Topic 17.3 · Supplement
Meiosis
Meiosis exists to solve a problem that mitosis creates if it’s the only division available: if gametes kept the full diploid chromosome number, fertilisation would double the chromosome number every generation. As with mitosis, the current syllabus asks for the overall result of meiosis, not its named stages.
What meiosis produces
Meiosis is involved in the production of gametes. It is a reduction division, meaning the chromosome number is halved: a diploid parent cell gives rise to haploid cells. The cells meiosis produces are also genetically different from one another, unlike the identical daughter cells mitosis produces.
Those are the two required outcomes, and an exam answer should state both rather than only one: chromosome number is halved, from diploid to haploid, and the resulting cells are genetically different. The stages of meiosis are not required by the current Cambridge 0610 syllabus, so there’s no need to name or describe individual phases of the division — the emphasis is entirely on what goes in and what comes out.
Why the reduction has to happen
Sexual reproduction works by fusing two gamete nuclei at fertilisation. If each gamete carried the full diploid chromosome number, the resulting zygote would have double that number, and the number would keep doubling with every subsequent generation. Producing haploid gametes through meiosis avoids that: when two haploid gamete nuclei fuse, the diploid chromosome number is restored rather than increased. This is the same haploid/diploid relationship introduced on the chromosomes, genes and proteins page — meiosis is the specific process that gets a diploid cell down to haploid gametes in the first place.
Mitosis and meiosis, side by side
| Feature | Mitosis | Meiosis |
|---|---|---|
| Main result | genetically identical cells | genetically different cells |
| Chromosome number | maintained | halved |
| Ploidy change | none | diploid → haploid |
| Main purpose | growth, repair, replacement, asexual reproduction | production of gametes |
A useful one-line version to have ready for a comparison question: mitosis preserves chromosome number and genetic information to make more of the same kind of cell; meiosis halves chromosome number and produces genetically different cells so that sexual reproduction can restore the normal diploid number at fertilisation without it ever climbing generation on generation.
What this sets up next
Meiosis is also the reason offspring are not genetic copies of either parent — every gamete it produces carries a different combination of alleles. That’s the mechanism behind everything in the second half of this chapter: monohybrid inheritance, Punnett squares and pedigree diagrams are all, at heart, tools for predicting or interpreting which combination of alleles a particular gamete pairing produced.