Incomplete Inhibition of Synchronized Cell Division by Hydroxyurea and its Relevance to the Normal Cellular Life Cycle

Abstract

ONE of the more puzzling aspects of synchronized cell division in heat-treated Tetrahymena has been the relative asynchrony of DNA synthesis which persists following the synchronizing treatment. This phenomenon has been well documented in the past and is perhaps most simply demonstrated by the fact that only about 50 per cent of the cell nuclei label with thymidine between the end of the heat-treatment (EHT) and the first synchronous division (SD)¹. Because spectrophotometric measurements of the DNA content of EHT cells have shown that they contain two to three times the DNA found in an average log cell², the reasons for additional DNA synthesis after EHT have been even more obscure. In the past it has usually been suggested that DNA replication is complete at EHT and that this further synthesis in some cells merely represents redundant DNA accumulation by “mature” nuclei³ known to be polyploid⁴.