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Modern Genetic Analysis Chapter 15. Regulation of Cell Number: Normal and Cancer Cells The Cell Proliferation Machinery
Genetics In Process 15-1: Yeast as a model for the cell cycle

Cell cycle genetics of the budding yeast Saccharomyces cerevisiae has revealed a large array of genetic functions that maintain the proper cell cycle. These functions are identified as a specific subset of temperature-sensitive (ts) mutations called cdc (cell division cycle) mutations. When grown at low temperature, yeasts with these cdc mutations would grow normally. When shifted to higher, restrictive temperatures, the yeasts would stop growing. What made these cdc mutations novel among the more general class of ts mutations was that a particular cdc mutant would stop growing at a specific time in the cell cycle, and all the yeast cells would look alike. Consider some examples of S. cerevisiae, a yeast that divides through budding, a process in which a mother cell develops a small outpocketing, a "bud." The bud grows and mitosis occurs such that one spindle pole is in the mother cell and the other in the bud. The bud continues to grow until it is as big as the mother cell, and the mother cell and bud then separate into two daughter cells. Any run-of-the-mill ts mutation in S. cerevisiae, when shifted to restrictive temperature, stops growth at varying times in the cycle of bud formation and growth. In contrast, after a shift to restrictive temperature, one S. cerevisiae cdc mutation produces yeast cells with only tiny buds, whereas another produces yeast cells exclusively with larger buds, half the size of the mother cell. Such different cdc phenotypes are indicative of different defects in the machinery required to execute specific events in the progression of the cell cycle.

With the completion of the sequencing of the S. cerevisiae (see Chapter 12), we are in the unprecedented position of being able to identify the entire array of proteins of the cyclin and CDK families (22 and 5, respectively). These genes are now being systematically mutagenized and genetically characterized to understand how they each contribute to the cell cycle.

© 1999 by W. H. Freeman and Company. All rights reserved.