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.

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