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NEET 2026 / 2027
Class 11 Biology NCERT Chapter 10: Cell Cycle & Ploidy Analysis
Master the S-Phase Chromosome Fallacy, Anaphase 4n=92 Cleavage, and the 3 Checkpoints for NEET 2026/2027
⚠️ The S-Phase Fallacy (Most Common NEET Trap)
Students often assume that because DNA duplicates from 2C to 4C in the S-phase, the chromosome number doubles to 92. FALSE! The chromosome count remains strictly 2n = 46 because sister chromatids remain attached at a single centromere.
⚡ Golden NEET Rule: Number of Chromosomes = Number of Centromeres!
| Cell Cycle Phase | DNA Content | Chromosome Count | Ploidy State |
|---|---|---|---|
| G1 Phase | 2C | 46 (2n) | Diploid (2n) |
| S Phase (Synthesized) | 4C (Doubled!) | 46 (STAYS 2n!) ✅ | Diploid (2n) |
| G2 Phase | 4C | 46 (2n) | Diploid (2n) |
| Anaphase (Centromere Split) | 4C | 92 (Temporary 4n) ⚡ | Tetraploid (4n) |
| Telophase / Cytokinesis | 2C per daughter cell | 46 (2n) per cell | Diploid (2n) |
🚦 The 3 Critical Cell Cycle Checkpoints (NEET PYQ Favorite)
- 1. G1/S Restriction Gate: Controlled by Cyclin D + CDK 4/6. Phosphorylates Rb protein. Once crossed, the cell is committed to divide.
- 2. G2/M Integrity Gate: Regulated by Cyclin B + CDK 1 (MPF). Sensed by p53 tumor suppressor. Halts division if DNA is damaged.
- 3. M-Phase SAC Gate (Spindle Assembly): Senses kinetochore tension before anaphase. Prevents nondisjunction and aneuploidy.
⏱️ Video Timestamps & Key Moments:
00:00 - S-Phase Chromosome Fallacy (2n=46 vs 4C DNA)
00:18 - Master Ploidy Matrix & Anaphase Cleavage
00:40 - The 3 Critical Cell Cycle Checkpoints (G1/S, G2/M, SAC)
00:51 - NEET PCB Handwritten Notes Vault & Qbnexa Store
00:18 - Master Ploidy Matrix & Anaphase Cleavage
00:40 - The 3 Critical Cell Cycle Checkpoints (G1/S, G2/M, SAC)
00:51 - NEET PCB Handwritten Notes Vault & Qbnexa Store
📚 NEET PCB Handwritten Notes Vault (83 Chapters)
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