Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
DNA: Structure of Chromosomes and Genes
Native DNA molecules are extremely fragile

Early experiments demonstrated that the molecular mass of DNA does not exceed 10∙10⁶, which corresponds to approximately 15,000 Base Pairs. However, as Methods for isolating native DNA improved, it turned out that its molecular mass is considerably larger. Today we know that native DNA molecules, such as those from E. coli Cells, are so large that they are difficult to isolate in intact form because they are easily sheared by mechanical stress. Simple stirring or pipetting a solution of native DNA can cause breaks in the molecule, resulting in The formation of A large number of shorter fragments. Nevertheless, with careful handling, it is possible to obtain intact DNA from large Introduction/6.html">DNA-containing Viruses and determine their molecular masses using physical methods. Using an Electron microscope, individual DNA molecules can be directly observed and their lengths measured. Electron microscopic studies have shown that carefully prepared DNA samples are not merely a mixture of polymers of varying lengths; rather, they consist of molecules of identical length and nucleotide composition. Let us now examine the size of DNA molecules as well as other characteristics of DNA isolated from viruses, Bacteria, and Eukaryotic cells.