Why cells divide
Multicellular organisms grow by producing new cells. Tissues damaged by injury or normal wear are repaired by cell replacement. Some organisms (e.g. hydra, strawberry plants) reproduce asexually by producing new individuals from existing cells. In all these cases, the new cells must have exactly the same genetic information as the parent — mitosis achieves this.
Before mitosis begins, the cell copies all of its DNA during S phase (synthesis phase) of interphase. Each chromosome is now made of two identical sister chromatids joined at the centromere. See our cells and organelles guide for how DNA is packaged inside the nucleus.
The stages of mitosis
Mitosis is conventionally divided into four stages — remembered with the mnemonic PMAT.
Interphase
(not mitosis)DNA replicates. Cell grows. Two copies of each chromosome (sister chromatids joined at centromere) are produced.
Prophase
Stage 1Chromosomes condense and become visible. Spindle fibres form from centrioles. Nuclear envelope breaks down.
Metaphase
Stage 2Chromosomes line up along the cell equator (metaphase plate). Spindle fibres attach to centromeres.
Anaphase
Stage 3Sister chromatids are pulled apart to opposite poles by spindle fibres shortening. Each pole now has a full set of chromosomes.
Telophase
Stage 4Nuclear envelopes reform around each chromosome set. Chromosomes decondense. Cytokinesis divides the cytoplasm.
Mitosis vs meiosis
Both are forms of eukaryotic cell division, but they serve very different purposes:
- Mitosis produces two daughter cells, each diploid (2n — full chromosome set), genetically identical to the parent. Used in somatic (body) cells.
- Meiosis produces four daughter cells, each haploid (n — half chromosome set), genetically unique due to crossing over and independent assortment. Used to produce gametes (sperm and egg cells).
When a sperm and egg fuse at fertilisation, the diploid chromosome number is restored. For the molecular perspective on heredity, see our natural selection guide on how heritable variation drives evolution.
Cell cycle checkpoints
The cell cycle is regulated by checkpoints that detect DNA damage or errors in chromosome attachment before allowing division to proceed:
- G1 checkpoint — checks whether the cell is large enough, DNA is undamaged, and conditions are suitable for division.
- G2 checkpoint — checks that DNA replication was complete and accurate.
- Spindle assembly checkpoint (M checkpoint) — checks that all chromosomes are correctly attached to spindle fibres before anaphase begins.
Mutations in checkpoint genes (e.g. p53, BRCA1) allow damaged cells to divide, which can lead to cancer. CRISPR-based approaches are being investigated to restore tumour suppressor function. See our CRISPR gene editing article.