The molecular weight of lambda DNA

It seemed that DNA dried in vacuum after being attached to a film of protein retained the B configuration, and actinomycin was found to increase enormously the sensitivity to shear of λ DNA.

Abstract

The DNA of the bacteriophage λ wild type was extracted by osmotic shock and spread on a water surface with a film of cytochrome c according to the Kleinschmidt technique. Its length, measured in the electron microscope, was 17.3 ± 0.6 μ. The density mutant λcb2, with a lower density than the wild type, had a DNA molecule of 14.3 ± 0.5 μ. The density difference is due, therefore, to the deletion of a piece of DNA 3 μ long. Assuming the DNA to be in the B crystalline form, the length measurements give a molecular weight of 33 × 106 for λ wild type, and 27 × 106 for λcb2, The suicide rates, due to incorporated tritium, were measured for the phages T4vT−, λ wild type, and λcb2, The rates were consistent with the molecular weights calculated from length measurements. The thymine content per infectious particle was assayed by measuring the incorporated radioactivity in λ fully labeled with tritiated thymine. Lambda wild type contained 47700 thymine molecules, corresponding to a molecular weight of 30 × 106. Actinomycin was found to increase enormously the sensitivity to shear of λ DNA. Using the shorter molecule of polyoma it was found that actinomycin did not affect the length of the molecules seen in the electron microscope. Since actinomycin prevents dry DNA from assuming the A configuration, it seemed that DNA dried in vacuum after being attached to a film of protein retained the B configuration. Length measurements in the electron microscope provide, therefore, a very precise measurement of DNA molecular weight.