Yeast can afford to be selfish. As a single-celled organism, it need not worry about the welfare of any other cell. As long as it has enough foodripening grapes, sticky leaves, or almost anything sweetit just eats and reproduces every 90 minutes.
Only abnormal yeast cells stop growing while their food supply is still plentiful. In the 1970s, Leland Hartwell of the University of Washington, Seattle, seized on this fact for his groundbreaking studies of the basic cycle through which a cell grows and divides into two daughter cells.
Hartwell chose to work with baker's yeast because it grows by budding. Unlike the cells of fission yeast, which reproduce by becoming larger and larger and then dividing in half, the cells of baker's yeast never grow bigger than they are at maturity. Instead, a little knob, or bud, starts to protrude from the cell's surface, breaking its neat symmetry, as the cell enters its reproductive cycle. A mere pimple on the cell's surface at first, the bud soon grows and balloons out until it is almost as large as the mother cell. Then it splits off.
Any mutation that affects cell division will make the bud stop growing after it has reached a particular sizean easy symptom to recognize. The bigger the bud when growth stops, the farther the cell had progressed along the division cycle when the effect of the mutation kicked in. Cells whose buds stop at different sizes therefore contain mutations that affect different stages in the cell cycle.
Using bud size as their guide, Hartwell and his colleagues succeeded in identifying the first mutant gene (cdc, or cell division cycle mutant) that was clearly responsible for blocking the cell cycle. Then they identified another 32 genes that were involved in halting the growth of yeast buds at different stages.
What made these findings riveting to researchers was that human equivalents of the yeast genes were soon found. Even more interesting, these human genes turned out to play crucial roles in the development of cancerthough cancer does not occur in single-celled organisms such as yeast.
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A yeast cell grows a new bud (right) despite a scar from a previous birth (left). After dividing 20-30 times, yeast cells are covered with scars and cannot divide again.
Image: Eric Schabtach and Ira Herskowitz