Non-random distribution of high density chromatin detected at opposite ends of T-banded human metaphase chromosomes

Abstract

Previous research using microdensitometric scanning and computer graphic image analysis showed that T-banded segments of human metaphase chromosomes usually exhibit an asymmetrical distribution of high density (HD) chromatin between sister chromatids. Here, we employed the same methods to analyze HD chromatin distribution at opposite ends of T-banded human lymphocyte chromosomes. This study revealed that in most chromosomes with an asymmetrical distribution of HD chromatin at both ends, the highest densities of each arm were located in opposite chromatids. The frequency of this configuration was 0.792 per chromosome, indicating that the highest chromatin densities of the terminal segments of T-banded human chromosomes were non-randomly distributed at opposite chromosome arms. The possible relationship of this observation to the mode of replication of the terminal chromosome region is briefly discussed.Key words: human chromosomes, T-banding, chromatin structure, telomere, microdensitometry. It is well known that the usual banding procedures (C-, G-, R-and T-) reveal the underlying structure and composition of DNA and associated proteins in mitotic chromosomes (Therman and Susman, 1993). The specialized chromatin structure and the GC-richness of the telomeric/ subtelomeric DNA (de Lange, 2005;Riethman et al., 2004) confer high resistance to heat denaturation to the terminal regions of metaphase chromosomes which results in selective staining after T-banding (Dutrillaux, 1973). T-banding is a modification of R-banding procedure and is obtained through the incubation of chromosomes in a buffer at a high temperature followed by Giemsa staining. This procedure results in T-bands as darkly stained segments in lightly stained chromosomes. Scanning electron microscopy showed that T-bands are areas of highly aggregated chromatin fibres (Jack et al., 1986;Allen et al., 1988).Microdensitometric scanning and computer graphic image analysis of high-resolution microphotographs of Tbanded segments of human and Chinese Hamster Ovary (CHO) chromosomes allowed the detection of a differential distribution of HD sub-telomeric chromatin between sister chromatids. The different patterns observed were arbitrarily classified in three kinds of HD chromatin distribution, namely: Type I, HD segments were of similar size in both sister chromatids; Type II, HD predominated in one of the chromatids; and Type III, HD was detected in only one chromatid (Drets et al., 1992). Alternatively, the distribution of HD chromatin could be classified in a simplified way as symmetrical (pattern type I) or asymmetrical (patterns type II and III). These patterns were confirmed by scanning T-banded endoreduplicated CHO chromosomes in which the same interchromatid distribution of HD chromatin appeared in both sister chromosomes (Drets and Mendizábal, 1998). Nevertheless, quantitative computerized microdensitometrical analyses on the distribution of HD chromatin at opposite ends of T-banded chromosomes have not yet been reported. Therefore, we perform...