mitosis defined in 1951 year

mitosis - mitosis (karyokinesis);
mitosis - The usual process by which a nucleus divides into two. Each chromosome duplicates before beginning of mitosis, and mitosis involves separation of the resulting duplicates so that one goes into each daughter nucleus. As a result the two daughter nuclei have an identical complement of chromosomes and hence of genes. The process is divided into four stages: prophase, metaphase, anaphase, telophase. Prophase (2): Within the optically largely homogeneous resting nucleus chromosomes appear, at first as very long threads (the whole tangle is sometimes called a spireme) which shorten and thicken steadily by each coiling into a close spiral. Each chromosome is already double at its earliest appearance. Metaphase (3): Nuclear membrane dissolves. A spindle forms where the nucleus was, between the centrioles if these are present. The chromosomes lie (as an 'equatorial plate') at the equator of the spindle, attached to it by their spindle attachments. They pause in this position. Ana-phase (4): The duplicates of each chromosome (chromatids) separate and move rapidly towards the poles of the spindle. They do not reach the poles, but the spindle itself elongates, pushing the two groups of chromosomes further apart. Telophase (5): The chromosomes uncoil, elongating and finally disappearing. A new nuclear membrane forms. Spindle gradually disappears. The cytoplasm, if it divides, does so at this stage (See also: Cell-division). Time taken for mitosis varies a good deal but is usually between half and three hours. See also: Meiosis, Amitosis.

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