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Learn7402Paper 1 · Paper 1: Topics 1–4CellsMitosis and the cell cycle
Cells · Subtopic 6 of 6

Mitosis and the Cell Cycle

The cell cycle: interphase (G1 growth and protein synthesis; S phase DNA replication; G2 growth and checks), mitosis (nuclear division) and cytokinesis. In mitosis: prophase (chromosomes condense into two sister chromatids joined at the centromere; nuclear envelope breaks down; spindle forms), metaphase (chromosomes line up at the equator, attached to spindle fibres), anaphase (centromeres divide; chromatids pulled to opposite poles), telophase (nuclear envelopes reform). Result: two genetically identical diploid cells, for growth, repair and asexual reproduction. Mitotic index = cells in mitosis ÷ total cells. Telomeres protect chromosome ends; stem cells keep dividing; cancer results from mutations in genes controlling the cycle.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI12AQA 7402 Paper 1≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the stages of the cell cycle and mitosis.
  • Calculate mitotic index and stage durations.
  • Explain the roles of telomeres and stem cells.
  • Explain how cancer arises.
Pause and think

Why must DNA be copied before mitosis begins?

Section 2 of 14

Core idea

Start hereTwo from one

You started as one cell. Mitosis made the trillions you have now, each with the same DNA.

The big idea

DNA is copied in interphase, then mitosis shares it exactly between two nuclei, giving genetically identical cells.

The cell cycle

Interphase is the longest stage.

Worked exampleTiming a stage

Cell cycle 20 h; 15 of 300 cells are in anaphase. How long is anaphase?

Goal
Time.
Strategy
Proportion × cycle time.
  1. 15300=0.05\frac{15}{300}=0.05
    Fraction of cells in anaphase.
  2. 0.05×20=1.0 h=60 min0.05\times20=1.0\ \text{h}=60\ \text{min}
    Same fraction of time.
Answer60 minutes.
✓ CheckShort compared with interphase ✓.
Key takeawayCancer Mutations in genes that control the cycle cause uncontrolled mitosis and tumours.

Summary

  • G1, S, G2, M, cytokinesis.
  • PMAT.
  • Mitotic index.
Key words
Chromatid
One of two identical copies of a replicated chromosome.
Mitotic index
Proportion of cells undergoing mitosis.
HistoryFlemming's drawings. Walther Flemming stained dividing salamander cells in 1882 and named "mitosis" after the Greek word for thread.
Fun factHeLa cells. Cancer cells taken from Henrietta Lacks in 1951 still divide in labs today, because their telomeres are kept long by the enzyme telomerase.
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