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Microbial Genetics (Micr340)

Microbial Genetics (Micr340) . Lecture 4 Gene Expression: Translation. Proteins. Protein structure. Its basic monomer (“link” of a chain): amino acid (a.a) Amino acids are linked by peptide bonds, forming polypeptides (a short chain of amino acids - oligopeptide).

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Microbial Genetics (Micr340)

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  1. Microbial Genetics (Micr340) Lecture 4 Gene Expression: Translation

  2. Proteins

  3. Protein structure • Its basic monomer (“link” of a chain): amino acid (a.a) • Amino acids are linked by peptide bonds, forming polypeptides (a short chain of amino acids - oligopeptide). • Peptide chains have direction (orientation) too; N-terminus (amino terminus with an unattached amino group while C terminus has an unattached carboxyl group

  4. Peptide bond formation

  5. Protein structure • Primary structure: sequence of a.a. • Secondary structure: parts of peptide chain are held together by H-bonds – a helices and b-sheets. • Tertiary structure: various regions of peptide folds up on itself; hydrophobic a.a. inside, hydrophilic a.a. outside • Quaternary structure: proteins are made up by multiple polypeptides -multimeric

  6. Translation

  7. Translation (protein synthesis) • Occurs on ribosomes • Ribosome is huge: • 3 different rRNAs • over 50 proteins

  8. Composition of ribosome

  9. Translation (protein synthesis) • Open reading frame (ORF) • Three nucleotides form a codon • One codon encodes a specific amino acid • For a given DNA fragment, there are three different ways (or reading frames) that nucleotide sequences translated into protein sequences

  10. Aminoacylation of tRNA

  11. Translation initiation • Translational initiation regions (TIRs) • All TIRs have an initiation codon; usually AUG or GUG • Initiation codons encode methionine • Many genes have Shine-Delgarno sequence (S-D) 5 to 10 nucleotides upstream of initiation codon. • S-D sequence usually A, G rich

  12. Translation Initiation

  13. Translation Initiation

  14. Translation elongation • During translation, ribosome moves 3 nucleotides at a time along mRNA • This leaves space open for another tRNA to enter; which type depends on the anticodon matching the next codon on mRNA • EF-Tu helps the correct tRNA enter the A (acceptor) site on the ribosome

  15. Translation elongation • Ribozyme peptidyltransferase catalyzes formation of a peptide bond between the incoming a.a. at the A site and the growing polypeptide at an adjacent site (P site) • EF-G (translocase) enters ribosome and moves polypeptide-containing tRNA to P site • The tRNA which has been displaced then moves to yet another site, E site before it exits the ribosome

  16. Overview of translation

  17. Overview of translation

  18. Translation termination • When ribosome moves to one of 3 stop codons (UAA, UAG and UGA), translation stops • The stop codons do not encode any a.a. so they have no corresponding tRNA; they are called nonsense codons • Termination requires release factors

  19. Translation termination

  20. Polycistronic mRNA

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