DNA precursor imbalances are mutagenic. For example, if dGTP accumulates, it can compete with dATP for incorporation opposite dTMP in the template, leading to a transition mutation. Recently it was reported that a modest balanced increase in all four DNTPS, three- or fourfold, stimulated mutagenesis out of proportion to the DNTP pool change. Describe a mechanism by which this could осcur.

DNA precursor imbalances are mutagenic. For example, if dGTP accumulates, it can compete with dATP for incorporation opposite dTMP in the template, leading to a transition mutation. Recently it was reported that a modest balanced increase in all four DNTPS, three- or fourfold, stimulated mutagenesis out of proportion to the DNTP pool change. Describe a mechanism by which this could осcur.

Biochemistry

6th Edition

ISBN:9781305577206

Author:Reginald H. Garrett, Charles M. Grisham

Publisher:Reginald H. Garrett, Charles M. Grisham

Chapter28: Dna Metabolism: Replication, Recombination, And Repair

Section: Chapter Questions

Problem 13P: Chemical Mutagenesis of DNA Bases Show the nucleotide sequence changes that might arise in a dsDNA...

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DNA precursor imbalances are mutagenic. For example, if dGTP accumulates, it can compete with dATP for incorporation opposite dTMP in the template, leading to a transition mutation. Investigators have shown that a modest balanced increase in all four dNTPs, three- or fourfold, stimulated mutagenesis out of proportion to the dNTP pool change. Describe a mechanism by which this could occur.
A certain compound that is an analog of the base cytosinecan become incorporated into DNA. It normally hydrogen bonds just as cytosine does, but it quite often isomerizes to a form that hydrogen bonds as thymine does. Doyou expect this compound to be mutagenic, and, if so,what types of changes might it induce at the DNA level?
In generating mutations in a bacterial gene involved in antibiotic resistance, a number of point mutations are isolated that render the bacteria sensitive to the antibiotic. You would like to sequence the gene in order to characterize the mutations, but unfortunately, your lab partner just finished the last of the lab's supply of DNA polymerase. The only things at your disposal are materials for performing a western blot, allowing you to visualize the protein encoded by the gene. How would you identify which mutations are likely to be the result of a missense mutation, which are likely to be the result of a nonsense mutation, and which are likely to be the result of a frameshift mutation?
The Ames test uses the reversion rate (His− to His+)to test compounds for mutagenicity.a. Is it possible that a known mutagen, like proflavin,would be unable to revert a particular His− mutantused in the Ames test? How do you think that theAmes test is designed to deal with this issue?b. Can you think of a way to use forward mutation(His+ to His−) to test a compound for mutagenicity? (Hint: Consider using the replica plating technique in Fig. 7.6.)c. Given that the rate of forward mutation is so muchhigher than the rate of reversion, why does the Amestest use the reversion rate to test for mutagenicity?
A temperature-sensitive mutation is one in which the defect is not presented functionally until the temperature is raised. In the case described below, the enzymes function normally in bacteria at 37 °C, but are non-functional at 40 °C. Predict the detailed molecular consequences of a loss of function in a temperature-sensitive mutant for each of the following enzymes: a) DNA gyrase, b) DNA polymerase III, c) DNA ligase, d) DNA polymerase I.
Richard Boyce and Paul Howard-Flanders conducted an experimentthat provided biochemical evidence that thymine dimers areremoved from DNA by a DNA repair system. In their studies, bacterialDNA was radiolabeled so the amount of radioactivity reflectedthe amount of thymine dimers. The DNA was thensubjected to UV light, causing the formation of thymine dimers. When radioactivity was found in the soluble fraction, thymine dimershad been excised from the DNA by a DNA repair system.But when the radioactivity was in the insoluble fraction, the thyminedimers had been retained within the DNA. The following tableillustrates some of the experimental results involving a normalstrain of E. coli and a mutant strain that was very sensitive to killingby UV light: Explain the results found in this table. Why is the mutant strainsensitive to UV light?
A recent estimate of the rate of base substitutions atSNP loci is about 1 × 10−8 per nucleotide pair pergamete.a. Based on this estimate, about how many de novomutations (that is, mutations not found in the genomes of your parents) are present in your own genome?b. Where and when did these de novo mutations inyour genome most likely occur?
After treating cells with a chemical mutagen, youisolate two mutants. One carries alanine and the othercarries methionine at a site in the protein that normallycontains valine (Figure Q6–4). After treating these twomutants again with the mutagen, you isolate mutants fromeach that now carry threonine at the site of the originalvaline (Figure Q6–4). Assuming that all mutations involvesingle-nucleotide changes, deduce the codons that areused for valine, methionine, threonine, and alanine at theaffected site. Would you expect to be able to isolate valine-to-threonine mutants in one step?
During an Ames test, bacteria were exposed to a potential mutagen.Also, as a control, another sample of bacteria was not exposedto the mutagen. In both cases, 10 million bacteria were plated andthe following results were obtained:No mutagen: 17 coloniesWith mutagen: 2017 coloniesCalculate the mutation rate in the presence and absence of the mutagen.How much does the mutagen increase the rate of mutation?
You are working with a newly discovered mutagen, andyou wish to determine the base change that it introduces into DNA. Thus far, you have determined that themutagen chemically alters a single base in such a waythat its base-pairing properties are altered permanently.To determine the specificity of the alteration, you examine the amino acid changes that take place after mutagenesis. A sample of what you find is shown here:Original: Gln–His–Ile–Glu–LysMutant: Gln–His–Met–Glu–LysOriginal: Ala–Val–Asn–ArgMutant: Ala–Val–Ser–ArgOriginal: Arg–Ser–LeuMutant: Arg–Ser–Leu–Trp–Lys–Thr–PheWhat is the base-change specificity of the mutagen?
"The molecule serving as the genetic material is expected to absorb at the wavelengths shown to be mutagenic." Explain this statement ?
Homologous Recombination, Heteroduplex DNA, and Mismatch Repair Homologous recombination in E. coli leads to the formation of regions of heteroduplex DNA. By definition, such regions contain mismatched bases. Why doesnâ€™t the mismatch repair system of E. coli eliminate these mismatches?