. "B is a member selected from the group consisting of the following nitrogenous heterocyclic bases: ##STR3## where X1, X2, X3, X4, X5, X6, X7, X8 =H, NH2 or halogen, ##STR4## where X9, X10, X11, X12, X13, X14 =H, CH3, or halogen and R1 and R2 can be the same or different and may be hydrogen or acyl protecting groups." . . .