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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    \ V 2. MITOSIS AND THE HUMAN MITOTIC CHROMOSOMES 11 A. Interphase The chromosomes of the interphase cell are not normally visible to the light microscope, but their presence can be demonstrated by techniques which use the higher resolution of the electron micro¬ scope. During interphase the chromosomes are active in RNA synthesis. At a certain, rather fixed time before the cell next divides they also replicate themselves and thus double the amount of chromosomal material in the nucleus. The time when they are doing this is known as the S (synthetic) period. The period between the end of the previous cell division and the beginning of the S period is known as the Gj period, and the interval between the end of the S period and the beginning of the next cell division is called the G2 period. The length of the S period and the ensuing G2 period are rather constant for any particular type of cell; thus for human lymphocytes or fibroblasts in culture the S period lasts about 7 hours and the G2 period 4 to 6 hours. Figure 2 summarizes the stages of Fig. 2. Digram of mitotic cycle (based on behaviour of human lymphocytes in vitro). P = prophase, M = metaphase, A = anaphase, T = telophase. G, = period between end of cell division and beginning of diromosome replication. S = synthetic period during which diromosomes replicate. Gj = period between end of replication and start of next cell division. All these periods except the G, period are fairly constant in duration.