High order structure of Chironomus polytene chromosomes by scanning electron microscopy

Polytene chromosomes isolated from Chironomus salivary gland cells were processed with the OSO4/TCH technique and examined by scanning electron microscopy (SEM) mainly using stereo techniques. When chromosomes were somewhat extended for preparation, band and interband regions became clearly visible. The bands appear to be dense masses of kinky fibers while the interband regions consist of the 30nm chromatin fibers. On the other hand, in less extended chromosomes where band and interband regions were less distinctive, most of the component fibers appear to have coiled configurations and their diameters range 150 to more than 200nm. We conclude, therefore, that the natural chromonemata of the polytene chromosomes are about 200nm in diameter rather 30nm even in the interband regions. The polytene chromosomes seem to have a cable-like structure.