Chapter 18 - The mechanism of translation II: Elongation and termination. This chapter consider some fundamental questions about the nature of elongation: (1) In what direction is a polypeptide synthesized? (2) In what direction does the ribosome read the mRNA? (3) What is the nature of the genetic code that dictates which amino acids will be incorporated in response to the mRNA? | Molecular Biology Fourth Edition Chapter 18 The Mechanism of Translation II: Elongation and Termination Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Direction of Polypeptide Synthesis and mRNA Translation Messenger RNAs are read in the 5’ 3’ direction This is the same direction in which they are synthesized Proteins are made in the amino carboxyl direction This means that the amino terminal amino acid is added first 18- Strategy to Determine Direction of Translation 18- The Genetic Code The term genetic code refers to the set of 3-base code words (codons) in mRNA that represent the 20 amino acids in proteins Basic questions were answered about translation in the process of “breaking” the genetic code 18- Nonoverlapping Codons Each base is part of at most one codon in nonoverlapping codons In an overlapping code, one base may be part of two or even three codones . | Molecular Biology Fourth Edition Chapter 18 The Mechanism of Translation II: Elongation and Termination Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Direction of Polypeptide Synthesis and mRNA Translation Messenger RNAs are read in the 5’ 3’ direction This is the same direction in which they are synthesized Proteins are made in the amino carboxyl direction This means that the amino terminal amino acid is added first 18- Strategy to Determine Direction of Translation 18- The Genetic Code The term genetic code refers to the set of 3-base code words (codons) in mRNA that represent the 20 amino acids in proteins Basic questions were answered about translation in the process of “breaking” the genetic code 18- Nonoverlapping Codons Each base is part of at most one codon in nonoverlapping codons In an overlapping code, one base may be part of two or even three codones 18- No Gaps in the Code If the code contained untranslated gaps or “commas”, mutations adding or subtracting a base from the message might change a few codons Would still expect ribosome to be back “on track” after the next such comma Mutations might frequently be lethal Many cases of mutations should occur just before a comma and have little, if any, effect 18- Frameshift Mutations Frameshift mutations Translation starts AUGCAGCCAACG Insert an extra base AUXGCAGCCAACG Extra base changes not only the codon in which is appears, but every codon from that point on The reading frame has shifted one base to the left Code with commas Each codon is flanked by one or more untranslated bases Commas would serve to set off each codon so that ribosomes recognize it Translation starts AUGZCAGZCCAZACGZ Insert an extra base AUXGZCAGZCCAZACGZ First codon wrong, all others separated by Z, translated normally 18- Frameshift Mutation Sequences 18- The Triplet Code The genetic code is a set of .
