# RNA Types and Functions > RNA types and functions in MBBS Biochemistry: mRNA, tRNA, rRNA structure, snRNA, miRNA and ribozymes with exam-ready comparisons. - Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/rna-types-and-functions - Exam / course: MBBS · Subject: Biochemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "RNA Types and Functions", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/rna-types-and-functions ## Direct answer Messenger, transfer and ribosomal RNA divide the labour of gene expression between them: mRNA (about 5 percent of cellular RNA) carries the code as a cap-and-tailed transcript; tRNA (about 15 percent) is the cloverleaf adapter that reads codons through its anticodon loop and carries a specific amino acid esterified to its 3' CCA end; and rRNA (about 80 percent by mass) is both the structural scaffold and — through the 28S peptidyl transferase activity — the catalytic core of ribosomes. Beyond the big three, small nuclear RNAs splice pre-mRNA as part of spliceosomes, small nucleolar RNAs modify rRNA, microRNAs of about 22 nucleotides silence genes post-transcriptionally, and long non-coding RNAs regulate chromatin and imprinting. RNA's 2'-hydroxyl group makes it chemically more reactive than DNA and permits the catalytic structures called ribozymes. ## What you must remember - **mRNA architecture (eukaryotic):** 7-methylguanosine cap joined 5'-to-5' by a triphosphate bridge, coding sequences interrupted by introns, and a 3' poly-A tail of 100-250 adenylates added after AAUAAA-directed cleavage; monocistronic in eukaryotes, often polycistronic in bacteria. - **tRNA details:** 73-93 nucleotides; cloverleaf secondary structure with DHU, anticodon and T psi C loops, L-shaped tertiary fold; the 3' CCA is added post-transcriptionally and accepts the amino acid; unusual bases include pseudouridine, dihydrouridine and inosine. - **Ribosome numbers (eukaryote):** 80S ribosome splits into 60S (28S, 5.8S, 5S) and 40S (18S) subunits; bacterial 70S into 50S and 30S. - **Wobble chemistry:** inosine in the anticodon's first position pairs with U, C or A at the codon's third position, which lets about 40-50 tRNA species read all 61 sense codons. - **Small RNAs:** snRNA U1 recognises the GU donor at the 5' splice site within the spliceosome; snoRNA guides 2'-O-methylation and pseudouridylation of rRNA; miRNA assembles into RISC to degrade or repress partially complementary messages. - **Ribozymes:** peptidyl transferase (28S rRNA), RNase P, self-splicing group I and II introns and hammerhead motifs — evidence for an RNA world. - **Quality dimension:** mRNA half-lives range from minutes (c-fos, cytokines) to many hours (globin), and this turnover is itself a regulated control point. ## How to work through an RNA-based-therapy case Consider how the COVID-19 mRNA vaccines were possible and why your older textbooks said RNA could not be a drug. The engineering maps exactly onto tRNA and mRNA structure notes: the open reading frame was synthesised with modified nucleosides (N1-methylpseudouridine) so innate RNA sensors would not fire; the 5' cap and a long poly-A tail plus UTR optimisation gave translation stability; and lipid nanoparticles replaced the nucleus as a delivery vehicle because the cargo needs only cytosolic ribosomes, no genome entry. The same logic in reverse explains antisense oligonucleotides and siRNA drugs, which exploit Watson-Crick pairing to destroy or block a chosen message. Then take the exam-standard anticodon question: a tRNA mutated to read a stop codon as sense extends the protein past its normal terminus — nonsense suppression; the same wobble chemistry that makes the code economical makes mistranslation possible, which is why aminoacyl-tRNA synthetases carry editing sites. ## Where students slip Students give rRNA "the most types" and mRNA "the least": correct order to quote is rRNA most abundant (about 80 percent), tRNA next (about 15 percent), mRNA least (about 5 percent). Second, the eukaryotic ribosome's catalytic centre is the 28S rRNA of the large subunit — writing "ribosomal protein" as the peptidyl transferase is an error no examiner forgives, and it is also why linezolid and chloramphenicol, which act at that centre, are bacteriostatic or bactericidal by binding RNA, not protein. Third, the poly-A tail is added enzymatically after transcript cleavage and is not encoded in the gene; a surprising number of answers say it is templated. ## Frequently asked questions ### Which RNA is the most abundant in a cell? Ribosomal RNA, roughly 80 percent of total cellular RNA by mass, followed by transfer RNA and then messenger RNA. ### What structural features define a tRNA molecule? A 73-93 nucleotide chain folded into a cloverleaf with DHU, anticodon and T psi C loops, an L-shaped tertiary structure, unusual bases such as pseudouridine, and a 3' CCA amino-acid acceptor end. ### Which RNA catalyses peptide bond formation? The 28S ribosomal RNA of the large subunit, acting as a ribozyme — the peptidyl transferase centre contains no protein at the catalytic site. ### What do microRNAs do? About 22-nucleotide RNAs loaded into the RISC complex that degrade messenger RNA or block its translation when partially complementary, a major post-transcriptional control layer. ### Why does mRNA need a cap and a poly-A tail? The 7-methylguanosine cap enables ribosome binding and protects against 5' exonucleases, while the poly-A tail stabilises the transcript and aids nuclear export and translation efficiency.