Mitosis is the process whereby one cell di vides, giving rise to two daughter cells that are genetically identical to the parent cell (Fig. 1).

Fig1. Various stages of mitosis. ln prophase, chromosomes are visible as slender threads. Doubled chromatids become clearly visible as individual units during metaphase. At no time during division do members of a chromosome pair unite. Blue, paternal chromosomes; red, maternal chromosomes.

Each daughter cell receives the complete complement of46 chromosomes. Before a cell enters mitosis, each chromosome replicates its DNA. During this replication phase, chromosomes are extremely long, they are spread diffusely through the nucleus, and they cannot be recognized with the light microscope. With the onset of mitosis, the chromosomes begin to coil, contract, and condense; these events mark the beginning of prophase. Each chromosome now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere. Throughout prophase, the chromosomes continue to condense, shorten, and thicken (Fig. 1A), but only at prometaphase do the chromatids become distinguishable (Fig. 1B). During metaphase, the chromosomes line  up in the equatorial plane, and their doubled structure is clearly visible (Fig. 1C). Each is attached by microtubules extending from the centromere to the centriole, forming the mitotic spindle. Soon, the centromere of each chromosome divides, marking the beginning of anaphase, followed by migration of chromatids to opposite poles of the spindle. Finally, during telophase, chromosomes uncoil and lengthen, the nuclear envelope reforms, and the cytoplasm divides (Fig. 1D—F). Each daughter cell receives half of all doubled chromosome material and thus maintains the same number of chromosomes as the mother cell.

مواضيع ذات صلة

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مغزى حادثات التشطر وتشكل الأريمة

بنية الجنين أثناء التشطر وتشكل الأريمة

العوامل المؤثرة على التشطر – انماط التشطر

الخلية أثناء التشطر

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