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Structure and Functions of RNA

1. General Structure of RNA

RNA (ribonucleic acid) is a single-stranded, unbranched polymer composed of nucleoside monophosphates joined by 3'-to-5' phosphodiester bonds. Key differences from DNA:
  • Contains ribose sugar (not deoxyribose)
  • Contains uracil (U) instead of thymine (T)
  • Exists as a single strand that folds into complex 2D and 3D structures
  • Is considerably smaller than DNA
  • Adenine pairs with uracil (A-U pairing) wherever the strand loops back on itself - this intrachain base pairing is important for function

2. Ribosomal RNA (rRNA) - Structure and Function

  • Makes up ~80% of total cellular RNA - the most abundant type
  • Found in ribosomes (the sites of protein synthesis) in association with ribosomal proteins
  • Prokaryotes: 3 species - 23S, 16S, and 5S (S = Svedberg unit)
  • Eukaryotes: 4 nuclear species - 28S, 18S, 5.8S, and 5S; plus 2 mitochondrial species (12S, 16S)
  • Function: Structural scaffold of the ribosome; certain rRNA act as ribozymes (catalytic RNA) - e.g., peptidyl transferase activity in protein synthesis

3. Transfer RNA (tRNA) - Structure and Function

  • The smallest RNA (~4S); makes up ~15% of total cellular RNA
  • At least one specific tRNA exists for each of the 20 amino acids
  • Secondary structure: Characteristic cloverleaf pattern due to extensive intrachain base pairing
  • Tertiary structure: Folded L-shaped form in the cell
  • Contains a high proportion of unusual/modified bases (e.g., dihydrouracil, pseudouracil)
  • Function: Adaptor molecule - carries its specific amino acid attached to its 3' end, then recognizes the complementary codon on mRNA during translation
  • Clinical note: Mutations in mitochondrial tRNA genes cause diseases like MERRF and MELAS

4. Messenger RNA (mRNA) - Structure and Function

  • Makes up only ~5% of total RNA, but is the most heterogeneous in size and sequence
  • Function: Carries genetic information from DNA to the ribosome for protein synthesis (translation)
  • Structure of eukaryotic mRNA (5' to 3'):
    • 5' cap: 7-methylguanosine attached via a 5'-to-5' triphosphate linkage (added post-transcriptionally)
    • 5' UTR (untranslated region)
    • Coding region: Start codon → amino acid codons → stop codon
    • 3' UTR
    • Poly-A tail: Up to 200 adenine nucleotides at the 3' end (added post-transcriptionally)
  • Eukaryotic mRNA is monocistronic (one gene per mRNA); prokaryotic is polycistronic (multiple genes)
  • Precursor form = hnRNA/pre-mRNA, processed in the nucleus before export to cytoplasm

5. Non-coding RNAs (ncRNA) and Ribozymes

Beyond the 3 major types, several smaller RNA molecules perform specialized roles:
RNA TypeLocationFunction
snRNA (small nuclear)NucleusRNA splicing (part of spliceosome)
snoRNA (small nucleolar)NucleolusrRNA processing and modification
miRNA (microRNA)CytoplasmGene regulation - silences gene expression
RibozymesVariousCatalytic RNA - cleave other RNA or catalyze peptide bond formation
Ribozymes are RNA molecules with enzymatic activity - they can catalyze reactions (e.g., self-splicing introns, rRNA peptidyl transferase). This shows RNA serves both informational and catalytic roles in the cell.
Sources: Lippincott Illustrated Reviews: Biochemistry, 8th ed.; Basic Medical Biochemistry: A Clinical Approach, 6th ed.
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