页数：37页文件大小：1.24 MB来源： 分子生物学基因Geneppt课件
本课件为《分子生物学》第7章遗传密码的使用（Using the genetic code），依据 Gene VII 教材编排，沿Codon-anticod等小节推进，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 7 Using the genetic code
Stop codons are the three triplets (UAA, UAG, UGA) which terminate protein synthesis.
Figure 7.1 All the triplet codons have meaning: 61 represent amino acids, and 3 cause termination (STOP).
Figure 7.2 The number of codons for each amino acid does not correlate closely with its frequency of use in proteins.
Wobble hypothesis accounts for the ability of a tRNA to recognize more than one codon by unusual (non-GC, non-AT) pairing with the third base of a codon.
Figure 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.
Figure 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.
Figure 7.5 Codon-anticod on pairing involves wobbling at the third position.
Modification of DNA or RNA includes all changes made to the nucleotides after their initial incorp or at ion into the poly nucleotide chain.
Figure 7.6 All of the four bases in tRNA can be modified.
Figure 7.7 Modification to inosine allows pairing with U, C, and A.
Figure 7.8 Modification to 2-thiouridine restricts pairing to A alone because only one H-bond can form with G.
Figure 7.7 Modification to inosine allows pairing with U, C, and A.
Figure 7.9 Changes in the genetic code 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.
Figure 7.9 Changes in the genetic code 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.
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Figure 7.10 An aminoacyl-tRNA synthetase charges tRNA with an amino acid.
Figure 7.11 An aminoacyl-tRNA synthetase contains three or four regions with different functions. (Only multimeric synthetases possess an oligomeriz at ion domain.).
Figure 7.12 Crystal structures show that class I and class II aminoacyl-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.
Figure 7.13 A class I tRNA synthetase contacts tRNA at the minor groove of the acceptor stem and at the anticodon.
Figure 7.14 A class II aminoacyl-tRNA synthetase contacts tRNA at the major groove of the acceptor helix and at the anticodon loop.
Proofreading refers to any mechanism for correcting errors in protein or nucleic acid synthesis that involves scrutiny of individual units after they have been added to the chain.
Figure 7.15 Recognition of the correct tRNA by synthetase is controlled at two steps. First, the enzyme has a gre a ter affinity for its cognate tRNA. Second, the aminoacyl at ion of the incorrect tRNA is very slow.
Figure 7.16 When a synthetase binds the incorrect amino acid, proofreading requires binding of the cognate tRNA. It may take place either by a conform at ion change that causes hydrolysis of the incorrect aminoacyl-adenylate, or by transfer of the amino acid to RNA, following by hydrolysis.
Figure 7.17 The accuracy of charging tRNAIle by its synthetase depends on error control at two stages.
Missense mutations change a single codon and so may cause the replacement of one amino acid by another in a protein sequence.
Figure 7.18 Nonsense mutations can be suppressed by a tRNA with a mutant anticodon, which inserts an amino acid at the mutant codon, producing a full length protein in which the original Leu residue has been replaced by Tyr.
Figure 7.19 Nonsense suppressor tRNAs are generated by mutations in the anticodon.
Figure 7.20 Missense suppression occurs when the anticodon of tRNA is mutated so that it responds to the wrong codon. The suppression is only partial because both the wild-type tRNA and the suppressor tRNA can respond to AGA.
Figure 7.21 Nonsense suppressors also read through natural termination codons, synthesizing proteins that are longer than wild-type.
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Figure 7.22 A +1 frame shift is required for expression of the tyb gene of the yeast Ty element. The shift occurs at a 7 base sequence at which two Leu codon(s) are followed by a scarce Arg codon.
Figure 7.22 A +1 frame shift is required for expression of the tyb gene of the yeast Ty element. The shift occurs at a 7 base sequence at which two Leu codon(s) are followed by a scarce Arg codon.
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1.The sequence of mRNA read in triplets 53 is related by the genetic code to the amino acid sequence of protein read from N-to C-terminus.2.Multiple tRNAs may respond to a particular codon.3.Each amino acid is recognized by a particular aminoacyl-tRNA synthetase, which also recognizes all of the tRNAs coding for that amino acid.
4.Aminoacyl-tRNA synthetases vary widely, but fall into two general groups according to the structure of the catalytic domain.5.Mutations may allow a tRNA to read different codons; the most common form of such mutations occurs in the anticodon itself.6.Frameshifts of the +1 type may be caused by aberrant tRNAs that read "codons" of 4 bases.
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