Electron microscope study of the denaturation of Polyoma virus DNA

Structural heterogeneity of mitochondrial DNA molecules within the genus Drosophila

We have determined by electron microscopy the molecular weight of circular mitochondrial DNA (mtDNA) molecules from 39 species representing 13 groups of five subgenera of the genus Drosophila. mtDNA molecules of all species examined, other than members of the melanogaster group, had, with one exception, molecular weights in the rather narrow range 9.90 X 10(6). The one exception was D. robusta, which had a molecular weight of 10.61 X 10(6). In contrast, mtDNA molecules from species within the melanogaster group had molecular weights covering the considerably greater range 9.92 X 10(6) to 12.35 X 10(6). Applying the electron microscope denaturation mapping technique of Inman to mtDNA molecules of eight species of the melanogaster group, we found each of them to contain a region [presumably rich in adenine and thymine (A+T)] which denatured at a specific temperature (40 degrees) at which most of the remainder of the molecule remained undenatured. The size of the A+T-rich region was constant for mtDNA molecules of a species, but varied from 0.62 X 10(6) to 3.41 X 10(6) for mtDNA molecules of different species. It was demonstrated that the differences in molecular weights of the A+T-rich regions can almost completely account for the differences in total molecular weights of the mtDNA molecules from species of the melanogaster group.

Heart denaturation studies of rat liver mitrochondrial DNA. A denaturation map and changes in molecular configurations

The effect of progressive denaturation of open circular molecules (component II) and supercoiled covalently closed circular molecules (component I) of rat liver mitochondrial DNA has been followed by heating in the presence of formaldehyde and examination in the electron microscope. After heating at 49 degrees C, two, three, or four regions of strand separation were visible in 25% of the component II molecules. Comparisons of the patterns of distribution of these regions in individual molecules indicated that they occurred at at least three specific positions around the molecule. Also, these regions, which were assumed to be rich in adenine and thymine, were within a segment which was less than 50% of the length of the molecule. After heating at 50 degrees C, up to 14 regions of strand separation were observed, but when comparisons were made no clear groupings were found. At 51 degrees C, component II molecules were completely separated into a single-stranded circle and a single-stranded linear piece of similar length. Strand separation was accompanied by shortening of the molecule. At 70 degrees C, single-stranded circles had a mean length of 2.7 micro, compared with 5.0 micro for native molecules. Progressive heating of component I molecules resulted first in conversion to an open circle (I') and then to a second supercoiled form (I''). Visualization of further denaturation products of component I was prevented by crosslinking of the molecule by formaldehyde at high temperatures.