Incorporation of adenine-8-C14 into chromosomal nucleic acids of Drosophila salivary glands has been observed with the autoradiographic technique. RNA-C14 and DNA-C14 were detected as the fractions extractable by ribonuclease digestion and resistant to ribonuclease, respectively. Extractions with desoxyribonuclease and acids were also used to identify the nucleic acids.

Time-course curves were determined from grain counts. RNA-C14 concentration reached a maximum in 2 hours, and decreased after the 4th hour. DNA-C14 concentration reached its maximum within 8 hours, and showed no decreases during a 48-hour experiment.

In the latter part of the period of observation, morphological differentiation of the gland occurred, the decrease in RNA-C14 became very rapid, and a large increase in DNA-C14 was observed. Marked decrease in RNA-C14 and increase in DNA-C14 were detectable in a few hours when isotope was administered shortly before visible differentiation of the gland.

Measurements of nuclear size indicated no significant decreases in RNA-C14 amount prior to the period of differentiation. During this later period, a large decrease in RNA-C14 amount occurs suddenly, and the same amount of C14 is added simultaneously to the DNA fraction, as expected if RNA-C14 is utilized in the synthesis of DNA.

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