Nucleic acid metabolism in the nucleus and cytoplasm of angiosperm meiocytes.

Over the course of flowering plant evolution many important stages in the life cycle have become compressed into the period between the premeiotic mitosis and gamete maturation. For this reason the changes in structure or chemistry that take place at this time are not easily ascribed to particular 'developmental stages'. During the meiotic process itself striking changes occur in levels of cytoplasmic and nuclear RNA, while DNA is synthesized in both the chromosomes and organelles. Evidence is presented indicating that a large proportion of the m- and rRNA is purged from the meiocyte cytoplasm by a combination of normal degradation and the activity of specific hydrolases. DNA synthesis in both the plastids and mitochondria precedes the re-differentiation of these organelles, while that in the chromosomes occurs in two major peaks, as has previously been reported (Hotta & Stern, 1971). High-resolution autoradiography did not indicate either peak of DNA synthesis to be localized within the synaptonemal complex, although final confirmation of this conclusion must await further investigation. Examination of these events in organisms where meiosis is removed from other stages of the life cycle suggests that the degradation of RNA and the chromosomal DNA synthesis are features which regularly accompany the meiotic process. The significance of these findings is discussed in terms of the hypothesis that there is at least one stage in the life cycle of all plants at which a purge of information-carrying molecules takes place.