Default cycle phases determined after modifying discrete DNA sequences in plant cells

After bromosubstituting DNA sequences replicated in the first, second, or third part of the S phase, in Allium cepa L. meristematic cells, radiation at 313 nm wavelength under anoxia allowed ascription of different sequences to both the positive and negative regulation of some cycle phase transitions. The present report shows that the radiation forced cells in late G1 phase to advance into S, while those in G2 remained in G2 and cells in prophase returned to G2 when both sets of sequences involved in the positive and negative controls were bromosubstituted and later irradiated. In this way, not only G2 but also the S phase behaved as cycle phases where cells accumulated by default when signals of different sign functionally cancelled out. The treatment did not halt the rates of replication or transcription of plant bromosubstituted DNA. The irradiation under hypoxia apparently prevents the binding of regulatory proteins to Br-DNA. (author)