Human chromosomes / E.H.R. Ford.

  • Ford, E. H. R. (Edward Hugh Rawlinson)
Date:
1973
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Human chromosomes / E.H.R. Ford. Source: Wellcome Collection.

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    2. MITOSIS AND THE HUMAN MITOTIC CHROMOSOMES 13 two halves of each chromosome, the chromatids, are completely separate from each other. The two sets of chromatids then begin to move away from each other fairly rapidly, under the influence of the spindle fibres, to opposite poles of the cell. The actual mechanism of this movement is still poorly understood. When the sets of chromatids have reached the opposite poles of the cell, anaphase is completed, and the next stage, telophase, begins. E. Telophase At telophase, the last stage of cell division, the cytoplasm undergoes cleavage to form two completely separate daughter cells. The chromosomes undergo in reverse the process through which they passed at prophase; they gradually elongate and become less distinct, while the nuclear membrane is reformed around them and the nucleoli reappear. The stage ends, and cell division is complete, when the cell has fully reassumed its interphase appearance. Since the metaphase chromosomes exist in pairs, and each chromosome of a pair has two equal chromatids, the metaphase cell contains four complete and comparable sets of genetic material. These are not all identical however, since one chromosome of each pair has come from the individual's father and the other from the mother. After cell division the nucleus contains only two sets, one from each parent. During the S period of interphase each of the chromatids in the nucleus manufactures another one identical to itself, so that when the chromosomes appear at the prophase of the next cell division they can be seen to be double once again. III. THE METAPHASE CHROMOSOMES A. General Appearance and Classification At metaphase in the human somatic cell it can be seen that there are normally 46 chromosomes (Tjio and Levan, 1956). Although there were some early reports of a different number in certain subjects and races, it is now generally agreed that these were mistaken. The 46 chromosomes form 23 pairs in female cells, and 22 pairs and two dissimilar chromosomes, the X and Y, in male cells. This number, 46, is referred to as the diploid chromosome number. The gametes formed after the reduction division of meiosis have only half this number (22 + X or 22 + Y) and this is referred to as the haploid number. Multiples of the haploid number other than diploid are sometimes found and are generally referred to as polyploid, or