microbial genetics (micr340)
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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). - PowerPoint PPT PresentationTRANSCRIPT
Microbial Genetics (Micr340)
Lecture 4Gene 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).
Peptide chains have direction (orientation) too; N-terminus (amino terminus with an unattached amino group while C terminus has an unattached carboxyl group
Peptide bond formation
Protein structure
Primary structure: sequence of a.a. Secondary structure: parts of peptide
chain are held together by H-bonds – helices and -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
Translation (protein synthesis)
Occurs on ribosomes Ribosome is huge:
3 different rRNAs over 50 proteins
Composition of ribosome
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
Aminoacylation of tRNA
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 Initiation
Translation Initiation
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
Overview of translation
Overview of translation
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
Translation termination
Polycistronic mRNA