Ribosome assembly takes place partly inside cell nucleus

Many of the transcribed but untranslated RNAs in the nuclear region participate in the manufacture of ribosomes. One such RNA is the snoRNA which is associated with the snoRNP. Wikipedia describes snoRNP


Small nucleolar RNAs (snoRNAs) are a class of small RNA molecules that primarily guide chemical modifications of other RNAs, mainly ribosomal RNAs, transfer RNAs and small nuclear RNAs. There are two main classes of snoRNA, the C/D box snoRNAs, which are associated with methylation, and the H/ACA box snoRNAs, which are associated with pseudouridylation. SnoRNAs are commonly referred to as guide RNAs but should not be confused with the guide RNAs that direct RNA editing in trypanosomes.

Here is a description of snoRNAs and snoRNPs

These snoRNPs are involved in the assembly of ribosomes inside the nucleus.


After transcription of the pre-ribosomal RNAs, most steps in eukaryotic ribosome synthesis occur within the nucleolus. Here, the pre-rRNAs are processed to yield the mature rRNA species (Fig. 3), which also undergo extensive covalent modification. In bacteria, rRNA modifications are made by conventional enzymes, but in eukaryotes most modification involves methylation of the sugar 2′ hydroxyl group (2′-O-methylation) or pseudouridine (psi) formation, which occur at sites that are selected by base pairing with a host of SMALL NUCLEOLAR RIBONUCLEOPROTEIN (snoRNP) particles55. Human cells contain over 100 species of snoRNP, and each pre-rRNA molecule must transiently associate with a member of each species. During pre-rRNA transcription and processing, many of the 80 or so ribosomal proteins assemble onto the mature rRNA regions of the pre-RNA. Many mutations known to inhibit ribosome synthesis in yeast are believed to act mainly at the level of ribosome assembly, but this process is poorly characterized.