DNA replication patterns in cultured female rat fibroblasts.

Abstract

Introduction The hypothesis of Lyon (1962) and Russell (1963) postulates that in female mammalian somatic cells one of the X-chromosomes is at random genetically inactivated at an early developmental stage. It has also been suggested that the Barr body, or sex chromatin, observed in resting nuclei represents this inactivated X (Ohno, Kaplan, and Kinosita, 1959; Ohno and Hauschlta, 1960). Cytogenetic evidence for this hypothesis has recently been provided by Ohno and Cattanach (1962) and Multherjee and Sinha (1964). It is a generally accepted n priori view that the heteropyknotic X (sex-chromatin) represents the late-labeling X observed in autoradiographic studies. Furthennore, Lyon's hypothesis has been helpful in interpreting a variety of genetic data and the autoradiographic studies of cells with abnormal sexchromosome constitutions (Hsu and Loclthart, 1965). Autoradio~ra~hic studies have shown that D N A svnthesis in cultured 0 1 ' . mammalian cells is asvnchronous in autosomes. as well as in sex-chromosomes. In female Chinese hakster cells, one X and tl1k long arm of the other are the last segments to conlplete their syntheses (Taylor, 1960; Hsu, 1964). Allocycly in the X-chromosome was also observed i i the following mammalian species and hybrids: human (Ailorishima, Gruinbach, and Taylor, 1962; Moorhead and Defendi, 1963; Schinid, 1963); Syrian hamster (Galton and Holt, 1964; Huang, personal communication) domestic cow (Multherjee and Sinha, 1963; Gartler and Burt, 1961); mouse (Galton and Holt, 1965); mule (Multherjee and Sinha, 1964; donltey-zebra hybrid (Benirschlte, et nl., 1964); and dog (Fraccaro, et 01.. 1965). Lyon's hypothesis embraces mammals in general but no studies have been reported on the sequence of DNA replication patterns in rat cells. The following report presents results of autoradiographic studies of euploid rat cells.