And Cytokinesis I

figure 8-11

Meiosis occurs in diploid reproductive cells. Before meiosis begins, the DNA of the diploid reproductive cells is copied. Meiosis I results in two haploid cells. Meiosis II results in four haploid offspring cells.

figure 8-12

Crossing-over occurs when chromosomes that make up a tetrad exchange portions of their chromatids during synapsis. Crossing-over results in an exchange of genes and in new combinations of genes.

figure 8-12

Crossing-over occurs when chromosomes that make up a tetrad exchange portions of their chromatids during synapsis. Crossing-over results in an exchange of genes and in new combinations of genes.

During synapsis, the chromatids within a homologous pair twist around one another, as shown in Figure 8-12. Portions of chromatids may break off and attach to adjacent chromatids on the homologous chromosome—a process called crossing-over. This process permits the exchange of genetic material between maternal and paternal chromosomes. Thus, genetic recombination results, because a new mixture of genetic material is created.

Metaphase I

During metaphase I (step ©), the tetrads line up randomly along the midline of the dividing cell, as shown in Figure 8-11. The orientation of the pair of chromosomes is random with respect to the poles of the cell. Spindle fibers from one pole attach to the centromere of one homologous chromosome. Spindle fibers from the opposite pole attach to the other homologous chromosome of the pair.

Anaphase I

During anaphase I (step ©), each homologous chromosome (consisting of two chromatids attached by a centromere) moves to an opposite pole of the dividing cell. The random separation of the homologous chromosomes is called independent assortment. Independent assortment results in genetic variation.

Telophase I and Cytokinesis I

During telophase I (step ©), the chromosomes reach the opposite ends of the cell, and cytokinesis begins. Notice that the new cells contain a haploid number of chromosomes.

During meiosis I, the original cell produces two new cells, each containing one chromosome from each homologous pair. The new cells contain half the number of chromosomes of the original cell. However, each new cell contains two copies (as chromatids), because the original cell copied its DNA before meiosis I.

0 PROPHASE II 0 METAPHASE II

Chromatids separate

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