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Lesson 3: Translation

Understand the process of translating RNA into proteins, decoding the genetic code, ribosome functions, initiation of synthesis, and protein regulation in eukaryotic cells. Learn about ribosomes, mRNA translation mechanisms, prokaryotic vs. eukaryotic mRNA, polyribosomes, and protein import by organelles.

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Lesson 3: Translation

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  1. Lesson 3: Translation

  2. From RNA to protein: translation The genetic code Three possible “reading frames” THE ABC FOR THE DNA Insertion (X) or Deletion (B) THE AXB CFO RTH EDN A THE ACF ORT HED NA

  3. Ribosomes in the cytoplasm of a eukaryotic cell

  4. Composition of eukaryotic ribosomes

  5. RNA-binding sites in the ribosome • Each ribosome has: • a binding site for mRNA • three binding sites for tRNA • A-site: aminoacyl-tRNA • P-site: peptidyl-tRNA • E-site: exit

  6. tRNA molecules: matching amino acids to codons in mRNA

  7. Translation of the genetic code: two adaptors that act one after another

  8. mRNA translation mechanism Step1: An aminoacyl-tRNA molecule binds to the A-site on the ribosome Step2: A new peptide bond is formed Step3: The small subunit moves a distance of three nucleotides along the mRNA chain ejecting the spent tRNA molecule Step4: The next aminoacyl-tRNA molecule binds to the A-site on the ribosome Step5: . . .

  9. The initiation phase of protein synthesis in eukaryotes • Initiation complex (small ribosomal subunit + initiation factors) • binds DNA and searches for start codon • Large ribosomal subunit adds to the complex • Translation starts • . . .

  10. Prokaryotic vs eukaryotic mRNA molecules

  11. Structure of a typical prokaryotic mRNA molecule

  12. 5´ end capping of eukaryotic mRNA molecules

  13. The final phase of protein synthesis • binding of release factor to a stop codon terminates translation • the completed polypeptide is released • the ribosome dissociates into its two separate subunits

  14. Polyribosomes: several ribosomes can simultaneously translate the same mRNA molecule

  15. Proteasomes: degradation of “unwanted” proteins in eukaryotic cells

  16. The production of a protein by a eukaryotic cell Many levels of regulation/variation

  17. Protein import by membrane-bounded organelles ATP !!!

  18. The role of signal sequences in protein sorting • proteins destined for the ER: N-terminal signal sequence that directs them • proteins destined to remain in the cytosol: no signal sequence

  19. Free and membrane-bound ribosomes

  20. ER signal sequence and SRP: directing ribosomes to the ER membrane

  21. Translocation of a soluble protein across the ER membrane

  22. Integration of a transmembrane protein into the ER membrane

  23. Integration of a double-pass transmembrane protein into the ER membrane

  24. Vesicular traffic

  25. Clathrin-coated pits and vesicles

  26. Selective transport: mediated by clathrin-coated vesicles

  27. Model of transport vesicle docking

  28. Transport vesicle fusion

  29. Protein glycosylation in the ER

  30. The Golgi apparatus

  31. Exocytosis: the regulated and constitutive pathways

  32. Exocytosis of secretory vesicles

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