Calibration of molecular weight scales for DNA

The molecular weights of T2, T5st +, T5st-0 and T7 DNA's were found by molecular autoradiographic determination of the phosphorus content of the phage DNA and/or the phage particle. The values obtained for the molecular weights of the sodium salts of these DNA's were 132 ± 12 × 10 6 a.m.u. † † Abbreviations used: a.m.u., atomic mass units; AET, 2-aminoethyl isothiuronium bromide hydrogen bromide; TCG, Tris-casamino acid medium. for T2, 83 ± 4 × 10 6 a.m.u. for T5st +, 77 ± 2 × 10 6 a.m.u. for T5st-0 and 25 ± 2 × 10 6 a.m.u. for T7. The sedimentation rates of these DNA's relative to T7 DNA (32.6 s) were determined in sucrose gradients. The S suc of T2, T5st + and T5st-0 were 57.0 s, 49.2 s and 48.1 s, respectively. These data and the relative sedimentation rates of T7 “whole” and “half” molecules were used to obtain the relationship: S suc = 0.0833 M 0.350. Emperical relationships for M versus [η] or S 20 · w 0 were obtained. These are: log 10 ([ η] + 5) + 2.774 = 0.653 log 10 M and log 10 (S 0 0 · w − 3.0) + 1.859 = 0.449 log 10M . These latter two relationships hold over a molecular weight range of 300,000 to 130 × 10 6 a.m.u. for duplex DNA. The mass per unit length of DNA molecules measured by the protein film technique was determined to be 19 ± 0.1 × 10 6 a.m.u. per micron for DNA in films cast on water.