Mitosis

Ceils divide to form daughter cells. Before they can divide however, the nucleus must divide so that each daughter cell will have the correct instructions (genes) to carry out its function. Division of the nucleus in somatic (body) cells is called mitosis. Division of the cell itself is called cytokinesis. Mitosis must occur before cytokinesis can occur, but cytokinesis does not have to occur after mitosis. For instance, some cells undergo mitosis but not cytokinesis and thus end up with more than one nucleus.

Although mitosis is a continuous process, it has historically been divided into four stages. These stages can be recognized by certain features. The acronym "PMAT" may help you remember the order in which the phases occur.

Exercise 4.1

As you read, label each diagram in Figure 4.1.

Prophase. In this stage the nuclear membrane breaks down and the chromosomes become visible. Chromosomes contain the genetic material, DNA, which contains the instructions for all of the body's anatomy and physiology. Each chromosome is composed of two chromatids which are held together by a centromere. Do not confuse the centromere with the centrioles which are larger organelles of the cell.

During prophase the two centrioles move to opposite ends of the cell and a spindle of visible fibers called microtubules forms from one pole of the cell to the other.

Metaphase. This phase is characterized by the movement of the chromosomes to line up in the middle of the spindle. The chromosomes attach to the spindle by means of the centromeres.

Figure 4.1 Mitosis and Interphase

Anaphase Centromere Centriole Chromatid Chromosome Interphase Metaphase Prophase Spindle Telophase

Anaphase. The metaphase chromosomes seem to pull apart by the action of some of the spindle fibers. The chromosome becomes two individual chromatids which, unfortunately, are now called chromosomes. The two anaphase chromosomes (formerly chromatids) move toward the poles of the cell.

Telophase. This phase is like prophase in reverse. The separated chromosomes begin to disappear and the nuclear membrane reappears. If cytokinesis is going to take place it usually begins during telophase. Remember that mitosis and cytokinesis are separate events, though they may occur at the same time.

It is often difficult to determine if a cell is in prophase or telophase. A general "rule of thumb" is that if two adjoining cells are in this state, they are probably daughter cells and the nuclei are daughter nuclei in telophase. If a single set of unaligned chromosomes is present then it is probably prophase.

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Essentials of Human Physiology

Essentials of Human Physiology

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