Fluctuations and the Cell Cycle in Yeast

Eric Siggia
Center for Studies in Physics and Biology, Rockefeller University

        Microcolonies of budding yeast have been imaged at 3 minute
intervals as they expand from one to 50-100 cells.  With the aid of
several fluorescent markers that define cell cycle events the
variability in timing has been measured as a function of ploidy
(copies of the genome).  The timing of certain events appears to be
limited by molecular noise, and the genes responsible were traced by
varying gene copy number.

        By imaging cells deleted for genes governing the transition from G1
to S phase, we have shown that there is a gene module that affects this
transition and functions as a unit.  The problem of how cells control
their size will also be touched upon, as well as the possibility of
phase locking the cell cycle to an external periodic signal.
These measurements illustratethe richer phenotypes obtained from
single cell measurements for mutations that were origionally defined at the
population level.