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The first experimental evidence that the DNA of eukaryotic chromosomes replicates semiconservatively was provided by Taylor et al. (1957). Root-tip cells of the broad bean, Vicia faba, allowed to incorporate 3Hthymidine during a period of DNA synthesis (an S phase), were fixed at the following mitosis and examined by radioautography. Each labeled chromosome showed radioactivity in both chromatids. Cells similarly labeled, but allowed to complete a further S phase in the absence of radioactive precursor and also given time to reach the next mitosis (i.e., the second mitosis after labeling) before fixation, showed radioactivity in only one chromatid at any location along each chromosome. These observations, now known to be valid for all eukaryotic chromosomes thus far studied, are evidence not only for semi-conservative replication of DNA as such but also for the proposition of uninemy, i.e., that each chromatid of a eukaryote contains only one double helix of DNA.
H. G. Callan (Tue,) studied this question.