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This lecture on Microbial Genetics (MICR340) delves into the fundamental processes of gene expression, focusing on protein translation and the structures of proteins. You'll learn about amino acids and their linkage through peptide bonds, leading to polypeptides. The lecture outlines the different levels of protein structure: primary, secondary, tertiary, and quaternary. Additionally, it covers the mechanics of translation, including ribosome composition, the role of tRNA, and the initiation, elongation, and termination phases of protein synthesis. Gain insights into how proteins are formed from nucleotide sequences.
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Microbial Genetics (Micr340) Lecture 4 Gene Expression: Translation
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
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
Translation (protein synthesis) • Occurs on ribosomes • Ribosome is huge: • 3 different rRNAs • over 50 proteins
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
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
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
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
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
