Timing of nuclear division cycle in Neurospora crassa.

The timing of the nuclear division cycle in Neurospora crassa was studied by inhibiting DNA synthesis with hydroxyurea added to exponentially growing cultures at concentrations that do not inhibit cytoplasmic growth. After the addition of hydroxyurea only the nuclei that have passed the S-phase divide, and therefore by counting the increase in the average number of nuclei per hypha, the stage in the cycle in which completion of S phase occurred was determined in different nutritional conditions of exponential growth. The stage at which DNA replication terminates is different in different conditions of exponential growth; however, when the length of G2+M phases is calculated a constancy of these phases is shown. Our data indicate that in N. crassa the coordination between cytoplasmic growth and the nuclear division cycle is achieved mainly through a growth-rate-dependent expansion of the G1 phase, according to a sizer-plus-timer model of control of the nuclear division cycle.