Once the nucleotides are synthesized they can exchange phosphates among one another in order to create mono-, di-, and tri-phosphate molecules.  The conversion of a nucleoside-diphosphate (NDP) to a nucleoside-triphosphate (NTP) is catalyzed by nucleoside diphosphate kinase, which uses ATP as the phosphate donor.  Similarly, nucleoside-monophosphate kinase carries out the phosphorylation of nucleoside-monophosphates.  Adenylate kinase is a specific nucleotide kinase used for regulating cellular energy fluctuations by the interconversion of 2ADP ⇔ ATP + AMP.