International A Level Biology / Mitosis, meiosis and reproduction
Meiosis: two divisions, four haploid cells
Edexcel International A Level Biology (YBI11), chapter 8: Mitosis, meiosis and reproduction, specification point 3.10. Free lesson.
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Meiosis makes four haploid cells through two divisions. It halves the chromosome number so fertilisation restores the diploid number instead of doubling it each generation.
Why gametes must be haploid
A diploid cell has two sets of chromosomes, arranged in homologous pairs. In humans, a body cell has 46 chromosomes in 23 pairs, with one chromosome of each pair from each parent. Homologous chromosomes carry the same genes in the same order. Meiosis makes haploid gametes with one chromosome from each pair. When sperm and egg nuclei fuse at fertilisation, their chromosomes combine to restore the diploid number. Human body cell: 2n = 46. Human gamete: n = 23. A diploid gamete would make a zygote with 92 chromosomes, and the number would keep doubling in later generations.
What separates in the two divisions
Before meiosis, DNA is copied during interphase. Each chromosome then consists of two identical sister chromatids, joined at a centromere. The two divisions separate different structures: meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids. At metaphase I, replicated homologous chromosomes line up in pairs at the equator, the middle of the cell. At anaphase I, one whole chromosome from each pair moves to each pole. The cell divides, making two haploid cells. Their chromosomes still have two chromatids each. In meiosis II, centromeres divide…
How chromosome number changes
The chromosome number halves in meiosis I, when homologous pairs are separated. Meiosis II separates chromatids, so the chromosome number stays haploid. The two divisions turn one starting cell into four cells. Meiosis I: 1 cell becomes 2, and the chromosome number changes from diploid to haploid. Meiosis II: 2 cells become 4, with the haploid chromosome number unchanged. In humans, the number goes from 46 to 23 in each cell after meiosis I, then remains 23 in each of the four final cells.
How DNA content changes
DNA content per cell follows a stepped pattern. In arbitrary units (a.u.), it starts at 2, doubles when DNA is replicated, then halves at the end of each division. It stays level during meiosis II before the final drop. The sequence is 2 a.u. → 4 a.u. → 2 a.u. → 1 a.u. After meiosis I, each new cell has 2 a.u. of DNA. DNA content stays at 2 a.u. through meiosis II, then falls to 1 a.u. in each final cell. DNA is copied only once, so the content does not rise above 4 a.u.
Apply the chromosome-number rule
To find the number in the final gametes, halve the body-cell number in meiosis I. Meiosis II separates chromatids but does not halve the chromosome number again. A horse body cell has 64 chromosomes. Each cell after meiosis I has 32 chromosomes, each still made of two chromatids. Each gamete has 32 chromosomes, now each a single chromatid.