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Genetic Code

Genetic Code. Codons composed of three nucleotides in RNA Codon specifies amino acid or stop Genetic code is redundant. Genetic Code Table. Reading Frames. One of three possible reading frames contains protein message. Mutations In Protein-Coding Sequences.

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Genetic Code

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  1. Genetic Code • Codons composed of three nucleotides in RNA • Codon specifies amino acid or stop • Genetic code is redundant

  2. Genetic Code Table

  3. Reading Frames • One of three possible reading frames contains protein message

  4. Mutations In Protein-Coding Sequences Missense Change to codon for different amino acid Silent Change to codon for same amino acid Nonsense Change to stop codon Insertion/ Usually disrupt reading frame deletion

  5. Transfer RNA • Adaptors for associating codons with amino acids • Cloverleaf structure • Anticodon recognizes codon by complementary base-pairing

  6. Wobble Base-pairing • Relaxed base-pairing at third position of codon • tRNA may recognize multiple codons

  7. Modified Nucleotides In tRNA • post-transcriptional modifications

  8. Aminoacyl-tRNA Synthetases • Different synthetase for each amino acid • Amino acid attached to tRNA by high energy ester bond

  9. Sequential Action Of Adaptors

  10. Editing By Synthetases • Active site may mistakenly bind related amino acid • Incorrect amino acid is hydrolyzed at editing site

  11. Growth Of Polypeptide • Stepwise growth from N-terminus to C-terminus

  12. Ribosome Structure • Large and small subunits • Composed of rRNA and proteins

  13. Translation On Ribosome • P site contains tRNA attached to growing polypeptide • A site binds incoming aminoacyl-tRNA • Peptide bond formation by peptidyl transferase of ribosome • Translocation of ribosome

  14. Elongation Factors • EF-Tu:GTP interacts with A site • Codon-anticodon interaction leads to hydrolysis of GTP and dissociation of EF-Tu • EF-G promotes ribosome translocation

  15. Diphtheria Toxin • Transfer of adenosine diphosphate ribose to EF-2 • Inhibits EF-2, which impairs ribosome translocation

  16. Initiation In Eucaryotes • Methionine-linked initiator tRNA to small ribosome P site (eIF-2) • Load onto mRNA 5’ end (eIF-4E, eIF-4G, polyA) • Scan for AUG • Assemble large ribosomal subunit

  17. Initiation In Bacteria • Formylmethionine-linked initiator tRNA • Small ribosome subunit binds to Shine-Dalgarno sequence • Polycistronic mRNAs

  18. Termination • Stop codons not recognized by tRNA • Release factors bind to A site • Addition of H2O to peptidyl-tRNA

  19. Polyribosomes • Multiple ribosomes translating same mRNA • Interaction of mRNA 5’ and 3’ ends

  20. Selenocysteine • Selenocysteine tRNA is charged with serine that is subsequently converted • Encoded by UGA codon followed by special signal

  21. Antibiotics Tetracycline blocks binding of aminoacyl- tRNA to A-site of ribosome Streptomycin prevents the transition from initiation complex to chain-elongating ribosome; causes miscoding Chloramphenicol blocks the peptidyl transferase reaction on ribosomes Erythromycin blocks the translocation reaction on ribosomes Rifamycin blocks initiation of RNA chains by binding to RNA polymerase

  22. Folding During Synthesis • Individual domains folded rapidly after their synthesis

  23. Creating Functional Proteins • Association with cofactors, proteins • Covalent modifications

  24. Molecular Chaperones • Prevent inappropriate aggregation during folding • Recognize exposed hydrophobic regions • Hydrolyze ATP • Heat shock proteins

  25. Hsp70 Chaperones • Act early • Repeated cycles of target binding & release mediated by ATP binding & hydrolysis

  26. Hsp60 Chaperones • Chaperonins • Isolation chambers • Cycles of target confinement & release mediated by ATP binding & hydrolysis

  27. Protein Quality Control • Exposed hydrophobic regions indicate misfolding • Selective degradation of proteins that cannot be correctly folded

  28. Proteasome • Protease activity in interior of cylinder • Caps function in ATP-dependent unfolding and as gates for selective entry

  29. Attachment Of Ubiquitin • Attached to lysine on target; multiubiquitin chains • Ubiquitin attaches to E1 • Transferred to E2 of ubiquitin ligase • E3 of ubiquitin ligase recognizes degradation signal

  30. Regulated Degradation • Modification of an E3 protein • Modification of a target protein

  31. Disease From Protein Aggregation • Aggregates of misfolded proteins • Cross-beta filaments • Neurodegeneration- Huntington’s, Alzheimer’s, Prion diseases (Creutzfeldt-Jacob, bovine spongiform encephalopathy)

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