Meiotic Cell Division

Premeiotic germ cell (2n)

DNA replication

Sister chromatids

Mitotic apparatus

Daughter cells (2n)

Paternal homolog

Maternal homolog

Premeiotic germ cell (2n)

DNA replication

Sister chromatids

Sister chromatids

Mitotic apparatus

Synapsis of homologs

Homologous chromosomes align

Mitotic apparatus

Daughter cells (2n)

▲ FIGURE 9-3 Comparison of mitosis and meiosis. Both somatic cells and premeiotic germ cells have two copies of each chromosome (2n), one maternal and one paternal. In mitosis, the replicated chromosomes, each composed of two sister chromatids, align at the cell center in such a way that both daughter cells receive a maternal and paternal homolog of each morphologic type of chromosome. During the first meiotic division, however, each replicated chromosome pairs with its homologous partner at the cell center; this pairing off is referred to as synapsis. One replicated chromosome of each morphologic type then goes into one daughter cell, and the other goes into the other cell in a random fashion. The resulting cells undergo a second division without intervening DNA replication, with the sister chromatids of each morphologic type being apportioned to the daughter cells. Each diploid cell that undergoes meiosis produces four haploid (1 n) cells.

Synapsis of homologs

Homologous chromosomes align

Mitotic apparatus

Gametes (1n)

Cell division II

Cell division II

Gametes (1n)

(a) Segregation of dominant mutation

Gametes

First filial generation, F n : all offspring have mutant phenotype

Mutant

Wild-type

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