Early replicating DNA involved in the G2 to M transition in Allium cepa L meristematic cells

Abstract

Summary— The involvement of genome portions replicating at different times of the S phase in the control of cell cycle events was analysed in Allium cepa L meristematic cells. 5‐Azacytidine (5‐azaC) was incorporated into discrete replicating DNA portions in synchronous cells. Cells treated with 5‐azaC during the late S phase went through normal cell cycles while cells treated with 5‐azaC during the early S phase were able to go through the immediately succeeding mitosis, as well as to begin the subsequent S phase. However, they were unable to enter a second mitosis. Thus, sequences replicating in the early S phase appear to code for a mitotic inhibitor which represses the emergence of a positive mitotic signal. This inhibition took place in the first half of the interphase (ie during G1) and resulted in a cell cycle blockage in G2.