What Is the Cell Cycle?
The cell cycle is the ordered sequence of growth and division phases a cell passes through to produce two genetically identical daughter cells. Checkpoints at key transitions ensure DNA is intact and correctly copied before the cell moves forward, protecting against errors that could lead to disease.
The cell cycle is the ordered sequence of growth (G1, S, G2) and division (mitosis, cytokinesis) phases a cell goes through to produce two genetically identical daughter cells.
- 1.G1 — Cell grows and synthesizes proteins and organelles
- 2.S — DNA replication doubles the chromosome number
- 3.G2 — Cell prepares for mitosis and checks for DNA errors
- 4.M (Mitosis) — Chromosomes separate into two identical nuclei
- 5.Cytokinesis — Cytoplasm divides, forming two daughter cells
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Step-by-step worked examples
A human cell has a 24-hour cell cycle: G1 = 11h, S = 8h, G2 = 4h, M = 1h. What percentage of the cycle is spent in S phase?
Percent = (S ÷ total) × 100 = (8 ÷ 24) × 100 = 33.3%
A cancer cell divides every 18 hours instead of 24. If G1 shrinks to 5h while S, G2, and M stay at 8h, 4h and 1h, what is the new total cycle length?
Total = G1 + S + G2 + M = 5 + 8 + 4 + 1 = 18 hours This matches the faster division rate — a shortened G1 is a hallmark of uncontrolled proliferation
A tissue sample has 1,000 cells; a mitotic index study finds 40 cells in M phase. What percentage of cells are actively dividing?
Mitotic index = (cells in M ÷ total cells) × 100 = (40 ÷ 1000) × 100 = 4%
Flashcards
Quick quiz
Q1.Which phase involves DNA replication?
Q2.What is the correct order of interphase?
Q3.What happens at the G2/M checkpoint?
Q4.Uncontrolled passage through cell cycle checkpoints can lead to…
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Common mistakes
Mitosis and the cell cycle are the same thing. — Correct: Mitosis is just one phase (M) of the larger cell cycle, which also includes G1, S, and G2.
Cells spend most of their time actively dividing. — Correct: Cells spend most of the cycle in interphase (G1, S, G2), not actively dividing.
DNA replicates during mitosis. — Correct: DNA replicates during S phase, before mitosis begins.
All cells cycle at the same speed. — Correct: Cycle length varies widely — skin cells divide often, mature neurons rarely divide at all.
FAQ
What is the cell cycle?
It's the series of stages (G1, S, G2, M) a cell passes through to grow and divide into two daughter cells.
What is the cell cycle formula for phase percentage?
Percent of cycle = (phase duration ÷ total cycle length) × 100.
What are examples of cell cycle checkpoints?
The G1/S checkpoint, G2/M checkpoint, and the spindle assembly checkpoint during metaphase.
How is cell cycle length calculated?
Add the durations of the G1, S, G2, and M phases together to get the total cycle time.