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Match List I (Phase of cell cycle) with List II (Activity): A. G₁ phase B. S phase C. G₂ phase D. M phase
Options
1
A-II, B-III, C-IV, D-I
2
A-III, B-IV, C-I, D-II
3
A-I, B-II, C-III, D-IV
4
A-IV, B-I, C-II, D-III
Correct Answer
Option 1 : A-II, B-III, C-IV, D-I
Solution
1

List I: A=G₁, B=S, C=G₂, D=M

List II: I=Actual cell division, II=Metabolically active+grows but no DNA replication, III=DNA synthesis (DNA doubles), IV=Proteins synthesized, cell growth continues

2

G₁ phase (A) → II: Metabolically active, continuous growth, NO DNA replication

S phase (B) → III: DNA synthesis, DNA amount doubles

G₂ phase (C) → IV: Proteins synthesised, cell growth continues (prepares for division)

M phase (D) → I: Actual cell division (mitosis/meiosis)

A(G₁) → II | B(S) → III | C(G₂) → IV | D(M) → I
Answer: A-II, B-III, C-IV, D-I
Theory: Cell Division
1. Cell Cycle — Overview

Cell cycle = series of events leading to cell division. Two main phases: Interphase (G₁+S+G₂) and Mitotic phase (M). Interphase = 'resting' phase (misnomer — actually very active). Duration: typical mammalian cell ~24 hours total. Interphase: ~23 hours. M phase: ~1 hour. G₁ (Gap 1): cell growth, metabolic activity, no DNA synthesis. Duration varies most (4-24+ hours). G₀: cells exit G₁ and become quiescent (neurons, muscle cells). S (Synthesis): DNA replication — DNA content doubles (2N → 4N amount, but still 2N chromosome number). ~8 hours. G₂ (Gap 2): protein synthesis, cell continues growing, preparation for mitosis. ~4-5 hours. M phase: actual division.

2. Mitosis — Stages

PMAT: Prophase, Metaphase, Anaphase, Telophase. Prophase: chromatin condenses → chromosomes visible. Nucleolus disappears. Spindle formation begins. Nuclear envelope breaks down (prometaphase). Metaphase: chromosomes align at metaphase plate (equatorial plane). Kinetochores attach to spindle fibres. Shortest stage but clearest chromosome view — used for karyotyping. Anaphase: centromeres split, sister chromatids separate, move to poles. Cell elongates. Telophase: chromosomes reach poles, decondense. Nuclear envelope reforms. Nucleolus reappears. Cytokinesis begins. Duration: prophase longest, anaphase shortest.

3. Meiosis — Overview

Meiosis reduces chromosome number by half (diploid 2N → haploid N). Two divisions: Meiosis I (reductional) and Meiosis II (equational). Meiosis I: homologous chromosomes separate. Prophase I (longest, most complex): Leptotene→Zygotene→Pachytene→Diplotene→Diakinesis. Crossing over occurs in Pachytene (chiasmata form). Meiosis II: sister chromatids separate (like mitosis). Products: 4 haploid cells. Significance: (1) Maintains chromosome number across generations (with fertilisation). (2) Crossing over creates new genetic combinations → genetic variation.

4. G₁ Checkpoint (Restriction Point)

G₁ checkpoint: determines if cell should divide, pause, or enter G₀. Regulated by CDKs (Cyclin-Dependent Kinases) + Cyclins. Key proteins: Cyclin D + CDK4/6 → phosphorylates Rb (retinoblastoma protein) → releases E2F transcription factor → G₁→S progression. Rb is a tumour suppressor — mutations in Rb gene → cancer (retinoblastoma). At G₂ checkpoint: checks if DNA replication complete and undamaged. At M checkpoint (spindle assembly checkpoint): checks if all chromosomes attached to spindle.

5. DNA Content During Cell Cycle

G₁: 2C (diploid DNA content, 2 copies of genome). S: 2C → 4C (DNA synthesis doubles amount). G₂: 4C (doubled DNA, still 2N chromosomes — each chromosome has 2 chromatids). M (metaphase): 4C, 2N. After mitosis (each daughter): 2C, 2N. After meiosis I: 2C, N (each cell). After meiosis II: 1C, N (haploid). Flow cytometry measures DNA content — G₁ cells show 2C peak, G₂/M cells show 4C peak, S phase shows broad distribution.

6. Amitosis vs Mitosis vs Meiosis

Amitosis: direct cell division without spindle formation. Simple constriction of nucleus and cytoplasm. Example: Amoeba, some unicellular organisms, RBCs of some vertebrates. Mitosis: produces 2 genetically identical daughter cells. Occurs in somatic cells. Maintains chromosome number. Meiosis: produces 4 genetically variable daughter cells. Occurs in germ cells. Halves chromosome number. Endomitosis: DNA replicates without cell division → polytene chromosomes (e.g., Drosophila salivary gland cells). Endopolyploidy: multiple rounds of S phase without M phase → giant cells.

7. Crossing Over and Recombination

Crossing over occurs during Pachytene of Prophase I. Homologous chromosomes (synapsed in bivalents) exchange segments. Physical contact points = chiasmata (singular: chiasma). Mechanism: Holliday junction model → branch migration → resolution. Crossing over creates new combinations of alleles (recombination). Distance between genes (cM = centimorgan) = % recombination frequency. 1 cM = 1% chance of crossing over between two loci in one generation. Crossing over on same chromosome → parental types more frequent than recombinants (if closely linked).

8. Programmed Cell Death (Apoptosis)

Apoptosis = programmed cell death. Normal process: essential for development (finger separation, removal of tadpole tail), immune function (clonal deletion of autoreactive lymphocytes), tissue homeostasis. Characteristics: cell shrinkage, DNA fragmentation (DNA ladder on gel), membrane blebbing, formation of apoptotic bodies. Initiator caspases → executioner caspases → cell dismantled. Two pathways: intrinsic (mitochondrial — activated by DNA damage, Cytochrome C release → apoptosomes) and extrinsic (death receptor — Fas/FasL, TNF → caspase 8). Bcl-2 family: pro-apoptotic (Bax, Bid) and anti-apoptotic (Bcl-2, Bcl-xL).

Frequently Asked Questions
1. What is the difference between G₁ and G₂ phase? ⌄
G₁ (Gap 1): first gap phase after M phase. Cell grows in size, synthesises proteins and organelles needed for DNA replication. No DNA replication. Checkpoint here decides if cell should divide. Duration: 4-24+ hours (most variable phase). G₂ (Gap 2): second gap phase after S phase. Cell continues to grow. Proteins needed for mitosis (tubulin for spindle, cyclins) are synthesised. DNA damage is repaired. Duration: 4-5 hours. Both phases: cell is growing and metabolically active. Key difference: G₁ is before DNA replication, G₂ is after.
2. Why does DNA double during S phase but chromosome number stays the same? ⌄
In S phase, each chromosome's DNA is replicated → each chromosome now consists of two identical sister chromatids joined at centromere. Number of chromosomes (structures) stays the same (e.g., 46 in humans). But DNA content doubles (2C → 4C). During anaphase of mitosis: centromeres split → sister chromatids separate → each becomes an independent chromosome → chromosome number doubles momentarily → cytokinesis restores original number. In meiosis I: homologs separate → each pole has N chromosomes (with 2 chromatids each) → 2C, N.
3. What happens in prometaphase? ⌄
Prometaphase is often included as a substage between prophase and metaphase. Key events: nuclear envelope breaks down (nuclear lamins phosphorylated and depolymerised by CDKs). Spindle microtubules invade the nuclear region. Kinetochore microtubules attach to kinetochores on chromosomes. Chromosomes captured and moved toward metaphase plate. Dynamic instability of microtubules: growing/shrinking ends search for kinetochores. Error correction: unattached or incorrectly attached kinetochores activate spindle assembly checkpoint → delays anaphase until all chromosomes properly attached.
4. What is G₀ phase? ⌄
G₀ is a quiescent state where cells exit the cell cycle from G₁ and stop dividing. Cells in G₀ are metabolically active but non-dividing. Can be reversible (cells can re-enter G₁ on stimulation) or irreversible (terminal differentiation). Examples of permanently G₀ cells: neurons (most), cardiomyocytes, skeletal muscle fibres. Examples of reversibly G₀ cells: hepatocytes (can regenerate after injury), lymphocytes (stimulated to divide by antigens). G₀ cells: low CDK activity, high Rb protein activity (unphosphorylated Rb holds E2F). Most adult cells spend most of their life in G₀.
5. How does cyclin-CDK regulate cell cycle? ⌄
Cyclin-CDK complexes are the master regulators: Cyclin D + CDK4/6: active in G₁. Phosphorylates Rb → releases E2F → S phase genes transcribed. Cyclin E + CDK2: active at G₁/S transition. Cyclin A + CDK2: active in S phase → DNA replication. Cyclin A/B + CDK1: active at G₂/M transition → initiates mitosis (MPF = Maturation Promoting Factor = Cyclin B + CDK1). Cyclin B degradation by APC (Anaphase Promoting Complex) → exit from mitosis. Cyclins: synthesized and degraded in waves during cell cycle. CDKs: levels constant but activated only when bound to cyclin.
6. What is the significance of the spindle assembly checkpoint? ⌄
The spindle assembly checkpoint (SAC) ensures all chromosomes are correctly attached to spindle fibres before anaphase begins. Mechanism: unattached kinetochores generate 'wait' signal (MCC = Mitotic Checkpoint Complex) → inhibits APC → APC cannot degrade Securin → Separase inactive → cohesin intact → sister chromatids cannot separate. When all kinetochores attached: SAC satisfied → APC activated → Securin degraded → Separase active → cleaves cohesin → sister chromatids separate (anaphase). Cancer relevance: SAC mutations → chromosomal instability (CIN) → unequal chromosome numbers (aneuploidy) → common in cancer.
7. What is the difference between mitosis in plant and animal cells? ⌄
Animal cells: have centrioles that form asters. Spindle is astral (with asters at poles). Cytokinesis by cleavage furrow (actin-myosin ring contracts). Plant cells: no centrioles. Spindle is anastral (without asters). Cytokinesis by cell plate formation (phragmoplast — Golgi vesicles fuse along equatorial plane → new cell wall formed from inside out). Similarities: both have prophase, metaphase, anaphase, telophase with similar chromosome behaviour. Key test point: cell plate = plant; cleavage furrow = animal. Both have functional spindle apparatus despite different formation.
8. What are the products of meiosis? ⌄
Meiosis produces 4 haploid cells from one diploid cell. In males (spermatogenesis): 4 functional spermatids → sperm (all equal size). In females (oogenesis): 1 large oocyte (with most cytoplasm) + 3 small polar bodies (non-functional). First polar body may or may not divide. The asymmetric division conserves cytoplasm in the egg (needed by embryo). Polar bodies eventually degenerate. In males: 1 primary spermatocyte → 4 spermatozoa. In females: 1 primary oocyte → 1 egg + 3 polar bodies.
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