The relation of DNA synthesis and mitosis in tobacco pith tissue cultured in vitro

Summary DNA synthesis and mitosis were initiated in cultured tobacco pith tissue by means of IAA and kinetin. DNA classes were determined by microspectrophotometric measurements (Feulgen); autoradiographs (tritiated thymidine) served to ascertain whether or not nuclei had undergone DNA synthesis during culture.All mitoses in new cells (resulting from divisions in culture) were diploid and had been preceded by DNA synthesis in culture.Whereas many of the old cells (which had not previously divided in culture) found in diploid or polyploid mitosis had undergone DNA synthesis during culture, others had not. Such non-radioactive mitoses still occurred after 16 days.In view of this, a 4 C nucleus in differentiated tissue should be considered as potentially both diploid and tetraploid, for it appears impossible to predict whether it would, upon restoration of conditions conducive to DNA synthesis and mitosis, enter a diploid mitosis or, after undergoing DNA synthesis, a tetraploid one.A high nuclear DNA content seems to have a much more inhibiting effect on the onset of DNA doubling than on that of mitosis.Somatic polyploidization is understood as the result of two DNA doublings between which mitosis was omitted, or aborted, or in effect undone by a failure of cytokinesis leading to fusion during a later mitosis.This work has been supported by research grants to K. Patau from the U.S. Public Health Service (grant No. C-3313) and the American Cancer Society.