BIOCHEMICAL CONTROLS OF MEIOSIS.

This review is addressed to the biochemical events that underlie the early phases of the meiotic cycle. Late activities such as chromosomes disjunction and special processes involved in gametogenesis are excluded from this article. Of primary concern are the prophase stages of meiosis during which homologous chromosomes pair and presumably undergo crossing-over. These two events are fundamental to genetic recombination and constitute a major and universal feature of meiosis. In reviewing the field from the standpoint of biochemical mechanisms we have one general concern which is best to state at the outset. The sources of evidence on biochemical activities during meiosis are few. Biochemical analyses of meiosis in yeast are just beginning and will not be reviewed here. The bulk of our information comes from analyses of liliaceous plants, but their study has been pursued in very few laboratories. The credibility of the conclusions drawn from biochemical studies depends to a large extent upon the degree to which they are consistent with the information provided by genetics and cytology. Genetic approaches to intragenic recombination in yeast were surveyed two years ago. The cytology and fine structure of meiotic cells from various sources have been fully reviewed in the previous volume and genetic aspects of meiosis will be discussed in the succeeding one. No incontrovertible evidence yet exists for the occurrence of crossing-over during the pachytene stage and no demonstration is yet available that zygotene synapsis is an essential condition for crossing-over. The controversies surrounding the relationship of synapsis to crossing-over and/or disjunction are rather lively and have been competently presented elsewhere. No attempt will be made in this review directly to challenge the substance of these controversies. Instead, the reader will be put in company with appreciable circumstantial evidence which supports the simple tie between synapsies and crossing-over. In writing this artile, we have favored the view that crossing-over between homologous chromosomes follows and is dependent upon chromosome synapsis. The data presented may indeed have considerable bearing on the correctness of the underlying proposition. [ABSTRACT FROM AUTHOR]