Morphological analyses of active genes and chromatin.

Chromatin with nascent ribonuclear protein (RNP) fibers representing transcription of ribosomal DNA (top) and nonribosomal DNA (bottom). These two types of transcription can be distinguished on the basis of the size of transcription units, chromatin morphology and the inferred ratio of DNA packing, the frequency of RNP fibers (number of fibers per micrometer of chromatin), and the solitary v tandem repeat occurrence of fiber arrays. In some cases, the structure of RNP fibers is also distinctive for different transcription units. The micrographs presented herein, from Laird and Chooi, were taken of chromatin samples prepared similarly to the method described by Miller and Bakken. Cells and nuclei from Drosophila were ruptured in low ionic strength buffer; nuclear and cytoplasmic constituents were recovered on electron microscopic grids under conditions that facilitated spreading of chromatin and nascent ribonuclear protein fibers. Bar represents 1 micron.