Ribosomal RNA(rRNA) is a component of the ribosomes, the protein synthetic factories in the cell. It is the most abundant RNA and eukaryotic ribosomes contain four different rRNA molecules 18S,5.8S,28S, and 5SrRNA. First three of the rRNA molecules are synthesized in the nucleolus, and 5S one is synthesized elsewhere. rRNA molecules are extremely abundant. They make up at least 80% of the RNA molecules found in a typical eukaryotic cell.