Nucleic Acid
The
successive nucleotides of both DNA and RNA are covalently linked through
phosphate-group “bridges”. Specifically, the 5’-hydroxyl group of one
nucleotide unit is joined to the 3’-hydroxil group of the next nucleotide
by a phosphodiester linkage. Thus the covalent backbones
of nucleic acid consist of alternating phosphate and pentose residues,
and the characteristic bases may be regarded as side groups joined to
the backbone at regular intervals.
All
the phosphodiester linkages in DNA and RNA strands have the same orientation
along the chain, giving each linear nucleic acid strand a specific polarity
and distinct 5’ and 3’ ends. By definition, the 5’end
lacks a nucleotide at the 5’ position, and the 3’end
lacks a nucleotide at the 3’ position. Other groups (most often one or
more phosphates) may be present on one or both ends.