The duration of DNA synthetic (S) period in Zea mays: A genetic control

The nuclear cycle among several diverse genetic stocks of Zea mays root meristem cells was compared and it was found that there were no significant differences among the nuclear cycle durations and its component phases. The durations of various periods of their mitotic cycles were studied by autoradiography of cells pulse-labelled with tritiated thymidine (3H-TdR). The total nuclear cycle was 10 to 11.5 hours and mitosis was 0.81 to 1.34 hours at 25°C. The S period is the longest interval (50% of the total time) of the nuclear cycle; of the rest of the cycle, G2 is longer than G1 or mitosis among all stocks. The constancy of the nuclear cycle among several stocks was adduced as evidence for strict genetic control of the cycle. Furthermore, it is demonstrated the DNA synthesis period is not dependent upon the amount of DNA present.