Live images of RNA polymerase II transcription units.

The nucleus of an amphibian oocyte can be manually isolated in mineral oil where it maintains all its activities for several hours. These undisrupted (live) nuclei have been used successfully in recent years to analyze the dynamic organization of several non-chromosomal nuclear organelles. Here, we describe an improved procedure for imaging an oil-isolated nucleus by light microscopy and we use it to produce the very first images of lampbrush chromosomes in an in vivo–like condition. These chromosomes are morphologically identical to those observed in conventional nuclear spread preparations. Importantly, their lateral loops, which are active RNA polymerase II transcription units, are readily distinguished by differential interference contrast microscopy.