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Preparative techniques are detailed for isolating nuclear contents of amphibian oocytes and HeLa cells for electron microscopy, and the ultrastructure of presumptive ribosomal RNA (rRNA) genes and other active chromosomal loci is described. The rRNA genes from the two cell types present strikingly similar structural configurations, both being fully loaded with RNA polymerases and exhibiting gradients of short to long nascent ribonucleoprotein (RNP) fibrils. Other chromosomal loci in the two cell types are strikingly different; those on lampbrush chromosomes of amphibian oocytes show closely-spaced polymerases and extremely long RNP fibrils, whereas those on HeLa genomes have widely-spaced polymerases and relatively short attached RNP fibrils.
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Miller et al. (1972) studied this question.
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