Persistence in Early Amphibian Embryos of Informational RNA's from the Lampbrush Chromosome Stage of Oogenesis

Abstract

The ripe ovarian oocyte contains a large stockpile of informational RNA's, a direct legacy of the intense gene activity which occurs during the lampbrush chromosome stage of oogenesis. The RNA synthesized at this stage is accumulated so that in Xenopus some 47 mµg of lampbrush-synthesized, template-active RNA is finally present in the mature oocyte.[1] Estimated by comparing the template activity of oocyte RNA in a cell-free amino acid incorporating system to that of a known viral RNA standard, this amount of stored template-active RNA is close to 3900X the 4C amount of DNA expected in the terminal ovarian oocyte chromosomes. [1,2] Similarly, a massive accumulation of template-active RNA appears to take place in the course of sea urchin oogenesis, according to an estimate of the amount of such RNA in the unfertilized egg recently obtained in the same manner by Slater and Spiegelman.[3] The amount of genetic information potentially encoded in the informational RNA's of the mature oocyte is as impressive as is their quantity: although only 3 per cent of the total genetic information possessed by the organism is actually transcribed during the lampbrush stage in Xenopus, it can be calculated that the amount of DNA active in the chromosomes would be enough to provide no less than 8.2 X 10^5 genes the size of that coding for a hemoglobin β chain.[1,24] We know that at least 65 per cent of genetic information transcribed during the lampbrush stage is still present in stored RNA's of the terminal oocyte.[1]

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