The invention provides nucleic acid monomers with a 2'-modification that are useful for the incorporation of dyes or blocking groups. The monomers can be incorporated on the 3'-end of a dual labeled probe to inhibit PCR polymerase extension during PCR. The polymerase is inhibited from extending the probe at the 3'-hydroxyl group when the monomer is present; there is no need to add a chemical moiety to the 3'-hydroxyl or remove the 3'-hydroxyl. The monomers can also be incorporated internally or at the 5'-end of the oligonucleotide. A detectable label, such as a fluorescent or quenching dye, can be incorporated on the 2'-position of such monomers.