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This book contains the papers presented at the Twenty-Seventh Annual Biology Division Research Conference which was held April 1-4, 1974 in Gatlinburg, Tennessee. The topic of the symposium was Mechanisms in Recombination and it follows by exactly twenty years the previous Gatlinburg Symposium on Genetic Recombination. During this interval, and the preceding years as well, the process of recombination has remained a central and tantalizing problem for geneticists. The subject assumes added significance with the recent appeal by a committee of leading scientists for a moratorium on the construction of certain types of recombinant molecules. That autonomously replicating molecules linking portions of pro­ karyotic and eukaryotic DNA can now be produced in vitro attests to the technical advances that have taken place in this field. Nevertheless, the details underlying the process in vivo continue to be elusive. This symposium brought together individuals studying recombi­ nation in organisms as widely separated as bacteriophage and mammals and using disciplinary approaches of comparable diversity. Conse­ quently the present volume summarizes much of current strategies and concepts concerning the subject. The meeting was sponsored by the Biology Division of the Oak Ridge National Laboratory (operated by the Union Carbide Corporation for the U. S. Atomic Energy Commission) with the support and encour­ agement of its director, H. I. Adler. The organizing committee was chaired by J. K. Setlow and included R. F. Grell, R. D. Hotchkiss and E. Volkin. Special thanks are due to the speakers, to I. R.

Once per life cycle, mitotic nuclear divisions are replaced by meiosis I and II – reducing chromosome number from the diploid level to a haploid genome and recombining chromosome arms by crossing-over. In animals, all this happens during formation of eggs and sperm – in yeasts before spore formation. The mechanisms of reciprocal exchange at crossover/chiasma sites are central to mainstream meiosis. To initiate the meiotic exchange of DNA, surgical cuts are made as a form of calculated damage that subsequently is repaired by homologous recombination. These key events are accompanied by ancillary provisions at the level of chromatin organization, sister chromatid cohesion and differential centromere connectivity. Great progress has been made in recent years in our understanding of these mechanisms. Questions still open primarily concern the placement of and mutual coordination between neighboring crossover events. Of overlapping significance, this book features two comprehensive treatises of enzymes involved in meiotic recombination, as well as the historical conceptualization of meiotic phenomena from genetical experiments. More specifically, these mechanisms are addressed in yeasts as unicellular model eukaryotes. Furthermore, evolutionary subjects related to meiosis are treated.

Over the past decade, our laboratory and others have been concerned with molecular archaeological studies aimed at revealing the origins and evolutionary histories of permeases (1). These studies have revealed that several different families, defined on the basis of sequence similarities, arose independently of each other, at different times in evolutionary history, following different routes. When complete microbial genomes first became available for analysis, we adapted p- existing software and designed new programs that allowed us quickly to identify probable transmembrane proteins, estimate their topologies and determine the likelihood that they function in transport (2). This work allowed us to expand previously-recognized families and to identify dozens of new families. All of this work then led us to attempt to design a rational but comprehensive classification system that would be applicable to the complete complement of transport systems found in all living organisms (3). The classification system that we have devised is based primarily on mode of transport and energy coupling mechanism, secondarily on molecular phylogeny, and lastly on the substrate specificities of the individual permeases (4).