Molecular weight of DNA from coliphage T7 by electron microscopy

Abstract

DNA from the coliphage T7 was isolated by phenol extraction method and prepared for electron microscopy by protein monolayer technique, using two hypophases: 0.1 M ammonium acetate and water. In another experiment, the phages were disrupted by osmotic shock and the released DNA was studied by electron microscopy. The average lengths of the phenol-extracted DNA were found to be 12.0 ± 1.0 μ, and 13.1 ± 1.0 μ for 0.1 M ammonium acetate and water hypophases, respectively. From the osmotic shock preparations, the average molecular length was 12.8 ± 0.9 μ. Assuming B crystalline configuration for DNA on electron microscopic grids, the molecular weight of T7 DNA corresponding to the above experimental lengths were 23.0 ± 1.9, 25.2 ± 1.9, and 24.6 ± 1.7 millions, respectively. Comparison of these values with the molecular weights obtained by means of other techniques indicated that the figure corresponding to the water hypophase or osmotic shock prepparations was more reliable. It was, therefore, concluded that the molecular length of DNA prepared by the osmotic shock method was the same as that of isolated DNA prepared with a water hypophase, and that with the assumed conversion factor of molecular length to weight these methods led to the correct molecular weight.