DNA and RNA synthesis by fertilised, cleavage arrested sea urchin eggs.

Eggs of Lytechinus variegatus which have been fertilised and maintained at 10 degrees C do not undergo segmentation, but do replicate DNA. It has been found that RNA synthesis is initiated in these eggs following a period of DNA synthesis resulting in amounts of DNA/egg comparable to those amounts found in late blastulae (1000 genome equivalents). The RNA synthesised has been studied by autoradiographic and sucrose density gradient centrifugation. RNAse-sensitive grains appear in autoradiographs of cleavage-arrested eggs exposed to tritiated uridine beginning two days after fertilisation at 10 degrees C. Incorporation of labelled uridine into the 18 and 25 s region of sucrose gradients parallels the appearance of acid-insoluble radioactivity in the uridine autoradiographs. Localisation of the RNA in the autoradiographs was shown to be in close association with Feulgen-positive areas of the egg periphery. Since differentiation is generally thought to involve changing patterns of transcription of the zygote genome, transcription patterns in the absence of cleavage and normal morphogenesis is of considerable interest. The study of molecular events occurring in a time span during which there is no cell division may have bearing on the role of cell division in differentiation.