Unscheduled dna synthesis
(Science: cell biology, molecular biology) Any synthesis of dna that happens ouside the s phase of the cell cycle. The term is used to describe the replication of dna during the nucleotide excision repair of dna damage and thus distinct from the semi-conservative replication of dna which is confined to the s-phase of the eukaryotic cell cycle. Unscheduled dna synthesis can be detected eg. by an elevated incorporation of [3H]-thymidine into the dna of cultured mammalian cells during the repair of damage. This can be detected by autoradiography associated with image analysis. This method involves culturing cells on glass slides, exposing them to the test substance in the presence of medium containing high specific radioactivity [3H]-thymidine and observing the radiolabel incorporated during UDS into those cells that are not semi-conservatively replicating dna. The amount of dna replication associated with UDS is relatively low compared with the amount associated with semi-conservative replication. If autoradiography is applied to monitor this process, s-phase cells which are undergoing semi-conservative replication are usually easy to eliminate from the analysis due to their heavy labelling indices. The measure of radiolabelled thymidine incorporated during UDS can be done by either autoradiography or liquid scintillation counting. Unfortunately the latter method cannot distinguish between semi-conservative and repair replication in the cell suspension.