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