chapter 9 part A part 1

Roles of RNA in protein synthesis

• The template: Mature messenger RNA (mRNA)

  • Processed in the nucleus: Capped at 5' end, spliced to generate the full coding region (Open Reading Frame - ORF), and poly-adenylated at the 3' end.

  • Transported to the cytoplasm.

  • Coated with ribonucleoproteins (RNPs) in both nucleus and cytoplasm, involved in processing, transport, and stability.

• The translation "machines": Ribosomes

  • Large complexes of RNAs and proteins, composed of a large and a small subunit.

  • Eukaryotic ribosomes are 80S (S = Svedberg, a unit of sedimentation related to size and shape).

  • Large subunit (60S): 3 RNAs + 47 proteins.

  • Small subunit (40S): 1 RNA + 33 proteins.

  • Possess a common core with prokaryotic (70S) ribosomes.

  • RNAs within the core are key for ribosome function.

  • Capable of binding both the mRNA template and aminoacyl-tRNAs for translation.

• The amino acid carriers: Transfer RNAs (tRNAs)

  • Small RNA molecules with a specific structure:

  • 2D representation: "cloverleaf" with an anticodon loop (reads mRNA codon) and an acceptor stem (carries amino acid).

  • 3D structure: "L shape" with anticodon loop and acceptor stem on opposite ends.

  • At least one specific tRNA exists for each amino acid (50
    -100 different tRNAs in eukaryotic cells).

  • Form aminoacyl-tRNA complexes (aa-tRNAaa) by enzymes called aminoacyl tRNA synthetases, matching the amino acid to the proper codons.