Mitosis and the Cell Cycle

The cell cycle

Cells reproduce through the cell cycle — a controlled sequence of growth and division. It has two main phases:

• Interphase (the longest phase): the cell grows and carries out normal functions. Split into G1 (growth, making organelles/proteins), S (DNA replication — each chromosome becomes two sister chromatids), and G2 (further growth, preparing for division).
• Mitosis (M phase): the nucleus divides, followed by cytokinesis (the cytoplasm splits) to form two genetically identical daughter cells.

Checkpoints control the cycle, ensuring DNA is undamaged and correctly copied before division proceeds.

Mitosis

Mitosis produces two diploid daughter cells that are genetically identical to the parent (same number of chromosomes). It is used for growth, repair and asexual reproduction.

The four stages (PMAT)

1. Prophase: chromosomes condense and become visible (each as two sister chromatids joined at a centromere); the nuclear envelope breaks down; spindle fibres form.

2. Metaphase: chromosomes line up along the equator (middle) of the cell, attached to spindle fibres by their centromeres.

3. Anaphase: spindle fibres contract, pulling the sister chromatids apart to opposite poles of the cell (centromeres divide).

4. Telophase: chromosomes reach the poles and decondense; two new nuclear envelopes form.

Then cytokinesis divides the cytoplasm into two cells.

The mitotic index

mitotic index = number of cells in mitosis ÷ total number of cells

Used to study how quickly a tissue is dividing (a high index can indicate rapidly dividing tissue, such as a tumour).

Cancer

Cancer results when the cell cycle's control (checkpoints) fails, causing uncontrolled cell division and a tumour. Many cancer treatments target the cell cycle — e.g. disrupting DNA replication (S phase) or spindle formation (mitosis).

Worked example

In which stage do sister chromatids separate and move to opposite poles?

• Anaphase — spindle fibres contract and pull the chromatids apart to opposite poles. ✓

Common mistakes

• Getting the stage order wrong — remember PMAT (Prophase, Metaphase, Anaphase, Telophase).
• Confusing chromatids separating (anaphase) with lining up (metaphase).
• Forgetting DNA is replicated in interphase (S phase), before mitosis.

Exam tips

• Learn the four stages and one key event of each.
• Know that mitosis gives genetically identical, diploid cells (contrast with meiosis).
• Be able to calculate the mitotic index and link uncontrolled division to cancer.

Key facts to remember

• Cell cycle: interphase (G1, S = DNA replication, G2) then mitosis + cytokinesis.
• Mitosis stages PMAT: prophase (condense), metaphase (line up at equator), anaphase (chromatids to poles), telophase (nuclei reform) → two identical diploid cells.
• Mitotic index = cells in mitosis ÷ total cells; loss of cell-cycle control → cancer.