The effects of both α-particle and X-ray in radiation, on the
DNA
synthesis rate in mouse fibroblast and Hela cells in tissue culture is described. Tritiated thymidine autoradiography was used to indicate the rate of
DNA
synthesis in the single-layer cultures used. The results of these experiments show that: (1) The fraction of cells in a culture synthesizing
DNA
is not markedly affected by α-particles and X-rays in the dose used in the experiment. (2) The effect of either type of radiation is to reduce the rate of synthesis of
DNA
of the irradiated cells in synthesis. (3) The effect of a given dose of either type of radiation is to reduce the rate of synthesis of all the cells to a constant fraction of what it was in the unirradiated cells. (4) The rate of
DNA
synthesis is reduced to 37 % (1/
e
) by a dose of
ca
. 25 α/μ
2
or an X-ray dose of 14000 rads for mouse fibroblast cultures. In Hela cell cultures a dose of
ca
. 90000 rads is needed to reduce the rate of
DNA
synthesis to 37 % of the initial value. (5) The reduction in synthesis occurs not more than half an hour after irradiation and may be an immediate effect. From (4) above the target shape can be roughly calculated and if it is assumed to be cylindrical it appears to have dimensions 16 Å in one direction and 160000 Å in the other, i. e. a long thin thread with a mol. wt. of
ca
. 5 x 10
7
in the case of the mouse fibroblast experiments. In the case of the Hela cell experiments the target volume gives a mol. wt. of
ca
. 10
7
. These results are consistent with the view that the target may possibly be the
DNA
template (or maybe
DNP
because of the high value for the molecular weight in one case). If the effects described reflect damage to the
DNA
(or
DNP
) template during the exponented phase of synthesis then observations, (1) (2), and (3) above follow as obvious corollaries.
DEOXYRIBONUCLEIC ACID SYNTHESIS IN REGENERATING RAT LIVER STUDIED BY CYTOFLUOROMETRY IN COMBINATION WITH TRITIATED THYMIDINE AUTORADIOGRAPHY