Abstract
Pulse labeling of deoxyribonucleic acid (DNA) and radioautography have been used to study the effect of growth rate on nuclear replication in Aspergillus nidulans. When conidia were germinated in media supporting a fast growth rate, the radioactive pulse labeled either all of the nuclei in a cell or none of them. At slower growth rates, hyphae contained both labeled and unlabeled nuclei. Altering the growth rate thus changed nuclear replication from simultaneous to sequential. The time taken to duplicate the DNA in a nucleus, estimated from the ratio of labeled to total nuclei, remained constant at the different doubling times. The distribution of label showed that nuclei in the same hypha spent unequal times in both the postmitotic gap (G1) and the premitotic gap (G2) periods when grown at slow rates. These unequal G1 and G2 periods are considered to cause asynchrony. Once DNA synthesis was out of phase through growth on a poor medium, transferring the hypha to a rich medium did not resynchronize the nuclei. To interpret the data, two initiator mechanisms, one starting DNA synthesis and the other mitosis, are postulated to control nuclear replication in A. nidulans.
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