Meiosis makes gametes

1. Meiosis is a type of cell division that takes place only in the reproductive organs, the ovaries and testes in humans.
2. It produces gametes, the sex cells that carry genetic information from a parent to its offspring.
3. In animals the gametes are sperm cells and egg cells; in flowering plants they are pollen and egg cells.
4. Before meiosis begins, the cell copies all of its DNA, so every chromosome is doubled.
5. The cell then divides twice in quick succession, turning one parent cell into four gametes.

Haploid gametes and diploid cells

1. A normal body cell is diploid: it has two sets of chromosomes, one set inherited from each parent.
2. Meiosis halves this number, so each gamete is haploid and carries only a single set of chromosomes.
3. This is why meiosis is described as a reduction division.
4. In humans, a diploid body cell has $46$ chromosomes, so each gamete it makes has $23$.
5. Halving the number in the gametes keeps the chromosome number of the species constant from one generation to the next.

Fertilisation restores the number

1. At fertilisation, a haploid male gamete fuses with a haploid female gamete.
2. Their nuclei join, so the two half-sets of chromosomes combine into one full set.
3. This produces a single diploid cell called a zygote, with the full chromosome number restored.
4. The zygote then divides by mitosis to produce many genetically identical cells.
5. As this ball of cells grows into an embryo, the cells differentiate into the specialised cell types the organism needs.

Meiosis is not mitosis

1. Mitosis produces two daughter cells that are genetically identical to the parent cell and to each other.
2. Meiosis produces four daughter cells that are all genetically different from each other and from the parent.
3. Mitosis keeps the chromosome number the same, while meiosis halves it, from diploid to haploid.
4. Because the four gametes are genetically different, meiosis is a source of variation in the offspring.