28S

28S rRNA is a eukaryotic ribosomal RNA component of the large ribosomal subunit (60S). Along with 5.8S and 5S rRNAs, it contributes to the ribosome’s catalytic core and structural framework, enabling peptide bond formation and proper mRNA decoding during translation. The mature 28S rRNA is approximately 4,700 nucleotides long in many eukaryotes and is highly conserved across species.

Biogenesis and genomic context: In most organisms, the 28S rRNA is encoded within a larger precursor rRNA (the 45S or 47S transcript) transcribed by RNA polymerase I in the nucleolus. This precursor is processed to yield the mature small subunit rRNA (18S) and the large-subunit rRNAs (5.8S and 28S). The 5S rRNA is transcribed separately by RNA polymerase III. rRNA genes are organized in tandem repeats within nucleolar organizer regions on specific chromosomes, with the human clusters located on the acrocentric chromosomes 13, 14, 15, 21, and 22.

Structure and function: The 28S rRNA folds into multiple domains that assemble with ribosomal proteins to form the large subunit’s functional core. It participates in correctly positioning tRNAs and in peptidyl transferase activity, contributing to the ribosome’s catalytic center and the polypeptide exit tunnel.

Clinical and research notes: Variations in rDNA copy number, processing defects, or dysregulated ribosome biogenesis can affect cell growth and have been linked to diseases and cancer. The 28S rRNA gene region, especially the D1/D2 domains, is widely used in fungal systematics and phylogenetics. In laboratory practice, the integrity of rRNA often uses the 28S to 18S ratio as a quality assessment metric.