Abstract
Sequence of DNA synthesis and mitotic chromosome spiralization along heterochromatic portions of the sex (X1X2) and of some marker chromosomes in cultured Chinese hamster cells were studied, employing two methods: study of segmentation pattern caused in chromosomes with colcemid, and autoradiography with tritiated thymidine. The heterochromatic portions of all chromosomes studied were characterized by striking internal asynchrony of DNA replication. In particular, they had segments that replicated relatively early. The short arm of the X2 chromosome, heterochromatic in female somatic cells, had at least three such segments. Replication patterns of the long arms of the X1 and X2 chromosomes were different. In X1 this arm contains several segments showing relatively early replication. The long arm of X2 had no similar segments. The possible significance of the data obtained is discussed with regard to the problem of genetic inertness of heterochromatin. At the terminal stage of the S period, H3-thymidine seems to be incorporated into condensed chromatin of interphase nuclei. On the basis of the data obtained, it is proposed that during replication of heterochromatin consecutive despiralization of parts of it takes place.
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Zakharov, A.F., Egolina, N.A. Asynchrony of DNA replication and mitotic spiralization along heterochromatic portions of Chinese hamster chromosomes. Chromosoma 23, 365–385 (1968). https://doi.org/10.1007/BF00625285
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DOI: https://doi.org/10.1007/BF00625285