How Meiosis Creates Genetic Variety

Before meiosis begins, the cell copies its DNA so every chromosome consists of two identical sister chromatids. Matching chromosomes then pair up. These homologous chromosomes carry the same kinds of genes, although they may contain different versions of those genes.

The cell completes two divisions. During the first division, homologous chromosome pairs separate into different cells. During the second division, the sister chromatids separate. One starting cell usually produces four cells, each containing one chromosome from every original pair.

These final cells are haploid, meaning they contain half the chromosome number of the original diploid cell. In humans, most body cells have 46 chromosomes, while sperm and egg cells have 23. When fertilisation joins two haploid cells, the diploid number is restored.

Meiosis does more than reduce chromosome number. It also creates variation. When homologous chromosomes pair, matching sections can exchange pieces in a process called crossing over. The chromosomes that separate later may therefore contain new combinations of genetic material.

The chromosome pairs also line up independently during the first division. Each pair can face either direction, so the chromosome inherited from one parent does not always travel with the same set of other chromosomes. This independent assortment creates many possible combinations in the resulting cells.

The exact cells produced are shaped by chance. That does not make meiosis disorganised. The stages are carefully controlled, but several valid arrangements are possible. Biology can be orderly while still refusing to make siblings identical. Fertilisation adds another layer of chance because any one sperm may join any one egg.

Variation gives populations a wider range of inherited traits. If conditions change, some individuals may have combinations that help them survive and reproduce. Meiosis therefore contributes raw material for evolution, although natural selection acts on the resulting differences over generations.

Keep the main sequence clear:

• •DNA is copied before meiosis begins.
• •Homologous chromosomes pair and may exchange sections.
• •The first division separates chromosome pairs.
• •The second division separates sister chromatids.
• •Four genetically varied haploid cells are produced.

Meiosis produces haploid sex cells through two divisions and creates variation through crossing over and independent assortment. Follow what happens to chromosome pairs first, then what happens to sister chromatids, and the process becomes much easier to picture.