Integrity of parental DNA during replication

Induction and repair of DNA strand breaks in CHO-cells irradiated in various phases of the cycle

Induction and repair of DNA strand breaks was investigated in synchronized CHO cells by using the alkaline DNA-unwinding technique followed by chromatography on hydroxyapatite. The velocity of DNA denaturation in an alkaline solution (pH 12.1) was found to be independent of the position in the cycle. In unirradiated cells the number of DNA strand breaks per 3.6 x 10(12) dalton was found to vary with age in cell cycle amounting to about 250 breaks per cell for G1, 400 for S, and 500 for G2 + M cells. The number of induced DNA strand breaks per unit dose was found to be the same in all phases; an average of 340 strand breaks were induced per Gy and per 3.6 x 10(12) dalton. Within the first hour after irradiation about 90 per cent of all radiation-induced strand breaks are rejoined.

Purification of an alkaline nuclease from Physarum polycephalum

An alkaline nuclease was purified from microplasmodia of Physarum polycephalum. The nuclease, active on denatured DNA and RNA and free of contamination by other nucleolytic activities, appeared to be a zinc-metallo protein. The enzyme was only active under conditions, where Zn2+ were retained in the enzyme. Loss of zinc occurred by the chelating action of EDTA, EGTA or ampholines, by acid of highly alkaline pH conditions or by high ionic strength. The addition of ZnCl2 to compensate losses, restored all activity, while all other divalent cations caused inhibition. The nuclease, with a molecular weight of 32 000, was stable at neutral pH at high temperatures with a half-life of 20 min at 80 degrees C. It was inhibited by any salt of buffer concentration above the level of zero ionic strength and showed a special sensitivity towards phosphate ions. The possible similarity of this enzyme to nuclease S1 from Aspergillus oryzae is pointed out.

The effects of temperature and ethidium bromide on the banding of heat-denatured DNA in gradients of NaI

The effects of temperature and ethidium bromide on the banding of heat-denatured DNA was studied during equilibrium centrifugation in density gradients of NaI. Centrifugation at 10 degrees C prevents the partial renaturation of Escherichia coli DNA and Clostridium perfringens DNA that occurs at 20 degrees C. A centrifugation temperature of --5 degrees C is required to prevent renaturation of T7 phage DNA. Ethidium bromide decreases renaturation of Escherichia coli DNA during centrifugation at 20 degrees C and causes a small shift in the buoyant density of both denatured and native DNA. Equilibrium centrifugation at lower temperatures prevents DNA renaturation and permits increased utilization of the large buoyant density difference between native and heat-denatured DNA in gradients of NaI.