Phosphorylation
 

A phosphate simply is a phosphate group as one can find it on almost any regular 5'-end of biologically active DNA. As an OH-group is created during the synthesis of oligonucleotides using standard procedures, it can be necessary to add a terminal phosphate group e.g. for cloning or labelling, which can be done right during synthesis. Enzymatic coupling of phosphate groups is only possible on the 5'-end (T4 polynucleotide kinase). A phosphate group on the 3'-end of the oligonucleotide changes its degradability by nucleases. Additionally such an oligonucleotide is not elongated during PCR reactions.