Adjacent monomer units in nucleic acids are connected via phosphate groups attached to the hydroxyl on the 5' carbon of one unit and the 3' hydroxyl of the next one. This linkage is called a phosphodiester bond (Figure 4.1).

1. Phosphodiester bonds in nucleic acids are very stable to hydrolysis in the absence of a catalyst (such as an acid or a nuclease).

2. Synthesis of a phosphodiester bond in nucleic acids requires energy input. As a result, the nucleoside monophosphates in nucleic acids are built up from hydrolysis of nucleoside triphosphates. Cleaving a pyrophosphate from a nucleoside triphosphate yields a nucleoside monophosphate and enough free energy to make the formation of polynucleoside monophosphates (i.e., polynucleotides) thermodynamically favorable.