Polynucleotide kinase catalyzes the transfer of a phosphate group from ATP to the 5'-hydroxyl termini of polynucleotides. Selective labeling of the 5'-hydroxyl termini of DNA with polynucleotide kinase has been used to study the number and the identity of the 5'-terminal residues of bacteriophage DNA's, and to examine the nature of the phosphodiester bond cleavages produced by endonucleases and by sonic irradiation. The intact strands of T7 DNA bear 5'-phosphoryl end-groups; only deoxyadenylate and deoxythymidylate are present as 5'-terminal residues. The intact strands of native λ-DNA bear 5'-hydroxyl end-groups. M13 DNA, a circular molecule, cannot be phosphorylated. End-group labeling of DNA provides a method for determination of molecular weight; calibration against other DNA preparations is not required. The molecular weight of a single strand of T7 DNA, determined by end-group labeling, is 13.1 x 106; the molecular weight of a single strand of λ-DNA is 16.0 x 106. These values are in agreement with molecular weight estimates by sedimentation analysis and electron microscopy. Sonic irradiation of DNA has been shown to favor the production of polynucleotides terminated by 5'-phosphomonoester groups. All four deoxyribonucleotides are present as 5'-terminal residues of sonicated DNA.

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