Radiosensitivities of DNA molecules in lymphocytes from the circulating blood of man

The radiosensitivities of DNA molecules from lymphocytes of human circulating blood were examined by alkaline sucrose gradient centrifugation. The single-strand breaks of DNA per rad, including the breaks formed under the present alkaline condition, were 1.2 plus or minus 0.1 breaks per 10-12 daltons DNA per rad. When the cells were transformed, the number of breaks was found to increase to 1.8 plus or minus 0.2 breaks per 10-12 daltons DNA per rad. The lymphocytes are capable of rejoining radiation-induced single-strand breaks of DNA. The rate of rejoining was dependent upon types of the suspending medium. The rate increased to ten times of that of the non-transformed cells upon transformation.