Essential Key Points: Cell Cycle and Interphase

1. Definition and Structure The cell cycle is the ordered sequence of events by which a cell grows and divides. It is divided into Interphase (G1 + S + G2, ~95% of the cycle) and M phase (mitosis + cytokinesis, ~5% of the cycle).

2. G1 Phase — Growth

• Active RNA and protein synthesis
• DNA content: 2C; Chromosome number: 2n
• Variable duration; the "decision point" for cell division
• Cells may exit to G0 from G1

3. S Phase — DNA Replication

• Duration: 6–8 hours in mammalian cells
• DNA content doubles: 2C → 4C
• Chromosome number stays 2n — sister chromatids are joined at centromere
• NEET Critical: 4C DNA after S phase does NOT mean 4n chromosomes

4. G2 Phase — Final Preparation

• Duration: 3–4 hours
• Tubulin synthesised for spindle formation
• DNA = 4C, Chromosomes = 2n (unchanged)
• Spindle proteins, condensins, and cohesins are stockpiled

5. G0 Phase — Quiescence

• Cells that have exited the cell cycle from G1
• Permanently in G0: neurons, mature red blood cells (no nucleus), cardiac muscle
• Re-entry capable G0 cells: hepatocytes (liver), lymphocytes (after stimulation)
• G0 ≠ resting in terms of metabolism — cells may be metabolically active

6. M Phase — Division

• Karyokinesis (nuclear division by mitosis or meiosis) followed by cytokinesis
• Duration of mitosis: ~1 hour
• Checkpoint: Spindle assembly checkpoint (SAC) ensures proper chromosome attachment before anaphase

7. Why Interphase Is NOT a Resting Phase

• Massive DNA replication (3 billion base pairs in humans)
• Thousands of proteins synthesised
• Organelles duplicated (mitochondria, centrioles)
• DNA damage assessed and repaired at checkpoints

8. Cell Cycle Checkpoints (Overview)

• G1 checkpoint: monitors cell size, DNA integrity, nutrient availability → decision to divide
• G2 checkpoint: confirms DNA replication is complete → entry into M phase
• Spindle Assembly Checkpoint: confirms kinetochore attachment → allows anaphase

9. High-Yield NEET Table: DNA Content by Phase