Mitosis

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).

Cell Structure and Genetic Control 73

(a) Interphase

• The chromosomes are in an extended form and seen as chromatin in the electron microscope.

• The nucleus is visible

Chromatin

(b) Prophase

• 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.

(c) Metaphase

• 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.

(d) Anaphase

• The centromere split, and the sister chromatids separate as each is pulled to an opposite pole.

(e) Telophase

• The chromosomes become longer, thinner, and less distinct.

• New nuclear membranes form.

• The nucleolus reappears.

• Cell division is nearly complete.

Chromatin

Nucleolus

Nucleolus

Centrosomes

Chromatid pairs

Equator

Spindle fibers

Centriole m

Nucleolus

■ Figure 3.29 The stages of mitosis. The events that occur in each stage are indicated in the figure.

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