The Spontaneous Mutation Rate in E $\text { The Genetic Code }$

question 43

Essay

The spontaneous mutation rate in E.coli was determined to be 1 mistake for every 10⁹ nucleotides copied.This was determined by measuring the frequency of a particular AT-to-GC change.This was accomplished using a strain of E.coli that started out unable to produce histidine (His^-) because of an inserted UGA stop codon that disrupted the region coding for an enzyme required to produce histidine.When a spontaneous mutation arose that enabled the UGA stop codon to code for tryptophan, the E.coli cells were then able to produce the enzyme required for histidine production.
You discover that if the stop codon were to change to code for cysteine (instead of tryptophan), this change would also allow the bacteria to produce histidine.How would the previously calculated spontaneous mutation rate of 1 mistake every 10⁹ nucleotides copied change, given this new information? Explain.(The codon table is shown in Figure 9-48 to help you answer this question.) $\text { The Genetic Code }$
$\begin{array}{|l|l|l|l|l|}\hline & \text { U } & \text { C } & \text { A } & \text { G } \\\hline &\text { UUU Phe (F) } & \text { UCU Ser (S) } & \text { UAU Tyr (Y) } & \text { UGU Cys (C) } \\&\text { UUC - } & \text { UCC - } & \text { UAC - } & \text { UGC - } \\ \text { U } &\text { UUA Leu (L) } & \text { UCA -- } & \text { UAA Stop } & \text { UGA Stop } \\&\text { UUG - } & \text { UCG - } & \text { UAG Stop } & \text { UGG Trp (W) } \\\hline &\text { CUU Leu (L) } & \text { CCU Pro (P) } & \text { CAU His (H) } & \text { CGU Arg (R) } \\&\text { CUC - } & \text { CCC - } & \text { CAC - } & \text { CGC - } \\ \text { C}&\text { CUA - } & \text { CCA - } & \text { CAA Gin (Q) } & \text { CGA - } \\&\text { CUG }- & \text { CCG }- & \text { CAG }- & \text { CGG }- \\\hline &\text { AUU Ile (I) } & \text { ACU Thr (T) } & \text { AAU Asn (N) } & \text { AGU Ser (S) } \\&\text { AUC - } & \text { ACC - } & \text { AAC - } & \text { AGC - } \\ \text { A}&\text { AUA - } & \text { ACA - } & \text { AAA Lys (K) } & \text { AGA Arg (R) } \\&\text { AUG Met (M) } & \text { ACG - } & \text { AAG - } & \text { AGG - } \\\hline &\text { GUU Val (V) } & \text { GCU Ala (A) } & \text { GAU Asp (D) } & \text { GGU Gly (G) } \\&\text { GUC - } & \text { GCC - } & \text { GAC - } & \text { GGC - } \\ \text { G}&\text { GUA - } & \text { GCA - } & \text { GAA Glu (E) } & \text { GGA - } \\&\text { GUG - } & \text { GCG - } & \text { GAG - } & \text { GGG - } \\\hline\end{array}$
Figure 9-48

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Definitions:

Heterozygous Dihybrid

An individual possessing two different alleles for two traits, resulting from the cross of parents differing in two specific traits.

Genotype

The genetic constitution of an organism, determining specific characteristics by encoding instructions for proteins.

AaMm

Represents a genotype consisting of two different alleles, Aa, for one trait and two different alleles, Mm, for another trait, indicating a hybrid individual.

Dominant Allele

An allele that expresses its phenotype even in the presence of a different (recessive) allele at the same genetic locus in a heterozygote.