1、Chapter 7Using the genetic code7.1 Introduction7.2 Codon-anticodon recognition involves wobbling7.3 tRNA contains modified bases that influence its pairing properties7.4 (There are sporadic alterations of the universal code)7.5 tRNAs are charged with amino acids by synthetases7.6 Accuracy depends on
2、 proofreading7.7 Suppressor tRNAs have mutated anticodons that read new codons7.8 The accuracy of translation7.9 tRNA may influence the reading frameStop codons are the three triplets (UAA, UAG, UGA) which terminate protein synthesis.7.1 IntroductionFigure 7.1 All the triplet codons have meaning: 61
3、 represent amino acids, and 3 cause termination (STOP).7.1 IntroductionFigure 7.2 The number of codons for each amino acid does not correlate closely with its frequency of use in proteins.7.1 IntroductionWobble hypothesis accounts for the ability of a tRNA to recognize more than one codon by unusual
4、 (non-GC, non-AT) pairing with the third base of a codon.7.2 Codon-anticodon recognition involves wobblingFigure 7.3 Third bases have the least influence on codon meanings. Boxes indicate groups of codons within which third-base degeneracy ensures that the meaning is the same.7.2 Codon-anticodon rec
5、ognition involves wobblingFigure 7.4 Wobble in base pairing allows G-U pairs to form between the third base of the codon and and the first base of the anticodon.7.2 Codon-anticodon recognition involves wobblingFigure 7.5 Codon-anticodon pairing involves wobbling at the third position.7.2 Codon-antic
6、odon recognition involves wobblingModification of DNA or RNA includes all changes made to the nucleotides after their initial incorporation into the polynucleotide chain.7.3 tRNA contains modified bases that influence its pairing propertiesFigure 7.6 All of the four bases in tRNA can be modified.7.3
7、 tRNA contains modified bases that influence its pairing propertiesFigure 7.7 Modification to inosine allows pairing with U, C, and A.7.3 tRNA contains modified bases that influence its pairing propertiesFigure 7.8 Modification to 2-thiouridine restricts pairing to A alone because only one H-bond ca
8、n form with G.7.3 tRNA contains modified bases that influence its pairing propertiesFigure 7.7 Modification to inosine allows pairing with U, C, and A.7.3 tRNA contains modified bases that influence its pairing propertiesFigure 7.9 Changes in the genetic code in mitochondria can be traced in phyloge
9、ny. The minimum number of independent changes is generated by supposing that the AUA=Met and the AAA=Asn changes each occurred independently twice, and that the earlyAUA=Met change was reversed in echinoderms.7.4 There are sporadic alterations of the universal codeFigure 7.9 Changes in the genetic c
10、ode in mitochondria can be traced in phylogeny. The minimum number of independent changes is generated by supposing that the AUA=Met and the AAA=Asn changes each occurred independently twice, and that the earlyAUA=Met change was reversed in echinoderms.7.4 There are sporadic alterations of the unive
11、rsal codeCognate tRNAs are those recognized by a particular aminoacyl-tRNA synthetase.Isoaccepting tRNAs represent the same amino acid.7.5 tRNAs are charged with amino acids by synthetasesFigure 7.10 An aminoacyl-tRNA synthetase charges tRNA with an amino acid.7.5 tRNAs are charged with amino acids
12、by synthetasesFigure 7.11 An aminoacyl-tRNA synthetase contains three or four regions with different functions. (Only multimeric synthetases possess an oligomerization domain.)7.5 tRNAs are charged with amino acids by synthetasesFigure 7.12 Crystal structures show that class I and class II aminoacyl
13、-tRNA synthetases bind the opposite faces of their tRNA substrates. The tRNA is shown in red, and the protein in blue. Photographs kindly provided by Dino Moras.7.5 tRNAs are charged with amino acids by synthetasesFigure 7.13 A class I tRNA synthetase contacts tRNA at the minor groove of the accepto
14、r stem and at the anticodon.7.5 tRNAs are charged with amino acids by synthetasesFigure 7.14 A class II aminoacyl-tRNA synthetase contacts tRNA at the major groove of the acceptor helix and at the anticodon loop.7.5 tRNAs are charged with amino acids by synthetasesProofreading refers to any mechanis
15、m for correcting errors in protein or nucleic acid synthesis that involves scrutiny of individual units after they have been added to the chain.7.6 Accuracy depends on proofreadingFigure 7.15 Recognition of the correct tRNA by synthetase is controlled at two steps. First, the enzyme has a greater af
16、finity for its cognate tRNA. Second, the aminoacylation of the incorrect tRNA is very slow.7.6 Accuracy depends on proofreadingFigure 7.16 When a synthetase binds the incorrect amino acid, proofreading requires binding of the cognate tRNA. It may take place either by a conformation change that causes hydrolysis of the incorrect aminoacyl-adenylate, or by transfer of the amino acid to RNA, following by hydrolysis.7.6 Accuracy depends on proofreadingFigure 7.17 The accuracy of charging tRNAIle by
