Abstract
During the M phase of the mammalian cell cycle, transcription is completely silenced on highly condensed chromosomes. Transcription factors and RNA polymerases are displaced and the chromatin structure associated with active genes is lost. This is consistent with the generally accepted idea that mitotic chromosomes are metabolically inert. On the other hand, several reports have suggested active DNA repair on condensed mitotic chromosomes. We therefore examined the efficiency of the repair of DNA damage induced by both ionizing radiation and UV in nocodazole-arrested M-phase HeLa cells.
Static-field gel electrophoresis revealed that the majority of DNA double-strand breaks induced by 15 Gy of X-rays were repaired within 1 hr, which is in agreement with the previous reports. ELISA revealed that the majority of (6-4) photoproducts induced by 10 J/m2 UVC were also repaired within 1 hr.
Efficient DNA repair we observed suggests that the condensation of chromosomes during the M phase does not impede the processing of nucleic acids (or the access of functional proteins to target DNA), and implies the possibility that the condensation does not play a direct causal role in silencing of transcription.
