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 is the division of a cell’s nucleus to produce two nuclei with the same genetic information. It is usually followed by division of the rest of the cell, producing two genetically identical daughter cells.

Before mitosis, each chromosome is replicated, producing two identical copies joined together. During mitosis, the copies separate and move to opposite ends of the cell. A nucleus forms around each set, so each daughter cell receives the same genetic information and the same number of chromosomes as the original cell.

What mitosis enables

Mitosis increases cell number for growth. It also produces new cells to repair damaged tissues and replace cells that are lost or worn out.

In asexual reproduction, a single parent produces offspring without the joining of sex cells. Mitosis produces genetically identical cells, so the offspring have the same genetic information as the parent.

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.

Embryonic cell differentiation

As an embryo develops, its cells become specialised for different functions. This process is called differentiation. It produces different cell types with structures suited to the jobs they perform.

For example, a nerve cell develops long extensions that help it carry signals, while a muscle cell develops structures that allow it to contract. Their different structures help them perform different roles.

Differentiation is distinct from mitotic division. Mitosis increases the number of cells in the growing embryo; differentiation changes cells as they become specialised. As the embryo develops, both processes occur: the cell population grows and different cell types are produced.