large (~120 MDa) protein complexes (~2000 per nucleus) that perforate the nuclear envelope at sites where the two membranes fuse. Composed of ~30 different nucleoporins arranged with 8-fold symmetry. The central channel diameter is ~9 nm (passive diffusion of molecules <40 kDa) and ~39 nm (active transport with 120 nm total outer diameter).
Functions:- Passive transport: Ions, small metabolites, water diffuse freely
- Active import: Proteins bearing Nuclear Localisation Signals (NLS) are recognised by importins → importin-cargo complex docks at NPC → Ran-GTP drives import into nucleus → cargo released; importin recycled
- Active export: mRNA, tRNA, ribosomal subunits leave via Nuclear Export Signals (NES) and exportins
- Critical for gene regulation — transcription factors shuttle in/out based on signals
🔵 Nucleolus
A non-membrane-bound sub-nuclear compartment where rRNA genes (rDNA) are transcribed and ribosomal subunits are assembled. Visible as a dense dark body under light microscopy. Contains fibrillar centre (FC) — rDNA transcription sites; dense fibrillar component (DFC) — early rRNA processing; granular component (GC) — late rRNA processing and ribosome assembly.
A cell with high protein synthesis demand has a large nucleolus (e.g., pancreatic acinar cells, growing oocytes). Nucleolus disperses during mitosis and reforms at telophase at nucleolar organiser regions (NORs) on chromosomes 13, 14, 15, 21, 22 in humans.
Self-Assessment – Nucleus
Activities – Nucleus
Practical
Isolation and Staining of Nuclei
Lyse cells with 0.1% Triton X-100. Centrifuge at 1000×g for 10 min. Stain nuclear pellet with DAPI (if available) or Feulgen reagent. Observe and compare nucleoli under microscope in different cell types (e.g., onion root tip vs. liver cells from prepared slides).
Assignment
Nuclear Pore Complex
Draw a detailed diagram of the nuclear pore complex. Explain the mechanism of nuclear import of a transcription factor (e.g., NF-κB), including the role of IκB in cytoplasmic retention and signal-induced nuclear translocation.
Unit 3 · Topic 1
Chromosomes: Structure & Types
🦋 Giant Chromosomes
Polytene Chromosomes: Found in larval salivary glands of Drosophila (and other dipteran insects). Formed by repeated rounds of DNA replication without cell division (endoreduplication) — up to 1024 copies per chromosome. Sister chromatids remain paired (somatic pairing). Show distinct band-interband pattern — ~5000 bands in Drosophila; gene-dense regions appear as puffs (Balbiani rings — sites of active transcription). First described by Balbiani (1881).
Lampbrush Chromosomes: Found in oocytes of amphibians (and other vertebrates) during meiotic prophase I (diplotene stage). Large, extended bivalents with lateral loops extending from each chromomere. Loops are sites of intense RNA synthesis — support massive transcription needed for egg development. Length up to 800 µm (vs. 15 µm for normal mitotic chromosomes).
📍 Types Based on Centromere Position
| Type | Centromere Position | Arm ratio (p:q) | Example |
|---|---|---|---|
| Metacentric | Middle | 1:1 | Human chromosome 1, 3 |
| Submetacentric | Off-centre | ~1:1.7 | Human chromosome 4–12 |
| Acrocentric | Near one end | Short arm very small | Human chr. 13, 14, 15, 21, 22 |
| Telocentric | At one end | No short arm | Mouse chromosomes |
| Holocentric | Diffuse | — | Caenorhabditis elegans |
🧬 Euchromatin vs Heterochromatin & DNA Packaging
Euchromatin: Less condensed, transcriptionally active (gene-rich), early-replicating, appears light with Giemsa staining. Contains most protein-coding genes.
Heterochromatin: Highly condensed, transcriptionally inactive. Constitutive heterochromatin (centromeres, telomeres, satellite DNA — always condensed) vs. Facultative heterochromatin (inactivated X chromosome — Barr body; developmentally regulated silencing).
DNA Packaging — Nucleosome Model (Kornberg, 1974)
The human genome (3×10⁹ bp, ~2 m of DNA) is compacted into a nucleus (~6 µm) through hierarchical packaging:
- Nucleosome (~10 nm): 147 bp DNA wrapped 1.65 turns around a histone octamer (2× H2A, H2B, H3, H4). Linker DNA (~20-80 bp) connects nucleosomes. H1 binds linker DNA.
- 30 nm fibre (solenoid): Nucleosomes coiled with H1 — ~6 nucleosomes per turn. Compaction: ~40-fold over B-DNA.
- Loop domains (~300 nm): Radial loops attached to nuclear matrix (SAR/MAR sequences). ~50-100 kb loops.
- Coiled coils (~700 nm): Loops form rosettes; rosettes coil into larger structures.
- Metaphase chromosome (~1400 nm): Maximum compaction ~8,000-fold.
Video Resources – Chromosomes
DNA Packaging – HHMI BioInteractive
Nucleosome Structure – Khan Academy
Polytene Chromosomes – Yale OYC
Chromatin Remodeling – iBiology
Self-Assessment – Chromosomes
Activities – Chromosomes
Practical
Karyotyping Exercise
Using a printed/digital human karyotype, cut out and arrange chromosomes by size, centromere position, and G-banding pattern. Identify homologous pairs. Determine sex. Check for trisomies. Complete a karyotype worksheet.
Lab Observation
Polytene Chromosomes of Chironomus
Dissect salivary glands from Chironomus larvae. Squash on a slide in aceto-carmine stain. Observe banding pattern and puffs. Sketch and label. Compare with Drosophila polytene chromosome map.
Unit 3 · Topic 2
Cell Division & the Cell Cycle
🔄 Cell Cycle & Regulation
The cell cycle consists of interphase (G₁, S, G₂) and M phase (mitosis + cytokinesis).
- G₁ phase: Cell growth, protein synthesis; restriction point (Start in yeast) — commitment to division; p53, pRb active
- S phase: DNA replication — genome duplicated; ~6–8 hours
- G₂ phase: Continued growth; check for DNA replication errors
- M phase: Mitosis + cytokinesis; ~1 hour
- G₀: Quiescent/differentiated state; can be temporary (hepatocytes) or permanent (neurons)
Cell Cycle Regulation — Cyclins & CDKs
Cyclin-dependent kinases (CDKs) are activated by binding to cyclins (concentrations oscillate). Key checkpoints: G₁/S — Cyclin D/CDK4,6 + Cyclin E/CDK2; G₂/M — Cyclin B/CDK1 (MPF — maturation promoting factor); Spindle checkpoint — Mad2, BubR1 monitor kinetochore attachment. Tumour suppressors: pRb (blocks E2F transcription factor in G₀/early G₁); p53 (activates p21 CDK inhibitor in response to DNA damage).
🧫 Mitosis
P
Prophase
Chromatin condenses into visible chromosomes. Nucleolus disappears. Centrosomes separate. Spindle formation begins. Nuclear envelope begins to break down.
PM
Prometaphase
Nuclear envelope breaks down. Kinetochores capture microtubules. Chromosomes begin moving toward the plate. Dynamic attachments: amphitelic (normal), monotelic, syntelic, merotelic (corrected by Aurora B kinase).
M
Metaphase
Chromosomes align at the metaphase plate (cell equator). Spindle assembly checkpoint (SAC) — all kinetochores must be correctly attached before anaphase.
A
Anaphase
Anaphase A: Sister chromatids separate — cohesin cleaved by separase (activated by APC/C-Cdc20 degrading securin). Anaphase B: Spindle poles move apart.
T
Telophase & Cytokinesis
Nuclear envelopes reform around each chromosome set. Chromosomes decondense. Nucleoli reappear. Cytokinesis: contractile ring (actin + myosin II) forms cleavage furrow → midbody → cell division.
🧬 Meiosis
Two successive divisions (Meiosis I + II) with one DNA replication → 4 haploid cells. Critical for sexual reproduction and genetic diversity.
Meiosis I (Reductional division):- Prophase I (longest): Leptotene (chromosome condensation) → Zygotene (synapsis begins, synaptonemal complex forms) → Pachytene (crossing over, chiasmata) → Diplotene (desynapsis, bivalents held by chiasmata; lampbrush chromosomes in oocytes; arrested in human oocytes for years) → Diakinesis (terminalisation of chiasmata)
- Metaphase I: Bivalents at plate; random orientation (independent assortment)
- Anaphase I: Homologous chromosomes separate (NOT sister chromatids)
- Telophase I + Cytokinesis I → 2 cells, each with n chromosomes (but each chromosome = 2 chromatids)
Significance of Crossing Over
Exchange of segments between non-sister chromatids of homologous chromosomes at pachytene. Creates new combinations of alleles — genetic recombination. Frequency used to construct linkage maps. Catalysed by SPO11 (creates DSBs) and repaired by RecA homologs (DMC1, RAD51).Video Resources – Cell Division
Mitosis – HHMI BioInteractive
Meiosis – Khan Academy
Cell Cycle Regulation – Amoeba Sisters
Cyclins and CDKs – iBiology (Morgan)
Self-Assessment – Cell Division
Activities – Cell Division
Practical
Mitosis in Onion Root Tip
Fix root tips in Carnoy's fixative. Hydrolyse in 1N HCl at 60°C for 6 min. Stain with aceto-carmine/crystal violet. Squash under cover slip. Observe all stages of mitosis. Count cells in each phase. Calculate mitotic index.
Practical
Meiosis in Locust Testis
Dissect testis from adult male locust. Squash in aceto-carmine. Observe meiotic stages. Identify Prophase I sub-stages (leptotene through diakinesis). Draw and label bivalents, chiasmata, and other features.
Discussion
Cell Cycle & Cancer
Discuss how mutations in CDKs, cyclins, or checkpoint proteins (pRb, p53) lead to uncontrolled proliferation. Present specific examples: retinoblastoma, Li-Fraumeni syndrome.
Unit 3 · Topic 3
Cell-to-Cell Communication & Signal Transduction
📡 Types of Signalling
| Type | Distance | Mechanism | Example |
|---|---|---|---|
| Autocrine | Same cell | Self-stimulation | Cancer cells producing own growth factors (EGF) |
| Paracrine | Nearby cells | Local diffusion | Neurotransmitters at synapse, histamine |
| Endocrine | Distant cells | Blood circulation | Insulin, cortisol, thyroid hormones |
| Juxtacrine | Adjacent cells | Direct contact | Notch-Delta signalling |
| Synaptic | Across synapse | Neurotransmitters | Acetylcholine at NMJ |
🔑 Cell Surface Receptors
- G-protein coupled receptors (GPCRs): 7 transmembrane domains; largest receptor superfamily (~800 in humans); activate heterotrimeric G-proteins (Gα, Gβ, Gγ). Examples: β-adrenergic receptor, rhodopsin, odorant receptors.
- Receptor Tyrosine Kinases (RTKs): Single-pass transmembrane; ligand binding causes dimerisation → autophosphorylation → downstream signalling. Examples: EGFR, PDGFR, insulin receptor.
- Ion channel receptors (Ligand-gated): Ligand binding opens ion channel. Example: nicotinic acetylcholine receptor (nAChR) — Na⁺/K⁺ influx → depolarisation.
- Enzyme-linked receptors: Guanylyl cyclase (ANP receptor → cGMP production)
- Intracellular receptors: For lipophilic hormones (steroids, thyroid hormones) — nuclear receptors that act as transcription factors.
⚡ Second Messengers & Signal Transduction
Key second messengers: cAMP, cGMP, DAG (diacylglycerol), IP₃ (inositol trisphosphate), Ca²⁺, Phosphatidylinositol phosphates (PIPs).
Peptide Hormone Signalling (e.g., Glucagon via GPCR–cAMP pathway)
Glucagon → β-adrenergic receptor → Gαs activates → Adenylyl cyclase → cAMP ↑ → PKA activation → Phosphorylation of glycogen phosphorylase (activation) and glycogen synthase (inhibition) → Glycogenolysis. cAMP degraded by phosphodiesterase (PDE). Caffeine inhibits PDE → prolongs cAMP signal.
Steroid Hormone Signalling
Steroid hormones (cortisol, oestrogen, testosterone) are lipophilic → cross plasma membrane → bind intracellular receptor (in cytoplasm or nucleus) → Hormone-receptor complex translocates to nucleus → Binds Hormone Response Elements (HREs) on DNA → Activates/represses gene transcription → Hours to days effect. No second messenger needed.
RTK–MAPK Pathway (Growth Factor Signalling)
EGF → EGFR dimerisation → Autophosphorylation → Grb2-SOS docking → Ras activation (GDP→GTP) → Raf activation → MEK phosphorylation → ERK (MAPK) phosphorylation → Nuclear entry → Gene transcription → Cell proliferation. Ras is mutated in ~30% of cancers.
Video Resources – Cell Signaling
Signal Transduction Pathways – Khan Academy
GPCRs – Nobel Lecture 2012 (Lefkowitz)
MAPK Pathway – iBiology
Steroid Hormone Mechanism – Armando H.
Self-Assessment – Cell Signaling
Activities – Cell Signaling
Discussion
Drug Targets in Signal Transduction
Research: (a) Imatinib (Gleevec) targets BCR-ABL kinase in CML; (b) Herceptin (trastuzumab) targets HER2/EGFR in breast cancer; (c) Propranolol blocks β-adrenergic receptors. Explain mechanism of action in each case using signalling pathway knowledge.
Assignment
Pathway Diagram
Draw the complete cAMP second messenger pathway for adrenaline action on glycogen metabolism. Include: receptor → G-protein → effector enzyme → second messenger → protein kinase → substrate → physiological effect. Also show termination of signal.
Unit 3 · Topic 4
Cell Death: Necrosis & Apoptosis
⚡ Necrosis (Accidental Cell Death)
Uncontrolled, passive form of cell death caused by external injury (ischaemia, toxins, trauma). Features: cell swelling (oncosis), organelle swelling, membrane disruption, release of cellular contents → inflammation. DNA degradation is random (non-laddering). No energy required.
🌿 Apoptosis (Programmed Cell Death)
Controlled, energy-dependent process of cell self-destruction. Term coined by Kerr, Wyllie & Currie (1972). Nobel Prize 2002 awarded to Brenner, Horvitz & Sulston for apoptosis research in C. elegans. In C. elegans development, exactly 131 cells die by apoptosis (of 1090 generated).
Morphological features:- Cell shrinkage (opposite of necrosis)
- Chromatin condensation (pyknosis) and nuclear fragmentation (karyorrhexis)
- DNA ladder (180 bp multiples) — internucleosomal cleavage by CAD/DFF40
- Formation of apoptotic bodies — membrane-bound blebs
- Phosphatidylserine (PS) externalisation — "eat me" signal for macrophages
- No inflammation — apoptotic bodies phagocytosed cleanly
⚙️ Mechanisms of Apoptosis
Intrinsic Pathway (Mitochondrial)
Triggered by: DNA damage, oxidative stress, growth factor withdrawal, oncogene activation.
Mechanism: Pro-apoptotic Bcl-2 family members (Bax, Bak, Bad, Bid) — activated by p53 — insert into OMM → MOMP (mitochondrial outer membrane permeabilisation) → cytochrome c release into cytosol → cytochrome c + Apaf-1 + dATP form apoptosome → activates procaspase-9 → caspase-9 (initiator) → cleaves procaspases 3 and 7 → caspase-3 (executioner) → substrate cleavage (PARP, lamins, CAD inhibitor ICAD) → apoptotic features. Anti-apoptotic Bcl-2, Bcl-xL prevent MOMP.
Extrinsic Pathway (Death Receptor)
Ligands: FasL, TNF-α, TRAIL.
Mechanism: FasL binds Fas (CD95) → DISC (Death Inducing Signalling Complex) forms — FADD adaptor protein recruited → procaspase-8 → caspase-8 (initiator) → directly activates caspase-3 OR cleaves Bid → tBid → intrinsic pathway amplification.
Significance in Cellular Homeostasis
- Development: Digit formation (removal of interdigital webbing), neural development (50% neurons eliminated), immune selection
- Homeostasis: Balances cell proliferation; ~50–70 billion cells die per day in an adult human
- Immunity: Deletion of autoreactive T/B cells; killing of virally infected cells by CTLs
- Cancer: Dysregulation of apoptosis — Bcl-2 overexpression in follicular lymphoma (t(14;18)); p53 mutations in 50% cancers
- Therapeutic target: BH3 mimetics (venetoclax — Bcl-2 inhibitor) used in CLL treatment
| Feature | Apoptosis | Necrosis |
|---|---|---|
| Trigger | Programmed (intrinsic/extrinsic signals) | Accidental (external injury, toxin, ischaemia) |
| Cell volume | Shrinkage | Swelling |
| Membrane | Intact initially; PS externalisation | Rupture → loss of integrity |
| DNA | Ladder (180 bp multiples) | Random degradation (smear) |
| Energy | ATP-dependent | Passive (no ATP needed) |
| Inflammation | None (neat phagocytosis) | Yes (cellular contents released) |
| Key molecules | Caspases, Bcl-2 family, cytochrome c | Lysosomal enzymes |
Video Resources – Cell Death
Apoptosis Pathway – Khan Academy
Apoptosis Animation – HHMI
Nobel 2002 Lecture – Sulston, Horvitz
Bcl-2 family & Cancer – iBiology
Self-Assessment – Cell Death
Activities – Cell Death
Practical
TUNEL Assay (Conceptual)
Study the principle of TUNEL (Terminal deoxynucleotidyl transferase dUTP Nick End Labelling) assay for detecting apoptotic cells. Using published micrographs, identify apoptotic cells (brown TUNEL-positive nuclei) vs. viable cells. Calculate apoptotic index.
Discussion
Apoptosis — Hero or Villain?
Discuss scenarios where apoptosis is beneficial (embryonic development, immunity) vs. detrimental (neurodegeneration in Alzheimer's, HIV-induced T cell depletion). How can we harness apoptosis for therapy?
Assignment
Flow Chart of Caspase Cascade
Draw a detailed flowchart of both intrinsic and extrinsic apoptotic pathways, showing the convergence at executioner caspases. Indicate where each Bcl-2 family member (pro- and anti-apoptotic) exerts its effect.
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