Cambridge O Level Biology · Syllabus 5090 · Development of Organisms and Continuity of Life
Meiosis
What is Meiosis?
Reduction division in which the chromosome number is halved from diploid to haploid, resulting in cells that are genetically different from one another. Meiosis is involved in the production of gametes, and the diploid number is restored later at fertilisation.
This definition is part of the Development of Organisms and Continuity of Life chapter in Cambridge O Level Biology.
Meiosis in context
Life is continuous because cells divide, and the two kinds of nuclear division do opposite things to chromosome number. Mitosis maintains the chromosome number and produces genetically identical cells, which is how an organism grows, repairs damaged tissue, replaces dying cells and reproduces asexually. Meiosis is a reduction division: it halves the chromosome number from diploid to haploid and produces genetically different cells, which is how gametes are made. Fertilisation — the fusion of a male and a female nucleus — puts the two haploid sets back together and restores the diploid number. In humans that cycle reads 46 → 23 → 46 in every generation, which is exactly why chromosome number does not double each time a child is born.
Common mistakes with Meiosis
- “Mitosis halves the chromosome number.” Why wrong Mitosis maintains it — that is half the definition. Halving is what meiosis does. Say instead Mitosis: 46 → 46. Meiosis: 46 → 23.
- “Meiosis produces genetically identical cells.” Why wrong It contradicts the definition and removes the whole point of meiosis, which is variation. Say instead Meiosis produces genetically different cells; mitosis produces genetically identical ones.
- “The embryo grows by meiosis.” Why wrong If it did, the chromosome number would halve at every division and the embryo would soon have none. Say instead All growth after fertilisation is by mitosis. Meiosis happened earlier, to make the gametes.
- “Self-pollination is asexual reproduction.” Why wrong Gametes are still made by meiosis and two haploid nuclei still fuse. That is sexual reproduction, regardless of where the pollen came from. Say instead Self-pollination is sexual reproduction with less genetic variation than cross-pollination.
Examiner tips on Meiosis
- The single most valuable habit in this topic. Before writing any answer that mentions pollination, fertilisation, meiosis or mitosis, say the definition silently to yourself first. Most lost marks in Topic 16 are not gaps in knowledge — they are the right idea attached to the wrong word.
- What meiosis does not do. Meiosis is not fertilisation. It produces gametes; it does not join them. Writing “meiosis is when the sperm and egg join” loses every mark in the question, because it names the wrong process entirely. Meiosis halves; fertilisation restores.
Questions students ask about Meiosis
What is the difference between mitosis and meiosis?
Mitosis is nuclear division giving rise to genetically identical cells in which the chromosome number is maintained; it produces two cells and is used for growth, repair of damaged tissues, replacement of dying cells and asexual reproduction. Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells; it produces four cells and is used to make gametes. In humans, mitosis runs 46 → 46 while meiosis runs 46 → 23.
Why is the chromosome number not doubled in every generation?
Because meiosis halves it before fertilisation restores it. Gametes are made by meiosis, so each carries one set of chromosomes — 23 in a human. When two gametes fuse at fertilisation, the two sets combine to give the diploid number again, 46. The halving and the doubling are equal and opposite, and they alternate in every generation.