Ribosomal RNA Synthesis

Non-coding RNAs, such as ribosomal RNAs, transfer RNAs, snoRNAs, and micro RNAs are the RNAs that do not code for proteins but are, themselves, the final product. 

Ribosomal RNAs, the most abundant of the non-coding RNAs, are major structural components of the ribosomes.  Eukaryotic ribosomes contain rRNAs of four types: 5S, 5.8S, 18S, and 28S. 

Three of the four rRNA genes are encoded in a single DNA element, interspaced with transcribed spacer DNAs. The fourth rRNA, 5S rRNA, is encoded separately.

Within the nucleolus, the three rRNAs are transcribed together by RNA Polymerase I into a single large precursor rRNA called 45S precursor rRNA. 

After transcription, the precursor rRNA is modified extensively. Two common modifications include nucleoside methylations and pseudouridylation.

In nucleoside methylation, the 2 prime hydroxyl position on the nucleotide sugar is methylated. In pseudouridylation, the base uridine isomerizes, generating a different form called ‘pseudouridine.’

These modifications are catalyzed by a category of RNA-protein complexes called ‘small nucleolar RNA-protein complexes’ or ‘snoRNPs’.  Each snoRNP complex consists of a snoRNA and four or five proteins, in