Abstract
A crude, lightly sheared chromatin preparation obtained from a mixture of [methyl-3H] thymidine pulse and [2-14C] thymidine long-labeled sea urchin embryos (swimming blastulae), was centrifuged in metrizamide to form an isopycnic gradient. The buoyant density of the 3H pulse labeled chromatin was slightly higher than that of the 14C labeled bulk chromatin. The 3H/14C ratios in the higher and lower density regions of the overlapping radioactivity peaks, indicated the presence of fractions rich and poor in nascent DNA in these two density regions. After 15 min chase, the difference disappeared, indicating that the chromatin fractions with nascent DNA have a half-life shorter than 15 min.