Nucleic acid metabolism in oocytes and embryos of Urechis caupo

Abstract

The content and synthesis of nucleic acids were studied in mature oocytes and embryos of the echiuroid worm Urechis caupo. The mature oocyte contains 14 mμg of total RNA, including approximately 10 molecules of 4 S RNA and 1 molecule of 5 S RNA for each ribosome. The total DNA content of the tetraploid oocyte is 10 pg; individual sperm contain 1 pg. Nucleic acid synthesis was studied during the development of the fertilized egg to the 900-cell trochophore larva. During this period there is a 200-fold increase in DNA content, but no substantial change in either total RNA or 4 S RNA content. Net synthesis or turnover of RNA during this period of embryogenesis involves less than 10% of the embryo's 4 S RNA or rRNA. The rates of 4 S RNA and rRNA synthesis were studied in oocytes and during embryogenesis. The synthesis of both 4 S RNA and rRNA is reduced sharply upon fertilization. 4 S RNA is synthesized during cleavage, and its rate of synthesis per cell remains constant through the post-gastrula stage. In contrast, the rate of synthesis of rRNA increases at least 13-fold at later stages of embryogenesis relative to earlier stages. The synthesis of 4 S RNA is not coordinate with that of either rRNA or 5 S RNA.