The probability of G 1 cells to enter into S increases with their size while S length decreases with cell enlargement in Allium cepa

Abstract

The distribution of cell surface area projection (cell size) has been measured at birth and at initiation of DNA synthesis in steady-state populations of Allium cepa root meristems. The conditional probability, P( I G 1 ) , that initiation occurs given that the event of being in g 1 also occurs has been estimated from these data. p( I G 1 ) was found to increase when cells became larger. The distribution of G 1 duration has been constructed from indicated cell size distributions. The absolute frequencies of G 1 times showed a maximum in the zone of cells with short G 1 periods; about 14% of cells appear to enter into S with G 1 ∼- 1 h. These results suggest that the increase of p( I G 1 ) was due to cell enlargement and not to cell aging. By comparing the cell size distribution at initiation of S and at the end of this period, a drastic reduction of cell size variability during DNA replication was observed and both curves were seen as rather similar in shape although they obviously had different modal points. These observations support that there is a negative correlation between the initiation size and the duration of genome duplication, and that cells which initiate DNA synthesis with the same size have a similar replication time. From this hypothesis, a plot of S duration versus cell size at initiation of this period was constructed by comparing the distributions of cell size at start and end of replication; this plot was also consistent with the existence of a negative correlation between cell initiation size and S length.