Cell Biology

11.1 Microscopy and the History of the Cell Theory

The cell is the fundamental unit of life — a conclusion reached not in a single flash of insight but through two centuries of instrument-making and observation. Robert Hooke (1665) first applied the word "cell" to the box-like compartments he saw in cork slices under his compound microscope; those boxes were, of course, dead plant cell walls. Within a decade, Antonie van Leeuwenhoek (1674) ground lenses of unprecedented quality to observe living "animalcules" — bacteria and protozoa — in pond water, demonstrating that microscopic life was ubiquitous. The concept of the nucleus followed when Robert Brown (1831) reported a consistent opaque spot in the cells of orchid epidermis, naming it the nucleus. The unified cell theory crystallised from the independent botanical and zoological observations of Matthias Schleiden (1838) and Theodor Schwann (1839), and was completed by Rudolf Virchow (1855) with his axiom Omnis cellula e cellula — every cell from a pre-existing cell, demolishing spontaneous generation as an explanation for cellular origin. Walther Flemming (1882) described the choreography of chromosome movement in dividing cells, coining the term mitosis.

Hooke — cork "cells" 1665 · Leeuwenhoek — living microbes 1674 · Brown — nucleus 1831 · Schleiden — plant cell theory 1838 · Schwann — animal cell theory 1839 · Virchow — Omnis cellula e cellula 1855 · Flemming — mitosis 1882 · Singer & Nicolson — fluid-mosaic membrane 1972 · Hartwell, Hunt & Nurse — cell-cycle checkpoints, Nobel 2001

11.1.1 Light Microscopy

The compound light microscope stacks two lens systems — objective and eyepiece — to magnify a stained specimen up to ~1000×. The fundamental resolution limit was formalised by Ernst Abbe (1873): d = 0.61λ/NA, where λ is the wavelength of illuminating light and NA is the numerical aperture of the objective. For visible light (~550 nm) and a high-quality oil-immersion objective (NA ≈ 1.4), the practical resolution is ~0.2 µm — sufficient to distinguish organelles but not macromolecular complexes. Phase-contrast microscopy (Frits Zernike, Nobel 1953) converts differences in refractive index into differences in amplitude, enabling observation of living, unstained cells. Fluorescence microscopy excites tagged molecules with specific wavelengths and captures emitted light, underpinning modern cell biology (GFP Nobel 2008). Confocal laser scanning microscopy (CLSM) uses a pinhole to exclude out-of-focus light, building optically sectioned 3-D reconstructions of fixed or live material.

11.1.2 Electron Microscopy

To resolve structures below 0.2 µm the wavelength must be shortened below that of visible light. The electron beam used in transmission electron microscopy (TEM) has a wavelength ~0.005 nm, giving theoretical resolution ~0.1 nm — three orders of magnitude better than light. Ernst Ruska built the first practical TEM in 1931; he shared the Nobel Prize for Physics in 1986. Specimens are ultra-thin (50–100 nm) epoxy sections stained with heavy metals (osmium, uranyl acetate). Scanning electron microscopy (SEM) scans a focused beam across a gold-coated surface, collecting secondary electrons to build a 3-D surface image at up to ~20 nm resolution. TEM reveals internal ultrastructure; SEM reveals surface morphology. Cryo-EM (Henderson, Frank, Dubochet Nobel 2017) vitrifies samples in liquid ethane, eliminating fixation artifacts, and has now resolved membrane proteins at near-atomic resolution.

11.2 Prokaryotic vs Eukaryotic Cells

All cellular life divides into two fundamental domains of organisation. Prokaryotic cells (Domain Bacteria and Archaea) lack a membrane-bounded nucleus; their DNA is a single, typically circular, naked chromosome concentrated in an irregularly shaped region called the nucleoid. They have no membrane-bound organelles, though the plasma membrane may be elaborated into structures such as the mesosome (invaginations involved in cell-wall synthesis and DNA segregation in Gram-positive bacteria) and the photosynthetic lamellae of cyanobacteria. Prokaryotic ribosomes are 70S (50S + 30S subunits). Cell size ranges 1–10 µm.

Eukaryotic cells (Domain Eukarya — plants, animals, fungi, protists) possess a true nucleus enclosed by a double nuclear envelope perforated by nuclear pores. They compartmentalise biochemical reactions into membrane-bound organelles and have a cytoskeleton of microtubules, microfilaments, and intermediate filaments. Eukaryotic ribosomes are 80S (60S + 40S) in the cytoplasm, but mitochondria and chloroplasts retain 70S ribosomes — a key piece of evidence for endosymbiotic origin. Cell size 10–100 µm (some up to 1 mm).

11.3 Cell Wall & Plasma Membrane (Fluid Mosaic Model)

11.3.1 Cell Wall

The cell wall is a semi-rigid extracellular matrix that provides mechanical support, determines cell shape, and regulates turgor pressure. Its chemical composition varies across kingdoms:

• Plants: primary wall — cellulose microfibrils (~36-glucan chains) embedded in a hemicellulose-pectin matrix; secondary wall (in wood, fibres) — cellulose + lignin.
• Fungi: chitin (poly-N-acetylglucosamine) reinforced with β-glucans.
• Bacteria: peptidoglycan (murein) — alternating NAG and NAM residues cross-linked by short peptides; thick in Gram-positive, thin in Gram-negative (outer membrane overlies it).
• Algae: cellulose in most green algae; agar (Rhodophyta); alginic acid (Phaeophyta); silica frustule in diatoms.
• Archaea: pseudopeptidoglycan or S-layer glycoprotein — no true murein (reason archaea are intrinsically resistant to β-lactam antibiotics).

Plant cell walls have plasmodesmata — cytoplasmic channels (diameter ~30–60 nm, lined by desmotubule from ER) that connect adjacent cells, forming the symplast. The middle lamella, composed mainly of calcium pectate, cements adjacent cells together.

11.3.2 Plasma Membrane — Fluid Mosaic Model

The contemporary model of membrane structure is the fluid-mosaic model proposed by S. J. Singer and Garth Nicolson (1972), which replaced the earlier Davson–Danielli "pauci-molecular" sandwich model (1935) that erroneously placed protein layers coating the outside of a lipid bilayer. In the fluid-mosaic model, the membrane is a two-dimensional fluid of phospholipid molecules in which diverse protein molecules are embedded or associated — hence "fluid" (lipids diffuse laterally) and "mosaic" (proteins are scattered like tiles).

The phospholipid bilayer is the backbone: each phospholipid has a hydrophilic glycerol-phosphate head (faces aqueous environment) and two hydrophobic fatty-acid tails (face the membrane interior). In animal cell membranes, cholesterol molecules intercalate between phospholipids to modulate fluidity: at high temperatures cholesterol reduces fluidity (prevents excessive lateral diffusion); at low temperatures it prevents tight packing and maintains fluidity — it acts as a "fluidity buffer." Plants lack cholesterol and use phytosterols (stigmasterol, sitosterol) in the same regulatory role.

11.3.3 Membrane Proteins

Integral (intrinsic) proteins are embedded within the hydrophobic core of the bilayer, often spanning it completely (transmembrane proteins). They include ion channels, carrier proteins (transporters), G-protein-coupled receptors, and structural proteins. Many contain α-helical transmembrane domains of ~20 hydrophobic amino acids. Peripheral (extrinsic) proteins associate non-covalently with the polar heads of lipids or with integral proteins, mainly on the cytoplasmic face (e.g., ankyrin, spectrin in red blood cells). Glycoproteins bear oligosaccharide chains on the extracellular surface and form the glycocalyx — a carbohydrate-rich coat involved in cell-cell recognition, immune surveillance, and receptor function. Glycolipids (e.g., gangliosides, cerebrosides) are phospholipids with sugar groups and contribute to the glycocalyx.

11.3.4 Membrane Transport

Passive transport requires no metabolic energy: (i) Simple diffusion — small non-polar molecules (O₂, CO₂, ethanol, steroid hormones) dissolve and cross the bilayer along their concentration gradient. (ii) Osmosis — net movement of water across a semipermeable membrane from lower to higher solute concentration. (iii) Facilitated diffusion — hydrophilic molecules (glucose, amino acids) cross via specific carrier proteins or channel proteins (aquaporins for water; GLUT transporters for glucose), still down their gradient.

Active transport uses ATP to move solutes against their electrochemical gradient: the Na⁺/K⁺ ATPase pump ejects 3 Na⁺ out and imports 2 K⁺ per ATP hydrolysed, maintaining the resting membrane potential of neurons (~−70 mV). The proton pump (H⁺-ATPase) in plant cells and bacteria drives secondary active transport.

Vesicular (bulk) transport. Endocytosis brings material into the cell: phagocytosis ("cell eating") engulfs large particles (bacteria, debris) via pseudopods — characteristic of neutrophils and macrophages; pinocytosis ("cell drinking") takes up extracellular fluid in small vesicles; receptor-mediated endocytosis (RME) internalises specific ligands bound to clathrin-coated pit receptors (e.g., LDL uptake, Brown & Goldstein Nobel 1985). Exocytosis fuses secretory vesicles with the plasma membrane to export material (neurotransmitters, digestive enzymes, insulin).

11.4 Cytoplasm and Membrane-Bound Organelles

The cytoplasm is everything inside the plasma membrane excluding the nucleus: it includes the aqueous cytosol (~70% water, dissolved metabolites, ions, proteins, mRNAs) and all membrane-bound organelles. A variety of specialised compartments carry out distinct biochemical functions.

11.4.1 Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a continuous system of membrane-enclosed flattened sacs (cisternae) and tubules extending from the outer nuclear envelope throughout the cytoplasm. It exists in two functionally distinct domains. Rough ER (RER) is studded with ribosomes on its cytoplasmic face; it is the site of synthesis of all membrane-bound and secreted proteins. Newly synthesised polypeptides are co-translationally inserted into the RER lumen, where signal peptides are cleaved, N-linked glycosylation begins, and disulfide bonds form (with the help of protein disulfide isomerase). Smooth ER (SER) lacks ribosomes and specialises in: (i) lipid and steroid hormone synthesis (especially in liver and steroid-secreting adrenal cortex cells); (ii) detoxification of hydrophobic drugs and xenobiotics via cytochrome P450 enzymes (liver SER); (iii) Ca²⁺ storage and release in muscle cells (sarcoplasmic reticulum is specialised SER); (iv) glycogen metabolism.

11.4.2 Golgi Apparatus

Described by Camillo Golgi (1898), the Golgi apparatus is a stack of flattened membranous cisternae (typically 4–8 in mammalian cells) organised with a cis face (receiving side, near ER) and a trans face (sending/secretory side). Vesicles bud from ER and fuse with the cis face; modified vesicles depart from the trans face for lysosomes, the plasma membrane, or secretion. In the Golgi: (i) N-linked oligosaccharides added in ER are modified; (ii) O-linked glycosylation on Ser/Thr residues begins; (iii) proteolytic cleavage of propeptides (e.g., proinsulin → insulin); (iv) sulphation of tyrosine residues in secretory proteins; (v) lipid modifications. The Golgi is the post-office of the cell — sorting and addressing proteins for their final destination. In plant cells the Golgi contributes polysaccharide components of the new cell plate during cytokinesis.

11.4.3 Lysosomes

Lysosomes are membrane-bound vesicles (~0.1–1.2 µm) containing ~60 acid hydrolases (proteases, lipases, nucleases, glycosidases, phosphatases) that function optimally at pH 5, maintained by a V-type H⁺-ATPase proton pump in the lysosomal membrane. Discovered by Christian de Duve (Nobel 1974, shared with Albert Claude and George Palade). Functions: (i) autophagy — digestion of worn-out organelles (autophagosomes fuse with lysosomes); (ii) heterophagy — digestion of engulfed foreign material (phagosome + lysosome = phagolysosome); (iii) autolysis — programmed self-digestion in metamorphosis (tadpole tail resorption) and apoptosis. Lysosomal storage diseases (Gaucher's, Tay-Sachs, Pompe's) result from single enzyme deficiencies causing substrate accumulation.

11.4.4 Mitochondria

Mitochondria (singular: mitochondrion) are the sites of aerobic respiration and ATP synthesis, earning them the title "powerhouses of the cell." They have two membranes: the outer membrane is smooth and permeable (contains porin channels); the inner membrane is highly convoluted into cristae, increasing surface area for the electron-transport chain (ETC) and ATP synthase (Complex V / F₀F₁-ATPase). The space between membranes is the intermembrane space; the matrix is enclosed by the inner membrane and contains: the TCA (Krebs) cycle enzymes, fatty-acid oxidation enzymes, mitochondrial DNA (~16,569 bp circular in humans — a maternal-inheritance marker), 70S ribosomes (mitoribosomes), and tRNAs sufficient for a limited genetic code. The endosymbiotic hypothesis of Lynn Margulis (1967, 1970) proposes that mitochondria evolved from free-living alpha-proteobacteria engulfed by a proto-eukaryotic host cell ~1.5 Gya — supported by: (i) double membrane; (ii) 70S ribosomes; (iii) circular DNA; (iv) binary fission-like replication; (v) mitochondria-specific antibiotics (chloramphenicol blocks 50S).

11.4.5 Chloroplasts

Chloroplasts are the photosynthetic organelles of plant and algal cells, also of endosymbiotic origin (from cyanobacteria, ~1.2 Gya, Margulis). Structure: outer membrane (smooth), inner membrane (smooth, permeable), and an internal system of flattened sacs — thylakoids — stacked into grana (singular: granum) interconnected by unstacked stroma lamellae. The aqueous interior around the thylakoids is the stroma. Light reactions (PSI, PSII, ETC, ATP synthase) occur on the thylakoid membranes; the Calvin cycle (carbon fixation) occurs in the stroma. Chloroplast DNA (cpDNA) is circular (~150 kb in land plants), and 70S ribosomes are present. The chloroplast genome encodes ~80 proteins but imports ~95% of its ~3000 proteins from the cytoplasm.

Plastids interconvert: Proplastids (undifferentiated, in meristems) differentiate into: Chloroplasts (green, photosynthetic; contain chlorophyll a & b, carotenoids); Chromoplasts (red/orange/yellow; carotenoids without chlorophyll; in flowers and ripe fruits, e.g., tomato lycopene); Leucoplasts (colourless storage plastids) — Amyloplasts (starch; potato tubers), Elaioplasts (oils and fats; in floral organs), Proteinoplasts/Aleuroplasts (proteins). Chloroplasts can convert to chromoplasts (as tomatoes ripen), but the reverse (chromoplast → chloroplast) is rare.

11.4.6 Ribosomes

Ribosomes are non-membrane-bound ribonucleoprotein complexes, the universal machinery for protein synthesis. Prokaryotic 70S = large 50S subunit (23S rRNA + 5S rRNA + ~34 proteins) + small 30S subunit (16S rRNA + ~21 proteins). Eukaryotic cytoplasmic 80S = large 60S (28S + 5.8S + 5S rRNA + ~49 proteins) + small 40S (18S rRNA + ~33 proteins). Mitochondrial and chloroplast ribosomes are 70S, consistent with endosymbiotic origin; their rRNAs are used as evolutionary clocks. S = Svedberg units (sedimentation coefficient, a property of size and shape, not simply additive: 50S + 30S ≠ 80S). Free ribosomes synthesise cytoplasmic proteins; membrane-bound ribosomes (on RER) synthesise secretory and membrane proteins.

11.4.7 Centrioles, Cilia, and Flagella

Centrioles are cylindrical structures (~0.5 µm long, ~0.2 µm diameter) composed of nine triplets of microtubules (9+0 arrangement — no central pair). Present in animal cells and lower plant cells; absent in higher plant cells and most fungi. A pair of centrioles form the centrosome (MTOC — microtubule organising centre) which nucleates spindle microtubules during cell division. Centrioles also template the basal bodies of cilia and flagella. Cilia and flagella have an axoneme with 9+2 arrangement (nine outer doublets + two central singlet microtubules). Dynein arms on outer doublets use ATP hydrolysis to generate bending force. Primary cilia (non-motile, 9+0) function as cellular antennae for signalling (Hedgehog pathway, PDGFα receptor). Defects in ciliary structure cause primary ciliary dyskinesia (Kartagener's syndrome — bronchiectasis, situs inversus, infertility).

11.4.8 Vacuoles

Vacuoles are fluid-filled, membrane-bounded compartments (tonoplast = vacuolar membrane). A mature plant cell typically has a single large central vacuole occupying 80–90% of cell volume. It maintains turgor pressure, stores pigments (anthocyanins), secondary metabolites (alkaloids, tannins, glucosinolates), ions, and water. The vacuole is also a compartment for sequestering toxic metabolites away from the cytosol. Animal cells have smaller, temporary vacuoles: food vacuoles (from phagocytosis) and contractile vacuoles in freshwater protists (osmoregulation).

11.5 The Nucleus and Chromatin

The nucleus is the control centre of the eukaryotic cell — the repository of the cell's genetic information and the site of DNA replication and RNA synthesis (transcription). It is bounded by the nuclear envelope: two concentric phospholipid bilayers (outer and inner nuclear membranes) separated by the perinuclear space (~20–40 nm). The outer nuclear membrane is continuous with the ER and studded with ribosomes. The two membranes are fused at ~3000–4000 nuclear pore complexes (NPCs) per mammalian nucleus. Each NPC is an octagonally symmetric channel ~120 nm in diameter, composed of ~30 different nucleoporin proteins (~120 MDa total), and mediates the selective, signal-directed, active transport of proteins (histones, transcription factors, RNA polymerases) into the nucleus and of mRNA, rRNA, tRNA, and ribosomes out of it.

Inside the nucleus, DNA is not naked but is associated with histone proteins to form chromatin. The fundamental unit of chromatin is the nucleosome, described by Roger Kornberg (1974, Nobel 2006). Each nucleosome has a disc-shaped protein core — a histone octamer of 2× each of H2A, H2B, H3, and H4 — around which ~146 bp of DNA is wrapped 1.65 turns in a left-handed superhelix. Nucleosomes are connected by linker DNA (~20–80 bp) and the linker histone H1, which seals the DNA against the core particle, creating the "beads on a string" (11 nm) conformation visible by electron microscopy. Further coiling with H1 condenses chromatin into a 30 nm fibre. Loops of the 30 nm fibre attach to a protein scaffold to form the 300 nm fibre of interphase chromosomes. Further condensation during mitosis produces the 700 nm chromatid and finally the ~1400 nm fully condensed metaphase chromosome.

11.5.1 Euchromatin and Heterochromatin

Euchromatin is lightly staining, loosely packaged, transcriptionally active chromatin. It corresponds to gene-rich regions that are accessible to RNA polymerases and transcription factors. Heterochromatin is darkly staining, tightly packaged, largely transcriptionally silent. It comes in two varieties: Constitutive heterochromatin — permanently condensed throughout the cell cycle; found at centromeres, telomeres, and pericentromeric regions; typically composed of repetitive satellite DNA sequences with no coding function. Facultative heterochromatin — regions that are condensed in some cells or developmental stages but euchromatic in others; the classic example is the inactivated X chromosome (“Barr body”) in female mammalian cells — one of the two X chromosomes is compacted into a Barr body (Lyon hypothesis, 1961) in all somatic cells, maintaining dosage compensation. The decision as to which X is inactivated is random and irreversible in each cell, making female mammals mosaic.

11.5.2 The Nucleolus

The nucleolus (pl. nucleoli) is a non-membrane-bound organelle inside the nucleus, formed around nucleolus organiser regions (NORs) — specific chromosomal loci carrying tandemly repeated rRNA genes (rDNA). In humans, NORs are on the short arms of acrocentric chromosomes 13, 14, 15, 21, and 22 (the stalks carry rDNA). The nucleolus is the site of rRNA transcription (by RNA Pol I), processing of pre-rRNA (45S → 28S + 18S + 5.8S), and assembly of ribosomal subunits (with ribosomal proteins imported from the cytoplasm). During mitosis the nucleolus disappears (rRNA transcription ceases) and reforms in telophase. Large, prominent nucleoli are a hallmark of rapidly dividing cells (and cancer cells with high ribosome demand).

11.6 Chromosome Structure & Karyotyping

A chromosome (from Greek chroma = colour + soma = body) is the maximally condensed form of a single DNA molecule and its associated proteins. Each chromosome consists, during S/G2 and prophase, of two identical sister chromatids joined at the centromere. After anaphase, when sisters separate, each is an independent chromosome. The centromere is the site of kinetochore assembly — the proteinaceous structure on which kinetochore microtubules of the spindle attach. Chromosomes are classified by centromere position:

11.6.1 Telomeres

Chromosome ends are capped by telomeres — repetitive TTAGGG hexanucleotide sequences (in vertebrates) extending ~5–15 kb, bound by the shelterin protein complex. Telomeres prevent chromosomal ends from being recognised as double-strand breaks and prevent chromosome fusions. With each round of DNA replication, telomeres shorten (the "end-replication problem") because DNA polymerase cannot replicate the lagging strand to the very tip. In germline cells and stem cells, the enzyme telomerase (an RNA-dependent DNA polymerase with its own RNA template — TERC — and a protein catalytic subunit — TERT) adds back TTAGGG repeats. Telomere biology was elucidated by Elizabeth Blackburn, Carol Greider, and Jack Szostak (Nobel Prize in Physiology or Medicine, 2009). Telomere shortening in somatic cells is a molecular clock of cellular ageing; restoration of telomerase is a hallmark of cancer immortalisation.

11.6.2 Karyotyping

A karyotype is the full complement of chromosomes in a cell, arranged in homologous pairs by size and centromere position, typically photographed at metaphase. Standard technique: lymphocytes stimulated to divide → arrested at metaphase with colchicine (depolymerises spindle) → hypotonic treatment (cell swells, chromosomes spread) → fixation → G-banding (Giemsa stain after trypsin treatment) → photography → arrangement. Key karyotypes relevant to HPRCA:

Common human chromosomal abnormalities tested in HPRCA: Down syndrome (Trisomy 21, 2n = 47); Turner syndrome (45, X0 — monosomy X, female); Klinefelter syndrome (47, XXY — male, infertile); Patau syndrome (Trisomy 13); Edwards syndrome (Trisomy 18); Cri du chat (deletion of 5p). These arise from nondisjunction (failure of chromosomes or chromatids to separate during meiosis I or II) or chromosomal structural changes (deletion, duplication, inversion, translocation).

11.7 Cell Cycle & Cell-Cycle Checkpoints

The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides into two daughter cells. In actively dividing mammalian cells it takes ~24 hours. The cycle has two broad phases: Interphase (the period of growth and DNA replication, ~90–95% of cycle time) and M phase (mitosis + cytokinesis, ~5–10%).

Interphase is subdivided into three gaps and a synthesis phase:

• G1 (Gap 1): rapid cell growth; biosynthesis of proteins, organelles; preparation for DNA replication. Most variable in length. Differentiated cells that exit the cycle do so from G1 into G0 (quiescent state). Duration: 11–12 h in typical mammalian cell. DNA content: 2C (diploid). Chromosome number: 2n.
• S phase (Synthesis): DNA replication occurs; each chromosome is duplicated, producing sister chromatids. Histones synthesised in parallel. Duration: ~8 h. DNA content at end of S: 4C. Chromosome number remains 2n (chromatids still joined at centromere).
• G2 (Gap 2): further growth; repair of replication errors; synthesis of mitotic apparatus proteins (tubulins, cyclins). Duration: ~4 h. DNA content: 4C.
• M phase: mitosis (prophase → metaphase → anaphase → telophase) + cytokinesis. DNA returns to 2C per daughter cell.

11.7.1 Cyclin-CDK Regulation

Cell-cycle progression is driven by cyclin-dependent kinases (CDKs) — serine/threonine kinases that are constitutively expressed but inactive alone. They are activated by binding to their oscillating partner proteins, the cyclins, whose concentrations rise and fall in precise waves. Key complexes: Cyclin D–CDK4/6 (G1 phase; phosphorylates RB to release E2F transcription factor, permitting S-phase entry — the restriction point); Cyclin E–CDK2 (late G1 → S); Cyclin A–CDK2 (S phase); Cyclin B–CDK1 (MPF — M-phase promoting factor) drives entry into mitosis. Cyclins are degraded by the ubiquitin–proteasome system via APC/C (anaphase-promoting complex/cyclosome) or SCF ubiquitin ligases, resetting the cycle. This system was elucidated by Leland Hartwell (yeast CDC28), Paul Nurse (fission yeast cdc2), and Timothy Hunt (cyclin oscillation) — jointly awarded the Nobel Prize 2001.

11.8 Mitosis

Mitosis is equational cell division — a parent cell with 2n chromosomes (4C DNA) divides to produce two genetically identical daughter cells each with 2n (2C). Described first by Walther Flemming (1882). It is the basis of growth, repair, and asexual reproduction in eukaryotes. Mitosis is conventionally divided into four continuous stages (with cytokinesis added as a separate event):

11.8.1 Stages of Mitosis

Prophase: Chromosomes condense and become visible as sister chromatid pairs (each consisting of two chromatids joined at the centromere). The nucleolus disappears (rDNA transcription ceases). Centrosomes (in animal cells) migrate to opposite poles, nucleating the mitotic spindle from tubulin subunits. Spindle assembly begins outside the intact nuclear envelope.

Prometaphase: The nuclear envelope breaks down (NEB), allowing spindle microtubules to access chromosomes. Kinetochore microtubules attach to kinetochores (protein assemblies on centromeres). Chromosomes begin oscillating (congression) toward the cell equator. The spindle assembly checkpoint (SAC) is active until all kinetochores achieve proper bi-oriented attachment (amphitelic attachment with tension).

Metaphase: All chromosomes are aligned at the cell's equatorial plate (metaphase plate), equidistant from the two poles. Kinetochore microtubules from opposite poles attach to sister kinetochores. This is the stage used for karyotyping because chromosomes are maximally condensed and well-spread.

Anaphase: The SAC is satisfied; APC/C ubiquitinates securin → releases separase → separase cleaves cohesin (the protein complex holding sister chromatids together) → sister chromatids separate simultaneously and move to opposite poles. Two mechanisms power movement: (i) kinetochore microtubules shorten (poleward flux, driven by kinesin-13 depolymerases); (ii) polar microtubules elongate (antiparallel sliding by kinesin-5), pushing poles apart.

Telophase: Chromosomes arrive at poles and begin to decondense. Nuclear envelopes reform around each set of chromatids (now chromosomes again). Nucleoli reappear. The spindle disassembles.

Cytokinesis (division of cytoplasm) begins in late anaphase/telophase. In animal cells: a cleavage furrow forms as a contractile ring of actin filaments and myosin II constricts the cell equator, pinching off two daughters. In plant cells: no cleavage furrow (rigid cell wall prevents constriction). Instead, Golgi-derived vesicles carrying cell-wall components (pectin, cellulose precursors) are guided by the phragmoplast (set of microtubules + actin) to the cell equator, where they fuse to form the cell plate, which ultimately matures into the new cell wall and middle lamella separating the two daughters.

11.9 Meiosis & Genetic Variation

Meiosis is reductional cell division: a diploid cell (2n, 4C after S phase) undergoes two sequential divisions — Meiosis I (reductional) and Meiosis II (equational) — to produce four haploid (n, 1C) cells. It is the basis of sexual reproduction and the source of genetic variation in populations. Occurs exclusively in the gonads (germline): spermatogenesis (testes) and oogenesis (ovaries) in animals; microsporogenesis and megasporogenesis in plants.

11.9.1 Meiosis I — Reductional Division

Prophase I is the most complex and prolonged stage, subdivided into five sub-stages:

• Leptotene (leptos = thin): chromosomes begin condensing; they appear as thin threads.
• Zygotene (zygon = paired): homologous chromosomes begin pairing (synapsis) along their lengths, mediated by the synaptonemal complex (SC) — a proteinaceous ladder-like structure with lateral elements (SYCP3) and transverse filaments (SYCP1). Formation of bivalents (each bivalent = tetrad = 4 chromatids from 2 homologues).
• Pachytene (pachus = thick): SC fully formed; crossing over (recombination) occurs at chiasmata (singular: chiasma) via Holliday junctions — double-strand breaks introduced by SPO11, processed by MRN complex and strand-invasion via RAD51, resolved by resolvases. This physical exchange of segments between non-sister chromatids shuffles alleles between homologues. Most important stage for genetic variation.
• Diplotene (diploos = double): SC dissolves; homologues begin to repel but remain connected at chiasmata, which become visible. Chromosomes open into an X-shaped configuration. In oocytes arrested at diplotene (primary oocyte stage — arrest lasts years to decades in women).
• Diakinesis (dia = through): further condensation; chiasmata move toward chromosome ends (terminalisation); nucleolus disappears; nuclear envelope breaks down. Marks transition to Metaphase I.

Metaphase I: Bivalents align at the metaphase plate with homologues on opposite sides. Kinetochore microtubules attach to the kinetochores of each homologue (co-orientation of sister kinetochores toward the same pole — opposite to mitosis). Tension generated by chiasmata holding bivalents together is required for correct alignment.

Anaphase I: Homologues separate and move to opposite poles (cohesin is cleaved on chromosome arms by separase, but centromeric cohesin is protected by shugoshin/PP2A until Meiosis II). Sister chromatids remain joined at the centromere.

Telophase I & Cytokinesis: Two secondary cells form, each with n chromosomes (but each chromosome = 2 chromatids). Brief interkinesis follows (no DNA replication).

11.9.2 Meiosis II — Equational Division

Meiosis II resembles mitosis but starts with haploid cells: Prophase II → Metaphase II (haploid chromosomes align) → Anaphase II (centromeric cohesin cleaved → sister chromatids separate) → Telophase II + cytokinesis → four haploid cells. Each is genetically unique due to recombination and independent assortment.

11.9.3 Sources of Genetic Variation in Meiosis

Meiosis generates three distinct sources of genetic variation:

1. Crossing over (recombination) during Prophase I: New combinations of alleles are produced on individual chromatids. The average human meiosis has ~55 crossovers (about 2 per chromosome pair).
2. Independent assortment of homologues at Metaphase I: Each pair of homologues orients independently of all other pairs. For n chromosome pairs, 2ⁿ possible chromosome combinations exist in gametes. For humans (n = 23): 2²³ = 8,388,608 (~8.4 million) combinations from independent assortment alone.
3. Random fertilisation: When combined with independent assortment, the potential zygote combinations = (2²³)² = ~70 trillion — explaining why siblings (except identical twins) are genetically unique despite having the same parents.

11.10 Cancer — Causes, Types, Hallmarks

Cancer is a disease of uncontrolled cell proliferation arising from accumulated somatic mutations that deregulate the cell cycle, apoptosis, and tissue homeostasis. It is not a single disease but a heterogeneous collection of ~200 distinct diseases, unified by the same fundamental defect: cells that grow, divide, and spread inappropriately. The landmark framework describing cancer biology is the Hallmarks of Cancer, originally published by Douglas Hanahan and Robert Weinberg in 2000, revised and extended in 2011.

11.10.1 Hallmarks of Cancer (Hanahan–Weinberg)

The six original hallmarks (2000): (i) Self-sufficiency in growth signals — cancer cells produce their own growth factors or constitutively activate downstream signalling (e.g., Ras mutation in ~30% of all cancers); (ii) Insensitivity to anti-growth signals — loss of contact inhibition; RB pathway disabled; (iii) Evasion of apoptosis — upregulation of BCL-2 (anti-apoptotic), loss of p53; (iv) Limitless replicative potential — telomerase reactivation in ~85–90% of cancers allows indefinite division; (v) Sustained angiogenesis — tumours >1–2 mm require a blood supply; VEGF secretion induces new vessel growth; (vi) Tissue invasion and metastasis — epithelial-mesenchymal transition (EMT), loss of E-cadherin, matrix metalloproteinase secretion.

The 2011 revision added four enabling characteristics and two emerging hallmarks: Reprogramming energy metabolism (Warburg effect: aerobic glycolysis even in O₂ — produces lactate, fuels biosynthesis); Evading immune destruction (downregulation of MHC-I, expression of PD-L1 — target of checkpoint immunotherapy). Enabling characteristics: genomic instability and mutation; tumour-promoting inflammation. Emerging hallmarks: unlocking phenotypic plasticity; non-mutational epigenetic reprogramming; polymorphic microbiomes; senescent cells.

11.10.2 Types of Cancer

• Carcinoma: arising from epithelial cells (skin, gut lining, lung, breast, prostate). Most common type (~85% of all cancers). Examples: adenocarcinoma (glandular epithelium), squamous cell carcinoma (stratified squamous epithelium).
• Sarcoma: arising from mesenchymal/connective tissue (bone — osteosarcoma; cartilage — chondrosarcoma; muscle — rhabdomyosarcoma; fat — liposarcoma; blood vessels — angiosarcoma). Less common (~1% of cancers) but often aggressive.
• Leukaemia: cancer of blood-forming cells (bone marrow, blood). Abnormal white blood cells proliferate. Acute leukaemias (ALL, AML) progress rapidly; chronic (CML, CLL) progress slowly. BCR-ABL fusion oncogene in CML — target of imatinib (Gleevec).
• Lymphoma: cancer of lymphocytes in lymph nodes and spleen. Hodgkin lymphoma (Reed-Sternberg cells) vs Non-Hodgkin lymphoma (various B/T cell subtypes). Burkitt lymphoma — EBV-associated; c-Myc translocation t(8;14).
• Others: Melanoma (melanocytes), glioma (glial cells), hepatocellular carcinoma (liver), choriocarcinoma (trophoblasts).

11.10.3 Oncogenes and Tumour Suppressors

Cancer arises through a two-hit accumulation of mutations in two classes of genes. Proto-oncogenes are normal growth-promoting genes (growth factors, their receptors, signal transducers, transcription factors) that become oncogenes — dominant, gain-of-function mutations — through point mutation (RAS), amplification (MYC, HER2/ERBB2), or chromosomal translocation (BCR-ABL in CML: Philadelphia chromosome t(9;22)). Key oncogenes: Ras (GTPase, mutant stays permanently "on"), Myc (transcription factor, drives cell-cycle entry), HER2/neu (receptor tyrosine kinase, amplified in 20% of breast cancers — target of Herceptin/trastuzumab).

Tumour suppressor genes (TSGs) normally inhibit cell division or promote apoptosis; they are recessive (both alleles must be inactivated — "two-hit hypothesis," Knudson 1971). Key TSGs: RB (retinoblastoma protein) — keeps E2F sequestered in G1; phosphorylation by Cyclin D-CDK4/6 releases E2F, allowing S-phase entry; lost in retinoblastoma and many cancers. p53 (TP53 gene) — transcription factor and master regulator of the DNA-damage response; induces growth arrest (via p21), DNA repair (via GADD45), or apoptosis (via BAX, NOXA); mutated in ~50% of all human cancers; nickname "guardian of the genome." APC — degrades β-catenin (Wnt pathway); mutated in familial adenomatous polyposis/colon cancer. BRCA1, BRCA2 — DNA repair (homologous recombination); germline mutations confer high risk of breast and ovarian cancer.

11.10.4 Carcinogens

11.10.5 Apoptosis and Cancer

Apoptosis ("programmed cell death") is a controlled, energy-dependent process for eliminating unwanted or damaged cells without triggering inflammation, in contrast to necrosis (accidental, inflammatory cell death). Morphological features: cell shrinkage, chromatin condensation, membrane blebbing, formation of apoptotic bodies. Two main pathways: Intrinsic (mitochondrial) — triggered by DNA damage, oncogene activation, ER stress → p53 upregulates BAX (pro-apoptotic BCL-2 family member) → BAX oligomerises in outer mitochondrial membrane → cytochrome c released → apoptosome (APAF-1 + cytochrome c + dATP) → caspase-9 → caspase-3/7 (executioner). BCL-2 and BCL-XL are anti-apoptotic and are frequently overexpressed in cancer (BCL-2 translocation t(14;18) in follicular lymphoma). Extrinsic (death receptor) pathway: FAS-L or TRAIL bind cell-surface death receptors → caspase-8 → caspase-3. Venetoclax (BCL-2 inhibitor) is a targeted cancer drug exploiting this pathway (approved CLL).

11.11 Quick-Reference Tables

End of Chapter 11 · Cell Biology · HPRCA Biology Preparation Manual · © 2025 · All MCQ solutions verified against current NCERT, cell-biology textbooks (Alberts et al., Lodish et al.), and HPRCA previous-year patterns.