Meiosis

Living things that reproduce sexually need special sex cells called gametes. In humans these are the sperm (made in the testes) and the egg cells (made in the ovaries).

These gametes are not made by ordinary cell division. They are made by a special type of cell division called meiosis.

Two key facts to learn straight away:

• Meiosis makes gametes — the sex cells used in sexual reproduction.
• Meiosis happens in the reproductive organs — in humans, the ovaries and the testes.

When a sperm and an egg join at fertilisation, the new cell (the zygote) must end up with the correct, full number of chromosomes. In humans that full number is 46 chromosomes (23 pairs). To make this work, each gamete must carry only half the chromosomes — that is 23 chromosomes each. Meiosis is the division that halves the chromosome number, so that fertilisation restores the full number again.

Exam tip. A common one-mark question is "where in the body does meiosis take place?" — the answer is the reproductive organs (ovaries and testes). Another is "what type of cell does meiosis produce?" — the answer is gametes / sex cells.

To understand meiosis you must know two words: diploid and haploid.

• A diploid cell has the full number of chromosomes, arranged in pairs. Human body cells are diploid with 46 chromosomes (23 pairs).
• A haploid cell has half the number of chromosomes — only one chromosome from each pair. Human gametes are haploid with 23 chromosomes.

Meiosis starts with one diploid cell in a reproductive organ and ends with four haploid cells (the gametes). The chromosome number is halved during the division.

Why does halving matter? Look at what happens at fertilisation:

Stage	Cell	Chromosome number
Gamete (sperm)	haploid	23
Gamete (egg)	haploid	23
Fertilised egg (zygote)	diploid	23 + 23 = 46

If gametes were not halved, every generation would double the chromosome number, which would not work. By making haploid gametes, meiosis keeps the chromosome number constant from one generation to the next.

Exam tip. Remember the simple word match: haploid = half. Gametes are haploid; body cells and the zygote are diploid. If a question gives you the body-cell number (e.g. 46), the gamete number is half of it (23).

Meiosis does not just halve the chromosome number — it also makes the gametes genetically different from one another. This is one of the most important ideas in the topic.

One cell divides by meiosis to produce four cells, and every one of those four cells is genetically different — they each carry a different mixture of the parent's genetic information.

Here is meiosis shown as a simple diagram — one cell producing four genetically different cells:

This genetic variation is the whole point of sexual reproduction. Because every gamete is different, and because fertilisation randomly joins one sperm with one egg, the offspring are not identical to their parents or to each other. Variation helps a species survive when the environment changes.

Exam tip. When you describe the products of meiosis, use the full phrase the specification uses: "four genetically different haploid gametes." Each word earns marks — four, genetically different, haploid, gametes.

Students often mix up meiosis and mitosis because the names look almost the same. The two divisions do very different jobs, so it is worth learning the contrast carefully.

Feature	Meiosis	Mitosis
Number of cells made	four	two
Chromosome number	halved (haploid)	kept the same (diploid)
Are the cells identical?	No — genetically different	Yes — genetically identical
What it makes	gametes (sex cells)	body cells
Where it happens	reproductive organs (ovaries, testes)	all over the body
Used for	sexual reproduction	growth and repair (and asexual reproduction)

A simple way to remember the difference:

MEIOSIS makes the Mating cells → four, half the chromosomes, all different.

MITOSIS makes the MIne (more of the same) cells → two, same chromosomes, all identical.

So if a question asks about gametes, fertilisation, variation or sex cells, the answer involves meiosis. If it asks about growth, repair or identical copies, the answer involves mitosis.

Exam tip. "Compare" and "contrast" questions reward clear pairs of differences — for example: "Meiosis makes four cells but mitosis makes two." Marks are usually given for the differences in the number of cells, the chromosome number and whether the cells are identical or different.

Because meiosis produces genetically different gametes, the offspring made by sexual reproduction show variation — they are all slightly different from one another and from their parents.

This variation matters for two big reasons:

1. Survival of the species. If the environment changes (for example, a new disease appears), some individuals may have features that help them survive. Variation means there is a better chance that some of the offspring will cope. A population of identical individuals could all be wiped out by the same problem.
2. Inheritance. Because each gamete carries a different mixture of the parent's chromosomes, children inherit a unique blend of characteristics from both parents — which is why brothers and sisters resemble their parents but are not identical to each other.

Two sources of variation come together in sexual reproduction:

• Meiosis mixes up the chromosomes so that every gamete is different.
• Random fertilisation means any sperm can fuse with any egg, multiplying the number of possible combinations.

Meiosis → genetically different haploid gametes → variation in offspring → better chance the species survives.

Exam tip. If asked for an advantage of producing genetically different gametes, the safe marking points are variation and that variation gives a better chance of survival if the environment/conditions change.