Early and late replicating DNA involved in the G₁ to S transition in Allium cepa L meristematic cells

Abstract

The involvement of portions of the genome replicated at different times of the S period in the regulation of the G1 to S transition was analyzed in Allium cepa L meristem cells. For this, DNA bromosubstitution confined to discrete portions of a previous S period followed by anoxic UVA irradiation (300-400 nm light) was performed in synchronous cells. Sequences replicated in late S appeared to be involved in the positive regulation of the initiation of replication. Hence, cells were prevented from initiating replication if irradiated at mid G1 only when the DNA sequences replicated in the last third of the previous S period were bromosubstituted. Cycloheximide-induced inhibition of protein synthesis at late G1 also prevented the G1 to S transition. Sequences replicated in mid S appeared unrelated to any control of the initiation of replication. On the other hand, sequences replicated in the first third of the S period seemed to be involved in the negative regulation of the initiation of replication, since irradiation after previous bromosubstitution of early replicating DNA sequences advanced G1 cells into the next S phase and increased the proliferative fraction of the population. Finally, the simultaneous inactivation of DNA sequences involved in both positive and negative regulation of replication allowed the cells to enter into S.

The onset of cell proliferation is stimulated by ascorbate free radical in onion root primordia

“…Alternatively, some primordia were used for autoradiography. For this purpose, some primordia were fixed in the mixture ethanol-acetic acid (3:1) for 24 h after the pulse and processed for autoradiography as described elsewere [10]. An llford nuclear emulsion (type K5) was used, and the exposure time was 20 days at -30°C.…”

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

“…Previous studies have focused on the events involved in cell proliferation by incorporating 5-azaC into replicating DNA [ 13,181. This strategy is based on the following premises: i) DNA methylation in specific cytosines is one mechanism for controlling (repressing) gene expression in animal and plant cells (reviewed in [17,201); ii) the involvement of specific DNA segments in the progress of the cell cycle can be determined by allowing DNA modification (ie DNA methylation) for restricted intervals only during the S phase [4,8,9,13,171; and iii) the incorporation of base analogues during the S phase results in permanently modified replicated DNA. For instance, 5-azaC incorporation induces DNA hypomethylation and activates gene transcription selectively [ll, 13, 17,201. In the present study, the involvement of restricted DNA segments in the control of both the S phase and the mitotic onset was investigated.…”