Cell division: mitosis and meiosis

You started life as a single cell — a zygote — and now your body has tens of trillions of cells. Every day, billions of old cells in your skin, gut and blood are replaced by new ones. All of this happens thanks to cell division. But why doesn't a child have as many chromosomes as both parents put together?

Chromosomes

Before a cell divides, the long DNA molecules in its nucleus coil up tightly into chromosomes. Every species has a fixed number of them: human body (somatic) cells have 46 chromosomes, or 23 pairs. In each pair, one chromosome comes from the mother and one from the father; such pairs are called homologous chromosomes.

Before division the DNA is copied, so each chromosome consists of two identical chromatids joined at the centromere, giving it an X shape.

The cell cycle and mitosis

A cell spends most of its life in interphase: it grows, makes more organelles and copies its DNA. Then mitosis, the division of the nucleus, begins. Mitosis has four phases:

1. 1
   Prophase

   Chromosomes coil up, shorten and become visible under a microscope. The nuclear envelope breaks down and the spindle starts to form.

2. 2
   Metaphase

   Chromosomes line up in a row across the middle (equator) of the cell, and spindle fibres attach to their centromeres.

3. 3
   Anaphase

   The two chromatids of each chromosome separate, and the spindle fibres pull them to opposite poles of the cell.

4. 4
   Telophase

   New nuclear envelopes form at the poles and the chromosomes uncoil. Then the cytoplasm divides too (cytokinesis), giving two daughter cells.

Mitosis produces two identical diploid cells with the same set of chromosomes as the parent cell. Thanks to mitosis, organisms grow, wounds heal, worn-out cells are replaced and some organisms reproduce asexually.

Meiosis: making sex cells

Meiosis takes place in the sex organs and produces gametes. The DNA is copied once, but the cell divides twice in a row. In prophase of the first division, homologous chromosomes pair up and swap sections — this is called crossing over. The homologous chromosomes are then separated into different cells, and in the second division the chromatids separate, as in mitosis.

The result is four haploid cells, and thanks to crossing over each of them is genetically different. At fertilisation a haploid sperm and a haploid egg fuse: n + n = 2n. So the zygote gets back the full chromosome number, which stays constant from generation to generation.