Meiotic division sequence

Meiosis produces gametes with one set of chromosomes from a cell that has two sets. The original cell has chromosomes in homologous pairs: one chromosome of each pair comes from each parent. The two chromosomes carry the same genes, although they may carry different versions of those genes.

Before meiosis begins, the cell copies its DNA. Each chromosome is now made of two copies joined together. The cell then divides twice, with no further DNA copying between the divisions.

1. First division: the homologous chromosomes in each pair separate into two cells. Each cell receives one chromosome from each pair, but each chromosome still consists of two joined copies.
2. Second division: the two copies of each chromosome separate. The two cells divide again, producing four gametes.

Each gamete has one set of chromosomes: one chromosome from each original homologous pair. The gametes are genetically different from one another. In the first division, chromosomes from different homologous pairs can be distributed in different combinations, so gametes can receive different mixtures of the chromosomes inherited from the two parents.

Chromosome number in gametes and zygotes

A gamete has a haploid nucleus: its nucleus contains one set of chromosomes. When two gamete nuclei fuse at fertilisation, their chromosome sets combine in a new cell called a zygote.

The zygote has a diploid nucleus, containing two sets of chromosomes: one set from each gamete. For example, if each gamete has one set of 23 chromosomes, the zygote has two sets, giving it 46 chromosomes.

Gametes contain only one set, so fertilisation restores the two-set chromosome number rather than doubling it in each generation. This maintains the species’ chromosome number across generations.

Mitosis

Mitosis produces two new cells from one parent cell. The two new cells, called daughter cells, have the same number of chromosomes as the parent cell and normally have the same genetic information as the parent cell and each other.

Before the cell divides, it copies its DNA. During mitosis, the chromosome copies are separated into two nuclei, so each nucleus receives the same genetic information. The cell then divides to form two daughter cells, each with one of the nuclei.

Mitosis in asexual reproduction

Asexual reproduction produces offspring from one parent. It uses mitosis, so the offspring have the same genetic information as the parent: they are genetically identical clones. There is no fusion of gametes and no mixing of genetic information from two parents.