Examples of base pairs found in DNA and RNA. The standard GC base pair has three hydrogen bonds and the sugars (R) are oriented towards the same face of the base pair to define a major groove face and a minor groove face. A mismatched GT "wobble" base pair has only two hydrogen bonds. A GG Hoogsteen base pair holds together two purines with two hydrogen bonds. Reverse base pairs have their sugars in an antiparallel orientation and therefore do not have major and minor groove faces.