Human chromosome identification and the pattern of DNA replication in fibroblasts from an XXY male

Abstract

A statistical study of early and late DNA synthesis in segments of chromosomes from XXY fibroblasts was performed in order to establish the accuracy of karyotyping on a morphological basis in relation to the labelling patterns over the chromosomes. Cells were labelled with <sup>3</sup>H-thymidine after incomplete synchronisation with a DNA inhibitor (5-fluorodeoxyuridine). The late-S pattern was, in general, found to complement the early-S pattern, and 71 % of the chromosome segments were significantly either early or late labelling. The larger late-labelling segments, typified by those in chromosome 4 and the late-labelling X, continue synthesis after the smaller ones (Nos. 21 and 22 and the Y) have more or less reached completion. In 40 cells from late- or early-labelled samples, where chromosomes were first karyotyped without reference to labelling behaviour, the following chromosomes showed significantly distinct patterns and could be distinguished from each other or from other chromosomes in their Denver groups: Nos. 4 and 5, the early-labelling X, Nos. 9, 11, 13, 17 and 18 and the Y. In a sample of 20 cells labelled at an early or penultimate stage of late-S, other chromosomes, including Nos. 14, 15, 21 and 22, showed unique labelling patterns. At no stage could chromosomes 8 and 10 or 19 and 20 be distinguished from one another in their respective groups on the basis of labelling pattern. The study indicates that even with errors arising from misclassification of chromosomes and the inherent within- and between-cell variability in labelling, significant differences in labelling behaviour of chromosomes can be demonstrated in a partially synchronised population labelled at the correct stage of DNA synthesis.