Morphological Features of DNA Macromolecules as Seen with the Electron Microscope

Desoxyribosenucleic acid molecules isolated from salmon sperm were studied with the electron microscope. The essential step in the technique which makes it possible to visualize the individual molecules consists in a preparative step wherein the materials are supported on the extremely smooth surface of cleaved mica where they are shadow-cast with platinum, which is then backed with a supporting film and stripped for observation in the usual manner. The DNA, which was originally about 8 million molecular weight, was also examined after fragmentation by sonic vibration. The fragments show a certain degree of rigidity and the ends generally terminate abruptly, indicating that the double helices of the Watson-Crick model both break close to the same place. DNA molecules heated to temperatures between 90 and 100°C, coil up into amorphous patches, although a few apparently unaltered molecules survive such heating.