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DNA pulse labeling exposes biological cells to a detectable precursor for a bounded interval. After removal, later sampling can follow the labeled material and descendants. A short pulse asks about synthesis during that interval; overnight labeling instead accumulates evidence across many hours. Detectable synthesis does not by itself prove completion of a cell cycle, and DNA repair can also incorporate precursors.

DNA repair addresses damage to DNA. Incorporation of a labeled precursor during DNA repair need not indicate duplication of the whole genome in S phase.