DNA Synthesis and Cell Division in Germinating Onion I. Onset of DNA Synthesis and Mitosis

SUMMARYThe study of the onset of DNA synthesis and mitosis in the germinating onion reveals that the first few millimeters of root growth occur exclusively as a result of cell expansion. DNA synthesis and mitosis are initiated at the 1.4 and 2.8 mm. stages of growth, respectively. Stable indices are attained at approximately 7.0 mm. in the case of DNA synthesis, and 10.0 mm. for mitosis. Timing of both points of initiation suggests an S plus G2 interval of 9.2 hours, and labelling indices indicate that S is not quite three times as long as M. The work demonstrates that emergent root length is a reliable external criterion for the timing of internal cellular events.