For an organism to reproduce sexually, it needs to produce gametes (sex cells). Meiosis is a type of cell division in reproductive organs that produces gametes.
Sexual reproduction requires gametes (e.g. sperm cells and egg cells). These gametes only contain half the genetic material of a normal cell. For this reason, they are called haploid cells. When the two gametes combine, they form a normal cell, which grows to form an organism. As this cell has two sets of genetic information from each parent, it is called a diploid cell.
However, to make gametes, cells have to undergo meiosis. During meiosis, the number of chromosomes doubles, and then the cell divides twice to produce four gametes. Each gamete has a copy of each chromosome, and all of the gametes are genetically different from each other. This process leads to genetic variation.
An important aspect of meiosis to remember is that the cell undergoes two divisions, resulting in four gametes, each with a single set of chromosomes. These four gametes are non-identical haploids, which is the point that fertilisation starts.
1. Chromosomes create identical copies of themselves, forming X-shaped structures
2. Chromosome pairs line up along the centre of the cell
3. Sections of DNA get swapped
4. These chromosome pairs are pulled to opposite sides of the cell
5. The cell divides, forming two cells with one copy of each chromosome
6. Chromosomes line up along the centre of the cell
7. The arms of the chromosomes are pulled to opposite sides of the cell
8. Both cells formed during the first division divide again, producing four haploid daughter cells
During fertilisation, the male and female gamete join together and the nuclei fuse. This fusion results in a zygote (a fertilised egg cell) with a complete set of chromosomes. The zygote divides by a type of cell division called mitosis, which forms an organism.
All cells in the embryo are genetically identical initially. However, as the embryo develops, these cells differentiate into specialised cells.