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.