Meiosis

• Somatic cell — any body cell that is not a gamete (skin, muscle, liver). Human somatic cells are diploid.
• Gamete — a sex cell (egg or sperm in animals). Gametes are haploid.
• Zygote — the single diploid cell formed when two gametes fuse at fertilization.
• Homologous chromosomes (homologues) — a matching pair of chromosomes, one from each parent. They are the same size, have the same centromere position and carry the same genes in the same order, but may carry different versions (alleles) of those genes.
• Sister chromatids — the two identical copies of one chromosome made when DNA replicates before division, joined at the centromere.
• Diploid (2n) — having two sets of chromosomes, one set from each parent. Humans: $2n=46$. The number varies by species and says nothing about complexity: a fruit fly (Drosophila melanogaster) has $2n=8$, the garden pea (Pisum sativum) $2n=14$ and the domestic dog $2n=78$.
• Haploid (n) — having one set of chromosomes, with one member of each homologous pair. Humans: $n=23$.

Before meiosis begins, the cell passes through interphase, where every chromosome is replicated into two sister chromatids. Meiosis then has two divisions in a row, with no DNA replication in between. Meiosis I separates the homologous pairs, so it is the division that halves the chromosome number (a reduction division). Meiosis II separates sister chromatids, much like mitosis. One diploid cell ends as four haploid cells. The spindle, a structure of protein fibres (microtubules), attaches to the centromeres and pulls chromosomes apart in both divisions.

Crossing over happens in prophase I, while homologues are in synapsis. Non-sister chromatids (one from each homologue) break at matching points and swap segments. The crossover point is called a chiasma (plural chiasmata). The result is recombinant chromatids that carry a new mix of the mother's and father's alleles, combinations that neither parent chromosome had.

Independent assortment happens at metaphase I. Each homologous pair lines up independently of every other pair, so the maternal copy of chromosome 1 might go left while the maternal copy of chromosome 2 goes right. With $n$ pairs there are $2^n$ possible combinations of chromosomes in a gamete. For humans that is $2^{23}=8388608$ combinations, before crossing over is even counted. Random fertilization multiplies the variety again: any one of those eggs can meet any one of those sperm, giving $2^{23}\times 2^{23}=2^{46}\approx 7.0\times 10^{13}$ chromosome combinations for one couple's children.

Non-disjunction is the failure of chromosomes to separate properly during meiosis. It can happen in either division, and the timing changes the outcome. If a homologous pair fails to separate in anaphase I, both homologues go to one cell, so all four gametes are abnormal: two have an extra chromosome $(n+1)$ and two are missing one $(n-1)$. If sister chromatids fail to separate in anaphase II, only one of the two cells is affected, giving two normal gametes, one $(n+1)$ and one $(n-1)$. When an abnormal gamete is fertilized by a normal one, the zygote has 47 or 45 chromosomes in humans. Having three copies of a chromosome is called trisomy, and having only one is called monosomy. You will meet the resulting disorders in the last topic of this unit.