《分子生物学 Molecular Biology》课程教学课件（PPT讲稿，英文版）Chapter 18 The Mechanism of Translation II - Elongation and Termination

Chapter 18 The Mechanism of Translation Il: Elongation and Termination Copyright The McGraw-Hill Companies,Inc.Permission r d for reproduction or dis

Chapter 18 The Mechanism of Translation II: Elongation and Termination

18.1 Direction of Polypeptide Synthesis and mRNA Translation ·Messenger RNAs are read in the5'→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 18-2

18-2 18.1 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

Strategy to Determine Direction of Translation Copyright The McGraw-Hill Companies,Inc.Permission required for reproduction or display. (a) HJleucine just added Finish abeling 5N8 。8 period N 00000 0000。●0 1↑2↑3↑4↑5↑6 Trypsin,isolate peptides, plot label vs.peptide position (N-term.) Peptide number (C-term.) 18-3

18-3 Strategy to Determine Direction of Translation

18.2 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 18-4

18-4 18.2 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

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 18-5

18-5 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

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 ·Vould 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 18-6

18-6 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

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 18-7

18-7 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

Frameshift Mutation Sequences Copyright The McGraw-Hill Companies,Inc.Permission required for reproduction or display. 1.Wild-type: CATCAT CAT'CATCAT 2.Add a base: CAG TCA TCATCA TCA 3.Delete a base: CAT ICTCIATCIATCIATC 4.Cross #2 and #3:CAGITCT ICATICAT ICAT 5.Add 3 bases: CAGIGGTICAT ICAT ICAT 18-8

18-8 Frameshift Mutation Sequences

The Triplet Code The genetic code is a set of three-base code words,or codons -In mRNA,codons instruct the ribosome to incorporate specific amino acids into a polypeptide Code is nonoverlapping -Each base is part of only one codon Devoid of gaps or commas -Each base in the coding region of an mRNA is part of a codon 18-9

18-9 The Triplet Code • The genetic code is a set of three-base code words, or codons – In mRNA, codons instruct the ribosome to incorporate specific amino acids into a polypeptide • Code is nonoverlapping – Each base is part of only one codon • Devoid of gaps or commas – Each base in the coding region of an mRNA is part of a codon

Coding Properties of Synthetic mRNAs CopyrightThe McGraw-Hill Companies,Inc.Permission required for reproduction or display. (a) UCUCUCUCUCUc Ser Leu Ser Leu (b) DUCUUCUUCUUC or UUCUUCUUcUuc 0 DUCUUCUUCUUc PhePhe Phe Phe Ser Ser Ser LeuLeuLeu UAUCUAUCUAUC (c) Tyr Leu Ser lle 18-10

18-10 Coding Properties of Synthetic mRNAs