Comparing Cell Division and Variation

Objective: Compare replication, division count, chromosome-set outcomes, and roles.

Mitosis ordinarily consists of one nuclear division after DNA replication and maintains chromosome-set number in daughter nuclei; in multicellular organisms it contributes to growth, maintenance, and some forms of asexual reproduction. Meiosis typically consists of two divisions after one DNA replication and reduces chromosome-set number in the diploid starting-cell model. Homolog pairing and crossing over are characteristic of meiosis I, not ordinary mitosis. Mitosis usually produces two nuclei or cells, whereas the simplified meiosis model produces four. Neither pathway guarantees perfect genetic identity or viability. Some organisms have life cycles in which the timing and function of meiosis differ from the familiar animal example.

Objective: Explain how failed segregation can change chromosome number without overgeneralization.

Nondisjunction is failure of homologous chromosomes or sister chromatids to separate normally. In meiosis I, failure of a homologous pair to separate can affect all four products in the simplified model; in meiosis II, failure of sisters to separate can affect two of four products while two may have the expected chromosome number. Mitotic nondisjunction can produce different chromosome numbers in cell lineages of one organism. The consequences depend on the chromosome, stage, and cellular context and cannot be inferred fully from a single diagram. An abnormal number of individual chromosomes is aneuploidy; this differs from a whole extra chromosome set, called polyploidy.

Objective: Interpret diagrams and counts while distinguishing sources of variation.

Variation among meiotic products can result from independent assortment, crossing over, and mutations. Assortment rearranges whole homolog choices; crossover rearranges linked segments; mutation changes DNA sequence. A chromosome-stage image can support an inference about alignment or segregation but usually cannot reveal exact allele combinations without molecular markers. Chromosome counts can test ploidy and detect some segregation errors, but identical counts do not prove identical DNA sequences. Predictions such as 2^n rely on independent assortment and exclude recombination; model assumptions must be stated. Genetic diversity also depends on later combinations of meiotic products and population context.