Volume 2

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

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

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    loid sets. Nuclear membranes form around these, and the cytoplasm is di vided into two separate cells. Chromosomes appear in their most distinctive forms when they are highly condensed and it is only at this point in mitosis that the features of the in dividual members of the chromosome set can be distinguished. The criteria used to describe the chromosomes are size and position of the centromere, the point at which the chromosome is attached to the spindle and by which it is pulled to the poles. Chromosomes in which the centromere is located at one end take a rodlike structure and are said to be telocentric. Acrocentric chromosomes appear similar but possess a small arm on one side of the centromere. (There has been some controversy about whether chromosomes may be truly telocentric or must always have a short arm, even if this is too small to detect cytologically.) Metacentric chromosomes have arms of about equal sizes because their centromeres are located at a central position; these take up a V shape at metaphase. When the two arms are of unequal length, the product is an L-shaped submetacentric chromosome. This classifica tion of chromosomes, although useful in distinguishing the members of a set, probably has no functional significance. The termini of chromosomes—the telomeres —appear to be differentiated from internal regions. When chromosomes are broken by X irradiation, the resulting segments may be free to fuse together again; but they do not appear to do so with the telomeres of unbroken chromosomes (McClintock, 1940; Muller and Herskowitz, 1954). It is therefore possible that the telomere may have a structure that precludes its attachment to other chromosomes or parts of chromosomes. Little is known about the underlying structure of the chromosome during mitosis and meiosis. During the early stages when chromosome structure is less compact, it can be seen to take the form of a coiled fiber, the chro- monema. This may show alternating wide and narrow regions which give the chromosome the appearance of beads on a string. The beadlike struc tures are known as chromomeres and the position of each chromomere is relatively constant for each chromosome. Probably they reflect the local manner of coiling of the chromosome. During the most condensed stages, a characteristic series of bands can be generated in each chromosome by treatment with several types of agent. Although presumably this reflects the organization of the chromosome, the nature of the interactions respon sible for band formation is not clear. However, this does represent a way in which the chromosomes of a set may be distinguished unequivocally; and banding techniques have superseded earlier distinctions based on centro mere position and chromosome size. Some chromosome regions remain condensed during interphase. These identify the heterochromatin, contrasted with the much greater length of euchromatin, which uncoils and swells during the same period. Hetero chromatin often appears to be located at the centromere and may be in close contact with the nucleolus. Heterochromatic regions on different chromo-