RNA Structure and Types

RNA Structure Basics - Sugar, Base, Chain!

• RNA: Typically single-stranded polymer of ribonucleotides.
• Linkage: Covalent $3'-5'$ phosphodiester bonds form the backbone.
• Ribonucleotide = Ribose sugar + Phosphate group + Nitrogenous Base.
  • Ribose: Pentose sugar with a reactive $2'-OH$ group (DNA's deoxyribose has $2'-H$).
  • Bases:
    • Purines: Adenine (A), Guanine (G).
    • Pyrimidines: Cytosine (C), Uracil (U). 📌 RNA uses Uracil (U); DNA uses Thymine (T).
• Chain Polarity: Defined by $5'$ (phosphate) and $3'$ (hydroxyl) ends.

DNA vs. RNA

Major RNA Players - The RNA Trio

Three major RNA types are central to protein synthesis, each with distinct structures and functions:

⭐ Wobble Hypothesis: The third base of an mRNA codon can form non-canonical pairs with the first base of a tRNA anticodon. This allows a single tRNA to recognize multiple codons, optimizing translation.

Other Functional RNAs - RNA's Special Forces

• snRNA (Small nuclear RNA):
  • Function: Pre-mRNA splicing (spliceosome component).
  • Location: Nucleus.
• snoRNA (Small nucleolar RNA):
  • Function: rRNA modification (methylation, pseudouridylation).
  • Location: Nucleolus.
• miRNA (MicroRNA):
  • Function: Post-transcriptional gene silencing (mRNA degradation/repression).
  • Location: Cytoplasm.
• siRNA (Small interfering RNA):
  • Function: RNA interference (gene silencing, viral defense). Often exogenous.
  • Location: Cytoplasm.
• lncRNA (Long non-coding RNA):
  • Function: Diverse gene regulation (e.g., X-chromosome inactivation). >200 nt.
  • Location: Nucleus, cytoplasm.
• piRNA (Piwi-interacting RNA):
  • Function: Transposon silencing in germline cells.
  • Location: Germ cells (with Piwi proteins).

⭐ RNA interference (RNAi) mediated by siRNAs and miRNAs is a key mechanism for gene regulation and defense against viral infections.

RNA Folding Fun - Twists and Turns

RNA folds into complex 3D structures via:

• Stem-loop (Hairpin): Intrastrand base pairing forms a duplex stem and an unpaired loop.
• Internal loop: Unpaired bases between two helical regions.
• Bulge: Unpaired nucleotide(s) on one side of a helix.
• Junctions: Intersection of multiple helical segments.
• Pseudoknot: Loop bases pair with a distant single-stranded region.

  ⭐ Pseudoknots are crucial for telomerase RNA and some viral RNAs.

• Non-canonical base pairs: (e.g., G-U wobble, Hoogsteen) stabilize tertiary structures.

High‑Yield Points - ⚡ Biggest Takeaways

• RNA: Typically single-stranded; contains ribose sugar and Uracil (U) instead of Thymine.
• mRNA: Carries genetic code from DNA to ribosome for protein synthesis.
• tRNA: Cloverleaf structure with an anticodon loop; transports specific amino acids.
• rRNA: Most abundant RNA; structural and catalytic (ribozyme) component of ribosomes.
• hnRNA: Eukaryotic precursor of mRNA; undergoes splicing, capping, and polyadenylation.
• Small RNAs: snRNA (involved in splicing), miRNA/siRNA (gene silencing).