Sciences

Meiosis: Halving the Chromosomes to Make Gametes

Meiosis divides one cell twice to give four genetically different gametes with 23 chromosomes each. Compared with mitosis on every point that matters.

Meiosis

Meiosis is the cell division that makes gametes, dividing one cell twice to produce four genetically different cells with half the chromosome number.

Où les élèves le rencontrent: Grade 10 science, in the reproduction topic, taught immediately after mitosis and immediately before inheritance.

La réponse en bref
Meiosis is the cell division that produces gametes. One cell divides twice to give four cells, each with half the original number of chromosomes — 23 in humans rather than 46 — and each genetically different from the others. Fertilisation then restores the full number in the new individual.

Un exemple

1 cell (46) → 4 gametes (23 each); 23 + 23 = 46 at fertilisation

A cell in the testes with 46 chromosomes goes through two divisions and produces four sperm cells, each carrying 23. An egg cell carries 23 as well, so fertilisation gives a zygote with 46 — the number a human body cell should have. Halving is not an oddity of the process; it is the reason the process exists. If gametes carried 46, the next generation would have 92 and the one after 184.

Why the number has to be halved

Sexual reproduction combines two cells into one. If each contributed a full set of 46 chromosomes, every generation would double the total, which is plainly not what happens. Meiosis is the correction built into the system: gametes are made with one chromosome from each pair rather than both.

The vocabulary for this is worth getting right. A cell with the full set of pairs is diploid; a cell with one of each pair is haploid. Body cells are diploid, gametes are haploid, and a zygote is diploid again from the moment of fertilisation.

Two divisions, four cells, no two alike

Meiosis divides twice. The DNA is copied once beforehand, as in mitosis; then the first division separates the pairs of chromosomes, sending one of each pair to each new cell, and the second division separates the copies. One cell in, four cells out.

The genetic variation comes from two places. Which chromosome of each pair goes to which cell is decided independently for every pair, so the combinations multiply rapidly. Sections are also exchanged between the two chromosomes of a pair before they separate, a process called crossing over that appears on some syllabuses and not others.

Fertilisation adds a third layer of chance on top: any one of a great many sperm may reach the egg. Between them, these are the reason siblings resemble each other without being copies, and the reason sexually reproducing populations carry the variation that natural selection acts on.

Meiosis and mitosis, point by point

Exam questions on this pair are almost always comparisons, and they turn on six differences. Learning them as a set is faster than learning either process twice.

One point catches people out. Meiosis is not a different way of copying DNA. The chromosomes are replicated once beforehand, exactly as in mitosis, and the halving comes entirely from dividing twice afterwards without copying again in between.

Divisions — mitosis divides once; meiosis divides twice
Cells produced — two from mitosis; four from meiosis
Chromosome number — kept the same by mitosis; halved by meiosis
Genetic make-up — daughter cells identical after mitosis; all four different after meiosis
Where — mitosis throughout the body; meiosis only in the reproductive organs
Purpose — mitosis for growth, repair and asexual reproduction; meiosis for producing gametes

Genetically different gametes produced from one cell by meiosis, each with 23 chromosomes in humans: 4Genetically different gametes produced from one cell by meiosis, each with 23 chromosomes in humans

Questions fréquentes

What is the difference between meiosis and mitosis?

Meiosis divides twice and produces four genetically different cells with half the chromosome number, and it happens only where gametes are made. Mitosis divides once and produces two identical cells with the full chromosome number, and it happens throughout the body. Different cell counts, different chromosome numbers, different purposes.

How many chromosomes are in a human gamete?

Twenty-three, one from each pair. A sperm carries 22 ordinary chromosomes plus either an X or a Y; an egg carries 22 plus an X. At fertilisation the two sets combine to give 46, and whether the sperm delivered an X or a Y decides the sex of the child.

Where does meiosis take place?

Only in the reproductive organs: the testes and the ovaries in humans. In flowering plants it occurs in the anthers, producing pollen, and in the ovules, producing egg cells. No other tissue divides this way, which is why every cell in your body except the gametes carries the full 46 chromosomes.

Why are the four cells genetically different?

Because the chromosome pairs are separated independently of one another, so which of your two copies of each pair a gamete receives is decided at random for every pair. Sections of DNA are also swapped between the chromosomes of a pair before they part. Together these produce an enormous number of possible combinations.

Is meiosis the same thing as fertilisation?

No, they are opposite steps of the same cycle. Meiosis halves the chromosome number to make gametes; fertilisation fuses two gametes and restores it. Meiosis happens inside the reproductive organs of each parent before the gametes ever meet, and fertilisation happens afterwards when a sperm and an egg join.

Dernière mise à jour 21 août 2026