Double Strand Rejoining in Mammalian DNA

Abstract

FOLLOWING the observation of an enzymatic rejoining mechanism for radiation-induced single strand scission in mammalian DNA^1,2, much speculation has arisen concerning the existence of a mechanism for rejoining similarly induced breaks involving both sides of the phosphodiester backbone of DNA. The latter type of strand scission has been reported to occur at a rate of 0.15–0.5 breaks per cell-rad^3–5. This observation, coupled with the fact that a radiation dose of 50–100 rad of gamma radiation has little effect (in terms of lethality) on mammalian cells, suggested that a mechanism to repair double strand damage should exist if such damage is important to the cell. Evidence suggesting the presence of this repair mechanism was presented by us earlier⁶; however, the experimental data were variable and a definite conclusion could not be reached. Recent improvements in techniques have led to a series of experiments that convincingly demonstrate a double strand rejoining mechanism in mammalian cells. Following the observation of an enzymatic rejoining mechanism for radiation-induced single strand scission in mammalian DNA^1,2, much speculation has arisen concerning the existence of a mechanism for rejoining similarly induced breaks involving both sides of the phosphodiester backbone of DNA. The latter type of strand scission has been reported to occur at a rate of 0.15–0.5 breaks per cell-rad^3–5. This observation, coupled with the fact that a radiation dose of 50–100 rad of gamma radiation has little effect (in terms of lethality) on mammalian cells, suggested that a mechanism to repair double strand damage should exist if such damage is important to the cell. Evidence suggesting the presence of this repair mechanism was presented by us earlier⁶; however, the experimental data were variable and a definite conclusion could not be reached. Recent improvements in techniques have led to a series of experiments that convincingly demonstrate a double strand rejoining mechanism in mammalian cells.