The nucleolus is the most prominent structure in a cell nucleus. It is the site of ribosomal RNA (rRNA) transcription, pre-rRNA processing and ribosome subunit assembly (reviewed by Olson et al., 2002). The nucleolus is a dynamic structure that assembles around the clusters of rRNA gene repeats during late telophase, persists throughout interphase and then disassembles as cells enter mitosis. Owing to the difference in density between the nucleolus and the surrounding nucleoplasm, it is readily visible in either live or fixed cells viewed by phase contrast or differential interference contrast (DIC) optics (upper-left panel). Thanks to the advent of fluorescent protein (FP) technology, nucleoli can also be detected by fluorescence microscopy in cell lines expressing FP-tagged nucleolar proteins. An example is shown in the inset of the upper-left panel, in which PP1γ, a protein phosphatase that accumulates in the nucleolus, is tagged with YFP and stably expressed in...

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