Folded chromosomes of Saccharomyces cerevisiae: comparison of response to sporulation medium, arrest at start, and G0 arrest.

Folded chromosome phenotypes have been examined and compared in four cell-division-cycle (cdc) mutants during transitions between cycling and non-cycling states. The two start mutants, cdc 28 and cdc 25, can undergo G0 arrest at the restrictive temperature. Arrest at start, defined by the cdc 28 and cdc 25 block points, is distinguishable from G0 arrest. Arrest at the cdc 28 and cdc 25 block points can also be distinguished from each other: folded chromosomes appear to be destabilized at the cdc 25 block, but are stable at the cdc 28 arrest point. On the other hand, folded chromosomes from cdc 28 in sporulation medium at the restrictive temperature appear unstable, while chromosomes from cdc 25 are stable. The G1 arrest mutants, cdc 4 and cdc 7, can undergo G0 arrest at the restrictive temperature. In sporulation medium no meiotic replication form is detected at the restrictive temperature, although incorporation of labeled precursors into nuclear DNA does take place. A schematic model incorporating these various findings is presented.