Meiosis and Gamete Formation

Why gametes need half the chromosome number

Chromosomes are structures made of DNA that carry genetic information. In most body cells, chromosomes occur in pairs: one chromosome of each pair was inherited from the mother and the other from the father. A cell with these two sets of chromosomes is diploid.

Sexual reproduction involves gametes, or sex cells. In animals, these are sperm and egg cells. During fertilisation, their nuclei fuse to form the nucleus of a fertilised egg cell called a zygote.

If both gametes contained the full body-cell chromosome number, fertilisation would double it. Instead, gametes contain just one chromosome from each pair. They are haploid, meaning they have one set of chromosomes. Meiosis is the cell division that halves the chromosome number to make this possible.

For humans, a typical body cell has 46 chromosomes arranged in 23 pairs. A sperm or egg cell has 23 chromosomes. When their nuclei fuse, the zygote has 46 chromosomes again: 23 from each parent. Meiosis and fertilisation therefore maintain the chromosome number from one generation to the next.

Interphase: copying DNA before division

Meiosis begins with a diploid cell in a reproductive organ. Before it divides, the cell passes through interphase, during which it grows and replicates its DNA.

After replication, each chromosome consists of two identical copies called chromatids, joined together. This is often drawn as an X-shaped chromosome, with each half of the X representing one chromatid. The DNA has been copied, but the joined copies still count as one chromosome.

This preparation supplies the DNA needed for the cells produced by the two divisions that follow.

The first division separates chromosome pairs

The chromosomes arrange themselves in matching pairs. During the first meiotic division, the two chromosomes of each pair separate, moving to opposite ends of the cell. The cell then divides into two cells.

Each new cell receives only one chromosome from each original pair, so the chromosome number has now been halved. However, each chromosome still consists of two joined chromatids. In humans, each of these cells has 23 chromosomes, each still made of two chromatids.

The second division separates the copied DNA

There is no further DNA replication before the second meiotic division. In each of the two cells, the joined chromatids separate. Once separated, each becomes an individual chromosome. Both cells divide again, producing four haploid cells in total.

The sequence is therefore one starting cell, then two cells, then four cells. DNA is copied once, but the cell divides twice. Each final cell has half the chromosome number of the original diploid cell.

Meiosis and sexual reproduction fit together

Meiosis produces haploid cells for sexual reproduction rather than the diploid body cells needed for growth. It also produces new combinations of chromosomes inherited from the parents, contributing to genetic variation. The products of meiosis are therefore not all genetically identical, unlike the daughter cells normally produced by mitosis.

When haploid gamete nuclei fuse at fertilisation, the diploid chromosome number is restored. The zygote can then divide by mitosis as the new organism grows.

Purpose

Meiosis halves the chromosome number to produce haploid cells for sexual reproduction.

Human chromosome numbers: body cell 46 → gamete 23; fertilisation restores 46 in the .