Student Theses and Dissertations

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RU Laboratory

Cross Fred Laboratory


The cell cycle machinery of Saccharomyces cerevisiae consists of a central negative feedback oscillator comprising cyclin-CDK and its antagonist, APCCdc20. This oscillator is stabilized and tuned by positive feedback loops, and its frequency is modulated by checkpoint controls. Either by directly triggering events, or by entraining independent oscillators controlling events, the cyclin-CDK oscillator regulates the key events of the cell cycle. These events have an established order and timing within the overall cycle. The work I describe in this thesis concerns two fundamental questions: how is the order and timing of cell cycle events controlled, and what sets the intrinsic frequency of the cell cycle oscillator? I describe work on two major processes in the cell division cycle that reveals two very different modes of regulation. The first of these processes – Start – represents a pivotal commitment to divide. In collaboration with Gilles Charvin, I demonstrate that positive feedback in the molecular machinery underlying Start acts as a bistable switch that renders this regulatory transition irreversible. The second major process is Mitosis, a set of events all triggered by the same class of cyclin-CDKs and yet occurring in a set and reproducible order. I describe an ordering mechanism underlying this choreography that relies on the natural ramping-up of cyclin-CDK activity level. The observation that different events require different levels of cyclin-CDK activity leads to the question of how these thresholds are set. To begin to answer this, I discuss how mitotic cyclin-CDK triggers two different events – depolarization of growth and formation of the mitotic spindle – in two very different ways. The first relies on entrainment of an independent oscillator controlling growth polarization; the other may involve the simultaneous regulation of multiple targets. The observation that cyclin-CDK is rate-limiting for mitotic events suggests that increasing the level of this key cell cycle regulator above its endogenous range should accelerate Mitosis, and I show evidence that this is the case. Quite surprisingly, this increase in cyclin-CDK abundance also accelerates the frequency of the cell cycle oscillator as a whole through its effect on growth. This provides an intriguing new answer to the question of what sets the intrinsic frequency of the cell cycle oscillator. Together, this work underscores the central role of the mitotic cyclin-CDK regulator, which controls not only the relative timing of individual cell cycle events, but also the growth rate of the cell, and the overall frequency of the cell cycle oscillator.


A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

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