Primary structure of nucleic acids

Mononucleotides are linked by phosphodiester bonds in oligonucleotides and polynucleotides. Phosphate usually binds C5' of one pentose to C3' of the following saccharide, so the axis of the polynucleotide is a chain in which pentose and phosphate alternate. The described phosphodiester bonds are the cause of chain polarity. By convention, 5'-end is written on the left and 3'-end on the right.

Nucleic Acids are extraordinarily long molecules, yet methods have been found to solve their primary structure (order or sequence of nucleotides). The procedures are based on an appropriately chosen cleavage of the polynucleotide.