Question
Describe the different phases of the cell cycle. Explain what happens in G1, S, G2, and M phases. Approximately how long does each phase last in a typical mammalian cell?
(NCERT Class 11, commonly asked in NEET)
Solution — Step by Step
The cell cycle is the ordered series of events that a cell goes through from one division to the next. In a typical mammalian cell, one complete cycle takes about 24 hours.
The cell cycle has two main stages:
- Interphase (~95% of the cycle): G1 + S + G2 — the cell grows and prepares for division
- M phase (~5% of the cycle): Mitosis + Cytokinesis — the cell actually divides
Duration: ~11 hours
The cell grows in size, synthesises proteins and RNA, produces organelles, and carries out its normal metabolic functions. The cell has the diploid number of chromosomes, each with a single chromatid (unreplicated DNA).
At the end of G1, there is a critical checkpoint called the restriction point (R point). If the cell receives the right signals (growth factors), it commits to division and enters S phase. If signals are lacking, it exits the cycle and enters a resting state called G0 phase.
Cells in G0 are metabolically active but not dividing. Neurons, mature RBCs, and muscle cells are permanently in G0. Some cells like hepatocytes are in G0 but can re-enter the cycle if stimulated (e.g., after liver damage).
Duration: ~8 hours
The critical event: DNA replication occurs. Each chromosome replicates to form two identical sister chromatids joined at the centromere. The DNA content doubles from 2C to 4C (C = haploid DNA content).
Centrosome duplication also begins during S phase (important for spindle formation in M phase). Histone proteins are synthesised in large quantities to package the new DNA.
Duration: ~4 hours
The cell continues to grow and synthesise proteins needed for mitosis (like tubulin for the spindle). Centrosome duplication is completed. The cell checks for DNA replication errors at the G2/M checkpoint — damaged DNA must be repaired before entering mitosis.
At the end of G2, the cell has 4C DNA content and is ready for division.
Duration: ~1 hour
Mitosis divides the nucleus (karyokinesis) through four stages: prophase, metaphase, anaphase, and telophase. Cytokinesis divides the cytoplasm, producing two identical daughter cells, each with 2C DNA content and the diploid chromosome number.
After M phase, each daughter cell enters G1 of the next cycle (or exits to G0).
Why This Works
The cell cycle is a precisely regulated process with multiple checkpoints that ensure accurate DNA replication and equal chromosome distribution. The checkpoints act as quality control — if DNA is damaged or replication is incomplete, the cycle pauses until the problem is fixed.
Cancer occurs when cell cycle regulation fails — mutations in checkpoint genes (like p53 or Rb) allow cells to divide uncontrollably, bypassing normal growth signals.
NEET frequently asks about the duration of phases. Remember: G1 is the longest and most variable phase, S phase takes about 8 hours, G2 is relatively short, and M phase is the shortest (~1 hour). The total cycle time varies hugely between cell types — yeast divides in 90 minutes, while some human cells take 24+ hours.
Common Mistake
Students often write that “the cell divides during interphase.” Interphase is the period BETWEEN divisions — the cell grows and replicates DNA but does not divide. Division happens only during M phase. The common misconception arises because interphase occupies 95% of the cycle, so cells are almost always “caught” in interphase when observed.
Another error: confusing DNA content (C value) with chromosome number (n value). After S phase, DNA content doubles (2C → 4C), but the chromosome number remains the same (2n). Each chromosome now has two sister chromatids instead of one — but it is still counted as one chromosome until the centromere splits in anaphase.