Rate of synthesis of desoxyribonucleic acid and mitotic rate in liver and intestine.

The incorporation of radiophosphorus (P”“) into desoxyribonucleic acid (DNA) has been shown to parallel the incidence of mitosis in various tissues (14). In tissues without mitotic activity, such as brain cortex, skeletal muscle, and cartilage, injected P32 was not found in DNA (4). Accordingly, it is generally thought that the presence of P32 in DNA occurs as a result of the synthesis of additional DNA which accompanies mitosis (14). In the present work, the rate of incorporation of P32 into DNA and the rate at which new cells are formed by mitosis were measured quantitatively and the results compared. This was done in rat liver, an organ with a low mitotic activity, and in rat intestinal mucosa, a tissue with a high mitotic activity. In both cases, twice as much phosphorus appeared in DNA as could be accounted for by the synthesis of the additional DNA required for the new cells. This finding may be explained only by assuming that all of the DNA phosphorus (DNA P) of the cells arising from mitosis is newly formed.