We examined lampbrush chromosomes (LBC) prepared from chicken oocytes of 1-3-mm diam using both light and electron microscopy. Both macro- and microchromosomes form LBC with morphologies very similar to the well known newt and salamander LBC. In chicken LBC typical loops have a contour length of approximately 15 microns, although some loops range up to 50 microns. Multiple transcription units are present on some loops. Electron microscopic examination of Miller spread preparations reveals closely spaced nascent transcripts typical of LBC transcription. We used 3H-labeled chicken DNA as a probe for light microscopic level in situ hybridization to repetitive sequences associated with nascent RNA transcripts. Approximately 25 sites were labeled, primarily on the microchromosomes, plus sites on chromosome 2 and on the putative sex chromosome. The small genome size of the chicken (1.2 pg) presents a considerable advantage over that of newts or salamanders in further study of LBC structure and function.

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