Reproduction » Meiosis

The problem meiosis solves

Every body cell in a human has 46 chromosomes. Imagine a sperm cell and an egg cell that each had 46 chromosomes. When they joined, the new cell would have 92. The next generation would have 184. The number would keep doubling every generation, which does not happen.

Living things solve this with a special kind of cell division. It makes gametes with half the normal number of chromosomes. This is meiosis, which you met in Sexual and Asexual Reproduction. This lesson looks at exactly how it works.

Key terms:

• Reproductive organs: the parts of the body that make gametes, such as the testes and the ovaries in humans.
• Fertilisation: the joining of a male gamete and a female gamete to make a new cell.

Where does meiosis happen?

Meiosis does not happen in most of your body. Skin cells, muscle cells and liver cells all divide by mitosis. Only cells in the reproductive organs divide by meiosis, and they do it to form gametes.

What happens when a cell divides to make gametes

There are three key steps.

1. Copies of the genetic information are made. Before the cell divides, it copies its chromosomes, just as it does before mitosis.
2. The cell divides twice. This is the big difference from mitosis, which divides once. Two divisions in a row make four cells.
3. Each of the four gametes has a single set of chromosomes. The chromosomes were copied once but shared out over two divisions, so each gamete ends up with half the number of the original cell.

In humans, a cell in a reproductive organ starts with 46 chromosomes. Meiosis makes four gametes, each with 23 chromosomes.

You do not need to know the stages of meiosis. You only need to know what goes in and what comes out.

All four gametes are different

The four gametes made by one cell are genetically different from each other. Each one gets a different mixture of the genetic information. That is why two sperm cells from the same man are not the same.

This is where the variety in sexual reproduction starts: in the way the gametes are made.

Fertilisation restores the full number

When a sperm cell and an egg cell join, this is fertilisation. Each gamete brings half the chromosomes, so the new cell has the full number again.

Half of the chromosomes in the new cell come from the mother and half from the father. That is why the offspring has features from both parents.

From one cell to an embryo

The fertilised cell now divides by mitosis, not meiosis. Each division makes identical copies, so every new cell has the full number of chromosomes. The number of cells increases and forms an embryo.

As the embryo develops, the cells differentiate. They become specialised for different jobs, such as nerve cells and muscle cells. Meiosis makes the gametes. After fertilisation, mitosis makes all the new cells for growth.

Exam-style question

(a) Where in the body does meiosis happen? [1 mark]

(b) A human cell in a reproductive organ has 46 chromosomes. How many chromosomes will each gamete have, and how many gametes are made? [2 marks]

(c) Explain why fertilisation is needed to keep the chromosome number the same in each generation. [2 marks]