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Replication: Mitosis & Meiosis

المؤلف:  Kim E. Barrett, Susan M. Barman, Heddwen L. Brooks, Jason X.-J. Yuan

المصدر:  Ganongs Review of Medical Physiology

الجزء والصفحة:  25th E, P15

2026-08-02

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At the time of each somatic cell division (mitosis), the two DNA chains separate, each serving as a template for the synthesis of a new complementary chain. DNA polymerase catalyzes this reaction. One of the double helices thus formed goes to one daughter cell and one goes to the other, so the amount of DNA in each daughter cell is the same as that in the parent cell. The life cycle of the cell that begins after mitosis is highly regulated and is termed the cell cycle (Figure 1). The G1 (or Gap 1) phase represents a period of cell growth and divides the end of mitosis from the DNA synthesis (or S) phase. Following DNA synthesis, the cell enters another period of cell growth, the G2 (Gap 2) phase. The ending of this stage is marked by chromo some condensation and the beginning of mitosis (M stage).

Fig1. Sequence of events during the cell cycle. A) Immediately following mitosis (M) the cell enters a gap phase (G1). At this point many cells will undergo cell arrest. G1 is followed by a DNA synthesis phase (S) a second gap phase (G2) and back to mitosis. B) Stages of mitosis are highlighted.

In germ cells, reductive division (meiosis) takes place during maturation. The net result is that one of each pair of chromosomes ends up in each mature germ cell; consequently, each mature germ cell contains half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full complement of DNA, half of which came from the father and half from the mother. The term “ploidy” is sometimes used to refer to the number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid just before division. Aneuploidy is the condition in which a cell contains other than the haploid number of chromosomes or an exact multiple of it, and this condition is common in cancerous cells.