Cells

In animals, meiosis produces gametes, which are haploid reproductive cells. Human gametes are sperm cells and egg cells. Sperm and egg cells each contain 23 (1n) chromosomes. The fusion of a sperm and an egg results in a zygote that contains 46 (2n) chromosomes.

Cells preparing to divide by meiosis undergo the Gp S, and G2 phases of interphase. During interphase, the cell grows to a mature size and copies its DNA. Thus, cells begin meiosis with a duplicate set of chromosomes, just as cells beginning mitosis do.

vocabulary synapsis tetrad crossing-over genetic recombination independent assortment spermatogenesis oogenesis polar body sexual reproduction

Because cells undergoing meiosis divide twice, diploid (2n) cells that divide meiotically result in four haploid (1n) cells rather than two diploid (2n) cells. The stages of the first cell division are called meiosis I, and the stages of the second cell division are called meiosis II.

Because cells undergoing meiosis divide twice, diploid (2n) cells that divide meiotically result in four haploid (1n) cells rather than two diploid (2n) cells. The stages of the first cell division are called meiosis I, and the stages of the second cell division are called meiosis II.

While reading about each phase of meiosis I, shown in Figure 8-11 on the next page, notice how these phases compare with the corresponding phases that occur in mitosis.

Prophase I

In prophase I (step Q), DNA coils tightly into chromosomes. As in the prophase of mitosis, spindle fibers appear. Then, the nucleolus and nuclear membrane disassemble. Notice that every chromosome lines up next to its homologue. The pairing of homologous chromosomes, which does not occur in mitosis, is called synapsis. Each pair of homologous chromosomes is called a tetrad. In each tetrad, chromatids of the homologous chromosomes are aligned lengthwise so that the genes on one chromosome are adjacent to the corresponding genes on the other chromosome.

Word Roots and Origins tetrad from the Greek tetras, meaning "four"

Crossing-over

Centromere (with kinetochore)

Crossing-over

Spindle

Tetrad

(pair of homologous chromosomes)

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