Distribution of repetitious DNA in randomly growing and synchronized Chinese hamster cells

Abstract

Diploid Chinese hamster cells were labelled with 32P for 2 days or synchronized and pulse-labelled with 3H-thymidine throughout the DNA synthetic (S) period. Total nuclear DNA and DNA fractions having different G + C contents were analysed, using an improved method of hydroxylapatite chromatography. Both uniformly labelled DNA (random DNA) and DNA pulse-labelled throughout the S period, contained a similar proportion (3–4%) of single-stranded sequences (SS-DNA). The amount and G + C content of SS-DNA were very much increased in the highest buoyant density DNA fraction analysed. After denaturing and renaturing at several C 0 t values, the amounts of reassociated DNA were determined. Using random DNA, the G + C content of the reassociated sequences did not differ from that of total DNA regardless of the C 0 t value. Using pulse-labelled DNA, the proportion of highly repetitious sequences (renatured at C 0 t 2) isolated from total DNA as well as from both high and low buoyant density fractions, did not vary throughout the S period, as compared with random DNA. However, both the heavy and light fractions of late S DNA contained a greater proportion of moderately repetitious sequences (renatured at C 0 t 10 and 50) than early S or random DNA. These results are compared with those in other animal species.