The Major Components of the Mouse and Human Genomes: 1. Preparation, Basic Properties and Compositional Heterogeneity

Main‐band DNA from mammals and birds can be resolved by density gradient centrifugation techniques into three or four families of fragments of different dG + dC contents. These major DNA components are similar in their buoyant densities and relative amounts in all species tested and are observed in DNA preparations ranging in M_r from 2 × 106 to over 200 × 106. In the present work, the four major components of mouse and human DNAs were prepared and characterized in several basic properties: relative amounts, dG + dC contents, buoyant densities and compositional heterogeneity. The results obtained lead to the following conclusions: (a) the major DNA components of mouse and man form at least 85% and possibly the totality of the main bands of these DNAs; (b) they have very low compositional heterogeneities over a wide molecular weight range; (c) they derive from very large chromosomal DNA segments of fairly homogeneous base composition, for which the name ‘isochores’ is proposed. A comparison of the compositional heterogeneity of main‐band DNAs from warm‐blooded and cold‐blooded vertebrates confirms our previous conclusion that these DNAs are characterized by a different sequence organization.