Reproduction and Chromosome Transmission

5 the following events occur during prophase i figure

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Unformatted text preview: ons. 5 The following events occur during prophase I (Figure 3.10). The chromosomes start to condense, forming threadlike structures. The homologous chromosomes recognize each other by a process known as synapsis. The chromosomes align along their entire lengths. At this point the pairs of homologous chromosomes are called bivalents. There are four sister chromatids in a bivalent. As the bivalent structure forms, the synaptonemal complex forms between the homologous chromosomes. The synaptonemal complex consists of lateral and central elements (Figure 3.11) that interact. The synaptonemal complex is not found in all species. The process of crossing over occurs between non-sister chromatids in the bivalent. The site of crossing over is called the chiasma. The synaptonemal complex dissociates. The individual chromatids are usually visible at this point, and the structure is called a tetrad. The events of prometaphase I (Figure 3.12) resemble those of mitosis. During metaphase I (Figure 3.12) the chromosomes align along a central line in the cell in the same manner as mitosis, except for the following: The bivalent chromosomes are aligned in a double row, rather than the single row of mitosi...
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