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Lecture

Chapter 9-3.docx

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Department
Biology
Course Code
BIOL 2000
Professor
Joseph Rasmussen

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Chapter 9-3 Genetic Code  Standard genetic code  Chain termination (‘Stop”) codons are shown in orange o UAA o UAG o UGA  Not for amino acids  Usual initiation “start” codon, AUG is shown in dark green Possible exam question o Look at mRNA and predict polypeptide formation o 5’ GAA ACA UGU 3’  Amino Terminus - Glu Thr Cys – Carboxy terminus o Where is the terminus  5’ end has amino terminus  3’ end has carboxyl terminus Degeneracy and the genetic code  Unambiguous o One codon, one amino acid  Degenerate o One amino acid, more than one carbon  More then one way of specifying the insertion of an amino acid Transfer RNA (tRNA) is the adaptor molecule  Base pairing “deciphers” the genetic code o mRNA codon : tRNA anti-codon interaction  Different tRNAs have different anti-codon sequences  At least 20 different tRNAs o Different tRNA for each 20 amino acids  Amino acids are covalently attached to tRNAs o Charging Structure of tRNA  Cloverleaf structure o This specific tRNA is always attached to Alanine o Always at the same site  Anti-codon sequence changes at the amino acid changes  Amino acid depends on the mRNA Specificity  Binds only one amino acid (cognate) o Cognate- the correct one  Binds only one cognate tRNA(s) o Looks at the R group  ATP is required during the charging reaction  The accuracy of attaching the correct amino acid to the correct tRNA is critical foe ensuring translation accuracy Structure of transfer RNA  Inverted L shape comes from loop interactions Genetic code is degenerate  Amino acid serine is specified by 6 different codons  How does the cellular tRNA set relate to the degeneracy? o Don’t need 6 to recognize all 6 codons  Usually only 3 tRNAs  Due to wobble base pairing & Watson-crick interaction  Wobble base pairing o When a G anti-codon serves a U codon  Unusual How does the cellular tRNA set relate to the degeneracy?  1) More then one tRNA species can be charged with the same amino acid o Isoa
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