Meiosis (source code)
= Meiosis
{wiki}
= Meioses
{synonym}
<Meiosis> reduces two sets of <chromosomes> to one. In the ordinary <diploid> life cycle, this makes it possible for <fertilization> of two <haploid> <gametes> to restore the <diploid> state rather than double the number of sets in every generation. Its two divisions perform different tasks: <meiosis I> separates <homologous chromosomes>, whereas <meiosis II> separates <sister chromatids>. One round of <DNA replication> precedes both divisions; there is no second round between them. See <Molecular Biology of the Cell, 4th edition>[Meiosis, “Duplicated Homologous Chromosomes Pair During Meiosis” and “Gametes Are Produced by Two Meiotic Cell Divisions”, Figures 20-6–20-7]{ref}.
Two <homologous chromosomes> carry corresponding <genetic loci>, potentially with different <alleles>; two <sister chromatids> originate by <DNA replication> of one <chromosome>. After <chromosomal crossover> between nonsister <chromatids>, the <sister chromatids> need not carry identical <alleles> everywhere. Separating the two relationships explains why a <haploid> product of <meiosis I> still contains replicated <chromosomes>.
\Image[/meiosis-chromosome-segregation.png]
{title=One <DNA replication> followed by <homologous chromosome> separation and separation of <sister chromatids>}
{description=Original simplified model with two <homologous chromosome> pairs, no <chromosomal crossover>, and symmetric divisions. The four final compartments represent <haploid> products, not four functional <ova>. Human <oogenesis> uses unequal divisions, retaining most <cytoplasm> in one developing <ovum> and producing <polar bodies>.}
The two divisions explain <Mendelian segregation>; variable orientation of different <homologous chromosome> pairs connects it to <independent assortment>. <Genetic recombination> adds another source of <allele> combinations. These mechanisms reshuffle inherited material without requiring a new <mutation>.
The symmetric diagram is useful for counting, but human <oogenesis> produces one functional <ovum> rather than four. The <polar bodies> receive discarded <chromosomes> and little <cytoplasm>. See <Biology 2e by Mary Ann Clark and others/Human Reproductive Anatomy and Gametogenesis> (§43.3, Oogenesis and Figure 43.13: unequal divisions and polar bodies).
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