What’s the difference between a phosphodiester and a phosphorothioate backbone?

A phosphorothioate linkage has a sulfur in a non-bridging location on the phosphate backbone. This modification is known to greatly retard nuclease degradation of oligonucleotides. Phosphorothioates also have a lower melting temperature, which is a measure of the association and disassociation rates. A phosphodiester linkage contains all oxygens on the phosphate backbone.