Cautionary note on the use of [methyl-3H]thymidine to measure rates of chloroplast DNA synthesis in Chlamydomonas reinhardtii.

Commercial [methyl-3H]thymidine preparations tested here contain about a 0.2% contaminant which is rapidly incorporated into Chlamydomonas DNA. This contaminant obscures the measurement of the rate of chloroplast DNA synthesis when methyl-labeled preparations are used. Such contaminants are not present in ring-labeled (either 3H or 14C) thymidine preparations. In ring-labeled thymidine preparations, a slower incorporation rate commensurate with cell density is observed. These slower, long-term incorporation kinetics would be expected for the utilization of bona fide thymidine into chloroplast DNA.