Meiosis

Objective: Trace chromosome sets through meiosis I and II.

Meiosis in many sexually reproducing eukaryotes starts after a single round of DNA replication and proceeds through two nuclear divisions. Meiosis I separates homologous chromosomes, usually reducing two chromosome sets to one; meiosis II separates sister chromatids. In the standard diploid starting-cell model, four haploid nuclei or cells result, although actual development and cytokinesis vary by organism. No complete S phase occurs between meiosis I and II. A replicated haploid chromosome after meiosis I still consists of sister chromatids; haploid describes sets of homologous chromosome types, not DNA molecule count.

Objective: Explain synapsis and recombinant chromatids with correct timing.

During prophase I, homologous chromosomes pair through a process called synapsis, bringing nonsister chromatids into close alignment. Crossing over exchanges corresponding DNA segments between nonsister chromatids of a homologous pair; visible chiasmata mark sites associated with exchange in many textbook models. Recombination can create chromatids with new combinations of alleles, though an exchange between identical sequences need not create an observable difference. Crossovers also help proper homolog segregation in many organisms. Crossing over takes place in prophase I, before homologs line up at metaphase I; it is not the same as DNA replication.

Objective: Connect metaphase I orientation to segregation outcomes.

At metaphase I, homologous pairs align at the spindle equator; the orientation of one pair can be independent of another, creating different combinations of maternal and paternal homologs in the resulting cells. At anaphase I, homologs segregate while sister chromatids generally remain joined. Meiosis II then separates chromatids. If a diploid cell has n distinguishable homologous pairs and assortment is independent, the basic model gives 2^n possible combinations of whole parental chromosomes before considering crossing over; linkage and species-specific behavior add complexity. Independent assortment is a source of variation, not a guarantee of equal survival or expression of all combinations.