Cell Division — NEET Biology
Types of Cell Division
1. Amitosis — Direct division (rare, in prokaryotes)
2. Mitosis — Somatic cell division (growth, repair)
3. Meiosis — Reproductive cell division (gametes)
Cell Cycle
G₁ → S → G₂ → M (Mitosis)
| Phase | Duration | Events |
|-------|----------|--------|
| G₁ | 10-12 hrs | Growth, protein synthesis |
| S | 6-8 hrs | DNA replication (2N→4N DNA content) |
| G₂ | 3-4 hrs | Preparation for division |
| M | 1 hr | Actual cell division |
G₀ phase = Quiescent/resting stage (neurons, RBCs)
Mitosis (Somatic Division)
Prophase
• Chromatin condenses into chromosomes
• Nucleolus disappears
• Centrioles move to poles
• Spindle formation begins
Metaphase ← Most Important
• Chromosomes align at metaphase plate (equatorial plate)
• Spindle fibers attach to centromeres (kinetochores)
• Chromosomes most visible — used in karyotyping
• Chromosome number can be counted
Anaphase
• Centromeres split
• Sister chromatids move to opposite poles
• "V", "J", "I" shapes of chromosomes
• Cell elongates
Telophase
• Chromosomes reach poles
• Nuclear envelope reforms
• Nucleolus reappears
• Chromosomes decondense
Cytokinesis
• Division of cytoplasm
• In animal cells: cleavage furrow (actin ring)
• In plant cells: cell plate forms (phragmoplast)
Result: 2 daughter cells with SAME chromosome number (diploid → 2 diploid)
Meiosis (Reduction Division)
Meiosis I (Reductional Division)
Prophase I — Longest, most complex:
1. Leptotene — Chromosomes become visible
2. Zygotene — Homologous chromosomes pair (synapsis) — form bivalents
3. Pachytene — Crossing over occurs at chiasmata
4. Diplotene — Chiasmata visible, chromosomes separate slightly
5. Diakinesis — Terminalization, nucleolus disappears
Metaphase I: Bivalents at equatorial plate
Anaphase I: Homologous chromosomes separate (NOT centromere split)
Telophase I: Nuclear envelope reforms
Result after Meiosis I: 2 cells with HALF chromosome number (haploid)
Meiosis II (Similar to Mitosis)
Prophase II → Metaphase II → Anaphase II → Telophase II
• Centromeres split
• Result: 4 haploid cells (gametes)
Significance
Mitosis
• Growth of organism
• Repair of worn out cells
• Reproduction in unicellular organisms
• Maintains chromosome number
Meiosis
• Formation of gametes (sperm, egg)
• Genetic variation through crossing over
• Maintains chromosome number across generations
• Source of variation for evolution
Key Points for NEET
1. DNA replication occurs in S phase (not during division)
2. Chromosomes most condensed at Metaphase
3. Crossing over occurs in Pachytene of Prophase I
4. Chiasmata = site of crossing over
5. In plant cells — cell plate forms (NOT cleavage furrow)
6. 2n → mitosis → 2n; 2n → meiosis → n
Disorders
• Non-disjunction in Meiosis I → Down syndrome (Trisomy 21, 47 chromosomes)
• Colchicine — inhibits spindle formation, stops at metaphase (used in polyploidy)
PYQ
Q: Draw stages of mitosis. What happens at metaphase?
Q: Distinguish between mitosis and meiosis.
Q: What is crossing over? State its significance.