How come you don't get deoxyribose backbones with U-bases, or ribose backbones with T-bases? It seems like the enzymes must know not to synthesize DNA or RNA with the wrong base there, even if the backbone segment is right. Or do they?
The enzymes involved in nucleotide synthesis are specific -- won't add ribose to T or deoxyribose to U. (Bases and sugars are made separately and then hooked up.) Then the enzymes for nucleic acid synthesis are specific too -- DNA polymerase won't use anything with U or ribose and RNA polymerase won't use anything with deoxyribose or T.