Like the nucleus itself, the nucleolus - the cell's ribosome factory - is dismantled in preparation for cell division. Its reassembly begins in telophase, and the compartment is fully formed by early G1 phase. What drives this process, and how is it regulated? In a Commentary onp. 2265, Danièle Hernandez-Verdun and co-workers review work that has shed light on the mechanics of nucleolar assembly and its coordination with the cell cycle. Studies some years ago suggested that assembly is driven by activation of the Pol I transcriptional machinery, the rRNA-processing machinery being recruited by the pre-ribosomal RNAs (pre-rRNAs) transcribed. More recent experiments,however, indicate that things are more complex. For example, inhibition of Pol I during the assembly process has revealed that partly processed rRNAs remain during mitosis and recruit processing components such as fibrillarin to forming nucleoli independently of Pol I transcription. Furthermore, whereas Pol I transcription is...

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