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By inhibiting the a-like DNA pofymerase, and therefore nuclear DNA synthesis, aphidicoiin induces accumulation of suspension cultured carrot ceils at the Gr/S boundary of the cell cycle. After a 24-h treatment with the drug the accumulation is complete, affecting all the cycling cells (95% of the population). Upon removal of the inhibitor, all cycling cells immediately resume nuclear DNA synthesis and move synchronously throughout the S-phase. Aphidicolin Synchronization Culfured plant cellDaucus carota Nuclear DNA synthesis

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Protoplasts from a diploid Nicotiana species were used to investigate the effects of gamma-rays and fast neutrons. The damage was assessed by scoring the relative plating efficiencies. The effects of single acute doses, as well as fractionated doses, are described for both qualities of radiation. The dose--r.b.e. relationship, expressing the effectiveness of fast neutrons relative to gamma-rays as a function of dose, appears similar to that of most animal systems, with higher values in the region of lower doses. The fractionation of gamma-ray doses, with sufficiently long time intervals between split doses, allowed a substantial recovery, as known also for X-rays. On the other hand, the fractionation of a fast neutron dose into low single fractions markedly increased the biological effectiveness of the total dose. This could not be explained by cell progression during resting time intervals and necessitated an additional hypothesis, possibly involving other mechanisms in the specific action of low doses of radiation.