Masked Messenger RNA and Cytoplasmic DNA in Relation to Protein Synthesis and Processes of Fertilization and Determination in Embryonic Development

Publisher Summary This chapter discusses masked messenger RNA and cytoplasmic DNA in relation to protein synthesis and processes of fertilization and determination in embryonic development. It has been demonstrated that mRNAs can occur in a masked form (mmRNA) in the mature unfertilized egg and become activated upon fertilization or upon artificial parthenogenesis of nonnucleate egg fragments. New mRNA is synthesized during early development also in an inactive form. The mmRNAs are found in protein-containing particles mostly smaller than the subribosomal units. They may also occur associated with polysomes of a kind that are not dissociable into monoribosomes by RNase unless pretreated with trypsin. The present view is that the primary event in the determination of a particular embryonic tissue is the production of the relevant mmRNAs. In favor of this is the occurrence, prior to the relevant differentiation, of a period of insensitivity to inhibition by dactinomycin, X-rays, or other agents that inhibit DNA-dependent RNA synthesis. Cytoplasmic DNA, measured by newer methods that eliminate interfering materials, is less in amount than previously reported but still several times the nuclear DNA in sea urchins and in other animals. It has the same priming capacity for RNA synthesis as nuclear DNA. Evidence that intrinsically synthesized protein may be exported from mitochondria provides a basis for possible morphogenetic action of these cell organelles.