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Relation of Uptake of Radiophosphorus into Nucleic Acids to Age of Developing Chick Embryos^*

Kingsley M. Stevens

( Department of Internal Medicine of the Bowman Gray School of Medicine, Wake Forest College, Winston-Salem, North Carolina)

1. The uptake of radiophosphorus into desoxyribonucleic acid (DNA) and pentosenucleic acid (PNA) in 9-16-day chick embryos has been determined.

2. The "relative specific activities" of both DNA and PNA rise rapidly to a peak on the thirteenth day, then fall abruptly to about 20 per cent of this peak value. The DNA activity then continues constant, while the PNA begins to rise again.

3. The turnover rates of both nucleic acids, but especially DNA, appear to be much higher in embryos than in adult tissues.

^* Supported, in part, by a contract between the Atomic Energy Commission and the Bowman-Gray School of medicine.

Atomic Energy Commission postdoctoral fellow in the Medical Sciences, 1948–50. Present address: Surgical Research Unit, Brooke Army Hospital, Fort Sam Houston, Texas.

Received 10/ 6/51.