Suppression of Mitosis and Macromolecule Synthesis in Onion Roots by Heavy Water

Exposure of microorganisms to high concentrations of D20 slows or stops growth, but the inhibition is often overcome after a time; in many instances, serial subculture leads to successful adaptation the many component processes of population growth, seems generally to be the most sensitive process. Shifts in the patterns of nucleic acid synthesis are sometimes seen (Flaumenhaft, Conrad, and Katz, 1960), but there is thus far scant evidence that DNA synthesis is especially sensitive. The situation in animal cells is different. Here, too, cell division is the first process to suffer a blockade (Gross and Spindel, 1960 a), but, in contrast to microorganisms, animal cells cease dividing very soon after immersion in D20-rich media. In sea urchin eggs, which are among the most thoroughly studied cell types, the effect on mitosis is virtually instantaneous (Gross and Spindel, 1960 b; Marsland and Zimmerman, 1963). There is no doubt that the speed of this effect depends upon the extremely short equilibration time between medium and intracellular water,-probably a few seconds (Tucker and Inou6, 1963),-and that it is related to the presence of a large and complex mitotic apparatus, whose mechanical properties are changed by deuteration (see, for review, Gross, Spindel, and Cousineau, 1964). But there is also a profound effect upon DNA synthesis. In sea urchin eggs, high concentrations of deuterium in the medium cause a rapid and reversible block of DNA replication (Gross and Harding, 1961). RNA synthesis in fertilized eggs is also inhibited, although to a smaller extent than DNA synthesis, while protein synthesis is even less affected (Cousineau, 1963). In fact, treatment of unfertil-ized eggs with D20 evokes protein synthesis which is ordinarily blocked, and the return of such treated eggs to normal sea water brings about parthenogenetic activation and prolonged mitotic activity (Gross, Spindel, and Cousineau, 1963). Over the intervals in which animal cells can survive in media containing 80 per cent or more D20, there appears to be no adaptive recovery. These facts lead to the suggestion that in higher