Meiosis and Crossing Over
Two divisions, four cells, endless variation.
Meiosis is completed in two divisions. The syllabus does not require its individual stages — what it requires is what happens, and why it matters.
Meiosis happens only in the reproductive cells, and only to make gametes. If gametes were made by mitosis, the chromosome number would double at every fertilisation.
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What this lesson covers
What you do
- Two mechanisms mix the genes here: crossing over, and which member of each pair happens to go to which cell. Find both in the sequence.
Challenges to clear
- Advance to the exchange that creates new combinations.
- Reach the division that actually halves the chromosome number.
- Step through the whole of meiosis.
The task
Meiosis is completed in two divisions. The syllabus does not require its individual stages — what it requires is what happens, and why it matters.
What this lesson covers
- Where and why — Meiosis happens only in the reproductive cells, and only to make gametes. If gametes were made by mitosis, the chromosome number would double at every fertilisation.
- Homologous chromosomes pair up — Each chromosome comes to lie alongside its partner — one inherited from the father, one from the mother.
- Crossing over — While the maternal and paternal chromosomes are separating, the chromatid material very often gets exchanged between two non-sister chromatids of a homologous pair. The point of attachment where this happens is a chiasma.
- Meiosis I — the reduction division — The homologous pairs separate, one member of each pair going to each daughter cell. The chromosome number is halved here: diploid becomes haploid. This is the reduction division.
- Meiosis II — the mitotic division — The second division is essentially mitotic: the chromatids separate, without any further reduction in number.
- Four haploid cells — One diploid cell has produced four haploid cells. Compare mitosis, which produces two diploid cells that are identical to the parent.
- Why it matters — Crossing over and the random assortment of maternal and paternal chromosomes mean no two gametes are alike. That is how the children of the same parents, however similar, are all different from each other.
Hint
The number is halved in meiosis I, not meiosis II — this is the single most commonly confused point in the chapter. Meiosis II simply separates chromatids, exactly as mitosis does.