There is much to be learned about how chromosomes are faithfully transmitted during mitosis and meiosis. Determining the detailed structure of an entire eukaryotic chromosome should yield much information about its function. As a first step toward this goal, we have been cloning and studying the sequence organization of the 260 kb DNA molecule from chromosome I, the smallest Saccharomyces cerevisiae chromosome. We have cloned approximately 75% of it and determined the physical location of most if its known genes. We propose to clone and restriction map the remaining DNA, further map all vegetatively expressed transcribed regions and physically map the not yet located known genes. These studies will provide the foundation for locating ARS elements and other sequences that could be involved in chromosome function. To address the question of meiotic chromosome function, the relationship between physical and recombination distances over the entire length of the DNA molecule will be examined and the total recombination map-length between molecularly defined ends will be determined. These experiments will enable us to estimate how often chromosome I homologues undergo recombination and will enhance our understanding of the meiotic behavior of a single chromosome. Hybridization studies suggest that chromosome I contains approximately 100 protein encoding genes. Despite intense effort, only 5 genes essential for growth on rich medium and 5 other genes were defined by classically induced mutations. Thus, like all other yeast chromosomes, the function of most chromosome I genes has yet to be determined. To define additional essential and nonessential genes on chromosome I, gene-disruption mutations in transcribed regions with no defined function will be induced by "shuttle mutagenesis". These studies will be used to estimate the total number of essential genes on chromosome I and should provide the basis for estimating the total number of essential genes needed for growth and division of a simple eukaryotic cell. This work is unique in that no complete eukaryotic chromosome has yet been cloned and characterized. The resulting information should increase our understanding of chromosome structure and function.
