Meiosis - Professor Davis PDF

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Animated Tutorial 7.2 Meiosis               

Before meiosis begins, in the preceding interphase, DNA replication takes place. The cell replicates each chromosome to form two chromatids. Meiosis consists of two cell divisions-meiosis I and II. As the diploid cell enters the first phase of meiosis I, called prophase I, its chromatic condenses into discrete chromosomes. The cell has two centrosomes, which form the mitotic spindle as the centrosomes migrate to opposite poles of the cell. The diploid cell contains homologous chromosomes, which pair up later in prophase I. The pairing is called synapsis. At this time, the nuclear envelope breaks down. Late in prophase I, the homologous chromosomes exchange genetic material, as indicated by crossing-over points called chiasmata. In this phase, microtubules attach to the chromosomes, directing them to the equatorial plate. At metaphase I, homologous chromosomes have lined up on the equatorial plate in a pair-wise fashion, with on homolog on either side of the plate. During anaphase I, chromosomes from each pair move to opposite poles of the cell. The centromeres do not divide, so each chromosome still consists of two sister chromatids, which now may not be genetically identical due to crossing over. In telophase I, chromosome decondense, nuclear membranes reform, and the cytoplasm divides in a process called cytokinesis. During the second half of meiosis, in prophase II, the chromatin again condenses into discrete chromosomes. There are now only a haploid number of chromosomes per cell. Each chromosome consists of two chromatids joined together by a centromere. At metaphase II, the chromosomes have lined up on the equatorial plate. Microtubules from opposite poles attach to each sister chromatid of a chromosome. During anaphase II, the centromeres divide, and both chromatids become independent chromosomes and move to opposite poles of the cell. During telophase II, chromosomes again decondense and nuclear membranes reform....