At the end of the G2 phase of the cell cycle, which is generally shorter than G1, each chromosome consists of two strands called chromatids that are joined together by a centromere (see fig. 3.28). The two chromatids within a chromosome contain identical DNA base sequences because each is produced by the semi-conservative replication of DNA. Each chromatid, therefore, contains a complete double-helix DNA molecule that is a copy of the single DNA molecule existing prior to replication. Each chromatid will become a separate chromosome once mitotic cell division has been completed.
The G2 phase completes interphase. The cell next proceeds through the various stages of cell division, or mitosis. This is the M phase of the cell cycle. Mitosis is subdivided into four stages: prophase, metaphase, anaphase, and telophase (fig. 3.29).
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• The chromosomes are in an extended form and seen as chromatin in the electron microscope.
• The nucleus is visible
• The chromosomes are seen to consist of two chromatids joined by a centromere.
• The centrioles move apart toward opposite poles of the cell.
• Spindle fibers are produced and extend from each centrosome.
• The nuclear membrane starts to disappear.
• The nucleolus is no longer visible.
• The chromosomes are lined up at the equator of the cell.
• The spindle fibers from each centriole are attached to the centromeres of the chromosomes.
• The nuclear membrane has disappeared.
• The centromere split, and the sister chromatids separate as each is pulled to an opposite pole.
• The chromosomes become longer, thinner, and less distinct.
• New nuclear membranes form.
• The nucleolus reappears.
• Cell division is nearly complete.
■ Figure 3.29 The stages of mitosis. The events that occur in each stage are indicated in the figure.
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