The cell cycle and mitosis

The cell cycle

• Interphase (most of the cycle): G1, the cell grows and makes organelles and proteins; S phase, DNA replicates so each chromosome becomes two sister chromatids; G2, further growth and checks.
• Mitosis: the nucleus divides.
• Cytokinesis: the cytoplasm divides, giving two cells.

Stages of mitosis

1. Prophase: chromosomes condense and become visible as two sister chromatids joined at a centromere; the nuclear envelope breaks down; in animal cells centrioles move to opposite poles and the spindle forms.

2. Metaphase: chromosomes line up at the equator, attached to spindle fibres by their centromeres.

3. Anaphase: centromeres divide and spindle fibres pull sister chromatids to opposite poles.

4. Telophase: chromatids reach the poles, nuclear envelopes re-form and chromosomes uncoil.

Why mitosis matters

Each daughter cell gets an exact copy of every chromosome, so daughter cells are genetically identical to the parent and to each other, with the same chromosome number. This is used for growth, repair and replacement of cells, and asexual reproduction (for example in strawberry runners or yeast budding).

Worked example

A cell has 20 picograms of DNA in G1. How much does it have at the end of S phase, and in each daughter cell after division?

1. S phase doubles the DNA: 40 pg.
2. Mitosis shares it equally: 20 pg each.

Answer: 40 pg after S; 20 pg in each daughter cell.

Check yourself

1. A human body cell with 46 chromosomes divides by mitosis. How many chromosomes are in each daughter cell?
   Show answer

   46. Mitosis keeps the chromosome number: 46.

2. A cell contains 12 units of DNA in G1. How many units does it contain in G2?
   Show answer

   24. DNA doubles in S phase: 24.

3. In which stage do chromosomes line up at the equator?
   • Prophase
   • Anaphase
   • Metaphase
   • Telophase
   Show answer

   Metaphase. Metaphase: middle.

4. What happens in anaphase?
   • Chromosomes condense
   • Centromeres divide and sister chromatids move to opposite poles
   • Nuclear envelopes re-form
   • DNA replicates
   Show answer

   Centromeres divide and sister chromatids move to opposite poles. Spindle fibres pull the chromatids apart.

5. In which phase does DNA replication take place?
   • Prophase
   • Metaphase
   • Cytokinesis
   • S phase of interphase
   Show answer

   S phase of interphase. S = synthesis of DNA.

6. Which is NOT a role of mitosis?
   • Growth
   • Producing gametes
   • Repair of tissues
   • Asexual reproduction
   Show answer

   Producing gametes. Gametes are made by meiosis.

7. Which event happens in prophase?
   • Sister chromatids separate
   • Cytoplasm divides
   • Nuclear envelopes re-form
   • The nuclear envelope breaks down
   Show answer

   The nuclear envelope breaks down. Chromosomes condense and the envelope breaks down.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.