Factors contributing to structural variation in DNA. (a) DNA nucleotide conformation is affected by rotation about seven different bonds. Six of the bonds rotate freely; rotation about bond 4 is constrained by the sugar ring, giving rise to the sugar pucker. (b) The syn and anti positions of deoxyguanosine, and the syn and anti positions of deoxythymidine. Note that pyrimidines are restricted to the anti position; the carbonyl at C-