Abstract
IT has recently been suggested that a genetic basis for cellular differentiation in multicellular organisms might be reflected in the distribution pattern of inactivated deoxyribonucleic acid (DNA) in the chromosomes1. Inactivated DNA is characterized by three features, namely, heterochromatinization, late replication at the time of doubling, and inactivity of synthesis of ribonucleic acid (RNA). To substantiate this hypothesis, we are now studying the pattern of DNA and RNA synthesis of chromosomes in human cells of various origin by means of autoradiography. In studying the synthesis of RNA by human chromosomes, we encountered a dilemma. The chromosomes are identifiable only in metaphase as are most somatic cell chromosomes in multicellular organisms. During metaphase and anaphase, however, synthesis of RNA completely ceases in all the cells of multicellular organisms so far investigated2–7. It is not, therefore, feasible at present to study the pulse labelling pattern of RNA on metazoan chromosomes except polytenic ones. Previous investigators6,8 have studied RNA synthesis mainly in meiotic chromosomes as these chromosomes remain visible throughout the meiotic cycle. However, they are not chromosomes of differentiated somatic cells in which we are interested. Hsu reported synthesis of RNA in intermitotic nuclei of mammalian somatic cells9, in which the Barr body was shown to be inactive in the synthesis of RNA. In general, however, the chromosome is not identifiable as a distinct morphological unit in the intermitotic phase of metazoan cells. To overcome this dilemma, we decided to label the cell just before division and examine the distribution of the label at metaphase which immediately follows the labelling. The interval between the labelling and the examination was chosen so as to be sufficiently short to prevent secondary diffusion of the labelled RNA from the site of the synthesis. If the labelled RNA is not released immediately after its synthesis and does persist during mitosis, it would be expected that the label incorporated at late G2 and prophase would also remain detectable on the metaphase chromosomes.
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FUJITA, S., TAKEOKA, O., KAKU, H. et al. Synthesis of Ribonucleic Acid by Human Chromosomes and a Possible Mechanism of its Repression. Nature 210, 446 (1966). https://doi.org/10.1038/210446a0
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DOI: https://doi.org/10.1038/210446a0
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