In eukaryotes, the site of synthesis of most of the ribosomal RNA (rRNA) is nucleolus. The nucleolar organizer contains many copies of ribosomal DNA (repetitive DNA). The RNA cistron of nucleolar DNA forms $45 \mathrm{~S}$ precursor with the help of RNA polymerase. This 45 S RNA undergoes cleavage with the help of nucleases to give 18S, 28S and 5.8S rRNA units. Out of different rRNAs, the 5S rRNA is not synthesized in nucleolus. It is synthesized outside it.