Incorporation of [3H]thymidine into mtDNA of ram spermatozoa.

Freshly ejaculated spermatozoa from 6 rams incorporated [3H]-thymidine only after a lag period of 4 h. The thymidine incorporation was dependent on the time after ejaculation regardless of the rate of fructolysis, which was controlled by cold shock. The DNA synthesis period lasted for 3 h followed by a degradation period of 2 h. This synchronized level of labelling of DNA with tritiated thymidine was not due to bacterial contamination, sperm cell mitosis, mitochondrial division, or DNA repair. The fact that the maximal level of the DNA synthesis was dependent upon the external pH of the seminal plasma, in a parabolic fashion with a minimum around pH 6.3, indicated that this activity was of a 'metabolic' type. The sensitivity of the sperm DNA synthesis to 10 micrograms ethidium bromide/ml indicated that it was of a mitochondrial origin.