Abstract: Compositions and methods for modulating the activity of RNA and DNA are disclosed. In accordance with preferred embodiments, antisense compositions are prepared comprising targeting and reactive portions. Reactive portions which act, alternatively, through phosphorodiester bond cleavage, through backbone sugar bond cleavage or through base modification are preferrably employed. Groups which improve the pharmacodynamic and pharmacokinetic properties of the oligonucleotides are also useful in accordance with certain embodiments of this invention. Delivery of the reactive or non-reactive functionalities into the minor groove formed by the hybridization of the composition with the target RNA is also preferrably accomplished. Therapeutics, diagnostics and research methods and also disclosed. Synthetic nucleosides and nucleoside fragments are also provided useful for elaboration of oligonucleotides and oligonucleotide analogs for such purposes.