the genetic code: 61 triplet codons represent 20 amino acids...
TRANSCRIPT
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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 stop
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Fig. 7.21
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Mutations
1. Substitution-1 base --> one of the three other bases
Transition: purine --> purine or pyrimidine --> pyrimidine A--> G or G--> A T--> C or C--> T
Transvertion: purine --> pyrimidine or vice versa A--> T, C; G -->T,C; T-->A, G; C-->A,G
2. Deletion or insertion-often causes frameshift mutation
causes missense, nonsense, silent, neutral or
splicing mutational effects
3. Chromosomal rearrangementinversion or translocation can change multiple genes
4. Dynamic mutations-caused by DNA replication slippageof trinucleotide repeats-leading to expansion of thetrinucleotide repeats (ie. Fragile-X-syndrome)
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Effects of point mutations
tyrosine TAT, TAC
TAT -> CAT tyr -> his missense (nonsynonymous)TAT -> TAA tyr -> stop nonsenseTAT -> TTT tyr -> phe neutral in many casesTAT -> TAC tyr-> tyr silent (Synonymous)
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Fig. 7.22
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Fig. 8.15
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Fig. 8.16
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Fig. 7.2
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Spontaneous mutation
Mutational process is random and isunrelated to adaptive advantages
Selective techniques merely select formutants that preexist in a population
Mutation rates vary widely from one geneto another; mutational hot spots aremore likely to be mutated than others
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Spontaneous mutations
Spontaneous 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 bone
Oxidative damages: G --> 8-oxodG (pair with A)
c. chemical changes (hydrolysis):
depurination; A,G --> O
deamination: C--> U
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Fig. 7.8
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Fig. 7.7
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Mutagen treatment greatly increases the mutation rate
Exposure to X-ray, UV light
Chemical 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 Orange
Transposons that insert into a gene and disrupt the normal reading frame
Mutagens
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Fig. 7.12a1Chemical Mutagens
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Fig. 7.12a2
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Fig. 7.12b1
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Fig. 7.12b2
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Fig. 7.12c1
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Fig. 7.12c2