Abstract
The incorporation of 3H-thymidine into the DNA of Rana catesbeiana tadpole liver was studied in vivo during spontaneous and 3,5,3′-triiodothyronine (T 3)-induced metamorphosis. Some early macromolecular responses occurring in the liver of T 3-treated tadpoles were elucidated by an improved technique for the fractionation of tadpole liver organelles. There is a transitory stimulation in the rate of 3H-thymidine and 14C-lysine incorporation into nuclear DNA and protein occurring 2–4 hours after T 3 administration. An enhanced rate of 3H-thymidine incorporation into liver mitochondrial DNA began 1 hr after hormone treatment of the tadpole and persisted for 3 days; thereafter the rate of incorporation declined to the control level. An increased rate of 3H-thymidine incorporation into tadpole liver nuclear DNA occurs 4 days after T 3 treatment and between stages XVII and XX in spontaneous metamorphosis. An increase of the thymine components in the acid-soluble pool of the liver coincides with the increased rate of 3H-thymidine incorporation. In both spontaneous and T 3-induced metamorphosis, the synthesis of tissue-specific proteins is preceded by, or coincides with, the enhanced nuclear DNA biosynthesis.
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