The Genetic Code: 61 tripletThe Genetic Code: 61 triplet codons codons represent 20 amino acids; represent 20 amino acids; 3 triplet3 triplet codons codons signify stopsignify stopFig. 7.21Mutations1. Substitution-1 base --> one of the three other basesTransition: purine --> purine or pyrimidine --> pyrimidine A--> G or G--> A T--> C or C--> TTransvertion: purine --> pyrimidine or vice versa A--> T, C; G -->T,C; T-->A, G; C-->A,G2. Deletion or insertion-often causes frameshift mutationcauses missense, nonsense, silent, neutral or splicing mutational effects 3. Chromosomal rearrangementinversion or translocation can change multiple genes4. Dynamic mutations-caused by DNA replication slippageof trinucleotide repeats-leading to expansion of thetrinucleotide repeats (ie. Fragile-X-syndrome)Effects of point mutations tyrosine TAT, TACTAT -> CAT tyr -> his missense (nonsynonymous)TAT -> TAA tyr -> stop nonsenseTAT -> TTT tyr -> phe neutral in many casesTAT -> TAC tyr-> tyr silent (Synonymous)Fig. 7.22Fig. 8.15Fig. 8.16Fig. 7.2Spontaneous mutationMutational process is random and isunrelated to adaptive advantagesSelective techniques merely select formutants that preexist in a populationMutation rates vary widely from one geneto another; mutational hot spots aremore likely to be mutated than othersSpontaneous mutationsSpontaneous mutation is rare: 2-12X 10-6 (per generation per gene)Spontaneous mutations can be caused by a. mistakes made during DNA replication (error rate 10-9)b. environmrntal effect: UV light: thymidine dimer X-ray: break sugar-phosphate DNA back boneOxidative damages: G --> 8-oxodG (pair with A)c. chemical changes (hydrolysis):depurination; A,G --> Odeamination: C--> UFig. 7.8Fig. 7.7Mutagen treatment greatly increases the mutation rateExposure to X-ray, UV lightChemical treatment: base analogs 5’-bromouracil (=T or rarely C) hydroxylating agent (add OH-group to C) alkylating agent such as E!MS (ethylmethane sulfonate) deaminating agent such as nitrous acid intercalating agent such as Acridine OrangeTransposons that insert into a gene and disrupt the normal reading frame MutagensFig. 7.12a1Chemical MutagensFig. 7.12a2Fig. 7.12b1Fig. 7.12b2Fig. 7.12c1Fig.
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