Thymine is used by animal cells primarily for the synthesis of DNA. A group of sea urchin eggs was fertilized in sea water containing radioactive thymine. Following fertilization, samples of embryos were removed at regular intervals and the radioactivity in the nucleic acid was measured in counts per minute. The results obtained are shown in the figure below.
The increase in radioactivity of the embryos with time probably results from
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synthesis of new proteins by the developing embryos
synthesis of radioactive thymine by the developing embryos
oxidation of radioactive thymine
incorporation of radioactive thymine in new cell membranes
incorporation of radioactive thymine in new DNA during replication
The graph shows an increase in the radioactive nucleotide, thymine, after fertilization. As the new sea urchin embryos grow, cell division occurs. The radioactive thymine is incorporated into new DNA in the growing embryos.