DNA Structure and the Effect of Methylation
The traditional B form of DNA is a right-handed antiparallel double helix. It is in this form that DNA exists in solution at physiological salt conditions. It should be borne in mind however that the structure of DNA even in the B form is far from regular, and the helical twist angle can vary between 40.0° and 27.7°, giving DNA molecules with an average of 10.4 to 10.6 base pairs per turn. In this form the base sugar linkage is normally anti with the C8 (purine) or C6 (pyrimidine) above the sugar ring, which has the endo configuration (see Figure 3.1) (Dickerson, 1983a; Wang, 1979; Rhodes & Klug, 1981; Trifonov, 1982).
KeywordsDouble Helix Cytosine Methylation Buoyant Density Sugar Ring Pyrimidine Purine
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