Tritiated thymidine routinely labels onion root cytoplasm during most of the cell cycle. One-third of this label could be cytochemically identified as DNA. The balance of the label was not RNA or a lipid, or attributable to labeled impurities in thymidine-3H. In electron microscope radioautographs one-third of the cytoplasmic silver grains was over organelles, presumably mitochondria and plastids. The other two-thirds of the silver grains in electron micrographs was distributed widely, 41% over ground cytoplasm and 10% over cell walls-cell membranes. Snake venom phosphodiesterase (SVDase) extracted a cytoplasmic fraction not degraded by DNase, and did not appear to extract nuclear DNA. The SVDase-extractable fraction may be DNA or a thymidine 5'-phosphoryl group in an ester linkage with another hydroxylic compound. The nature of the nonextractable fraction is considered. Possibilities discussed are: (1) technical problems such as the binding of an acid-labile nuclear DNA in the cytoplasm; (2) non-DNA, such as breakdown products, and thymine compounds other than DNA; (3) DNA, not extractable because of the nature of its binding to other compounds or because it is a "core" resistant to DNase. Until the chemical nature of this nonextractable fraction is known, cytoplasmic label following thymidine-3H treatment cannot necessarily be considered DNA, nor the assumption made that thymidine-3H exclusively labels DNA.

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