Double-stranded (ds) DNA was imaged by noncontact-mode atomic force microscopy in an ultrahighvacuum (UHV) after thermal annealing (<100
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C). The contour length of DNA measured (3300 Å) wasconsistent with that of the B-form DNA (1000 bp) we prepared. In addition, we resolved right-handedhelical turns with a spacing of 33 ± 2 Å. These observations showed both UHV condition and the thermalannealing did not make DNA take A-form but kept B-form.