Ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) is the most abundant class of RNA in the cell, constituting approximately 80% of total cellular RNA, and forms the structural and catalytic core of the ribosome by associating with ribosomal proteins. In prokaryotes, the ribosome (70S) contains three rRNA species: 16S rRNA (small 30S subunit), 23S rRNA, and 5S rRNA (large 50S subunit); eukaryotic ribosomes (80S) contain 18S (small 40S subunit), 28S, 5.8S, and 5S rRNAs (large 60S subunit). The 23S/28S rRNA harbours the peptidyl transferase centre — the ribosome's catalytic site for peptide bond formation — making rRNA a ribozyme.
Comparison of Prokaryotic and Eukaryotic Ribosomal RNA Components
| Organism Type | Ribosome Size | Subunit | rRNA Species | rRNA Size |
|---|---|---|---|---|
| Prokaryote | 70S | Small (30S) | 16S rRNA | ~1540 nt |
| Prokaryote | 70S | Large (50S) | 23S rRNA | ~2900 nt |
| Prokaryote | 70S | Large (50S) | 5S rRNA | ~120 nt |
| Eukaryote | 80S | Small (40S) | 18S rRNA | ~1900 nt |
| Eukaryote | 80S | Large (60S) | 28S rRNA | ~4700 nt |
| Eukaryote | 80S | Large (60S) | 5.8S + 5S rRNA | ~160 + ~120 nt |
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Related Terms
Transfer RNA (tRNA)
Transfer RNA (tRNA) is a small non-coding RNA molecule, typically 73–95 nucleotides long, that acts as a physical adaptor during translation by carrying a specific amino acid to the ribosome and recognising the corresponding mRNA codon via its anticodon loop. Each tRNA has a characteristic cloverleaf secondary structure that folds into an L-shaped three-dimensional conformation; the 3' CCA end accepts the amino acid (charged by aminoacyl-tRNA synthetase), while the anticodon loop base-pairs with the mRNA codon in the ribosomal A site. There are at least 61 functional tRNA species in cells (matching all sense codons), and their accurate aminoacylation is essential for translational fidelity.
Translation (biology)
Translation is the process by which a ribosome decodes the nucleotide sequence of a messenger RNA (mRNA) and synthesizes the corresponding sequence of amino acids to produce a polypeptide chain. It occurs in three phases — initiation, elongation, and termination — and takes place at ribosomes in the cytoplasm of both prokaryotes and eukaryotes. The genetic code, read in triplets called codons, determines which amino acid is incorporated at each step, with transfer RNA (tRNA) molecules acting as adaptors between the mRNA codons and the amino acids.
Transcription (biology)
Transcription is the first step of gene expression in which a specific segment of DNA is copied into RNA (messenger RNA, mRNA) by the enzyme RNA polymerase. The process occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes, and involves three stages: initiation at the promoter, elongation of the RNA strand, and termination at a specific sequence. The resulting pre-mRNA in eukaryotes undergoes processing (5' capping, polyadenylation, and splicing) before being exported to the cytoplasm for translation.
Ribosomal RNA combines "ribosome" (from ribose + -some, Greek for "body") with RNA. The term ribosome itself was coined by Richard Roberts at a 1958 symposium to replace the earlier "microsomal particle," and rRNA came into use concurrently as the RNA component of ribosomes was characterised.