Enzyme compositions and their method of use that provide ready-to-use master mixtures. The compositions comprise a modified thermophilic DNA polymerase lacking 5′-3′ and 3′-5′ exonuclease activity premixed with T4 DNA polymerase, Klenow fragment and T4 polynucleotide kinase and all other necessary components, including reaction buffer and nucleoside triphosphates, required to perform DNA blunting, phosphorylation, and single nucleotide extension reactions in one tube and in two steps. Among other benefits, the mixture of different enzymes, buffers and nucleoside triphosphates is stable during prolonged storage.