17. Mutation, Repair, and Recombination

Types of Mutations

Open Question
Many human genes are known to have homologs in the mouse genome. One approach to investigating human hereditary disease is to produce mutations of the mouse homologs of human genes by methods that can precisely target specific nucleotides for mutation.Numerous studies of mutations of the mouse homologs of human genes have yielded valuable information about how gene mutations influence the human disease process. In general terms, describe how and why creating mutations of the mouse homologs can give information about human hereditary disease processes.
Open Question
The human β-globin wild-type allele and a certain mutant allele are identical in sequence except for a single base-pair substitution that changes one nucleotide at the end of intron 2. The wild-type and mutant sequences of the affected portion of pre-mRNA are

Intron 2 Exon 3 _
wild type 5′-CCUCCCACAG CUCCUG-3′
mutant. 5′-CCUCCCACUG CUCCUG-3′

This is one example of how DNA sequence change occurring somewhere other than in an exon can produce mutation. List other kinds of DNA sequence changes occurring outside exons that can produce mutation. In each case, characterize the kind of change you would expect to see in mutant mRNA or mutant protein.
Open Question
The human β-globin wild-type allele and a certain mutant allele are identical in sequence except for a single base-pair substitution that changes one nucleotide at the end of intron 2. The wild-type and mutant sequences of the affected portion of pre-mRNA are

Intron 2 Exon 3 _
wild type 5′-CCUCCCACAG CUCCUG-3′
mutant. 5′-CCUCCCACUG CUCCUG-3′

Speculate about the way in which this base substitution causes mutation of β-globin protein.
Open Question
The fluctuation test performed by Luria and Delbrück is consistent with the random mutation hypothesis. Briefly describe their experiment and identify how the results match the prediction of the random mutation hypothesis. What would have to be different about the experimental results for them to agree with the prediction of the adaptive mutation hypothesis?
Open Question
Mutations in the CFTR gene result in cystic fibrosis in humans, a condition in which abnormal secretions are present in the lungs, pancreas, and sweat glands. The gene was mapped to a 500-kb region on chromosome 7 containing three candidate genes.

How would you prove that your chosen candidate is the CFTR gene?
Open Question
Mutations in the CFTR gene result in cystic fibrosis in humans, a condition in which abnormal secretions are present in the lungs, pancreas, and sweat glands. The gene was mapped to a 500-kb region on chromosome 7 containing three candidate genes.

Using your knowledge of the disease symptoms, how would you distinguish between the candidate genes to decide which is most likely to encode the CFTR gene?
Open Question
In a bacterial culture in which all cells are unable to synthesize leucine (leu⁻), a potent mutagen is added, and the cells are allowed to undergo one round of replication. At that point, samples are taken, a series of dilutions are made, and the cells are plated on either minimal medium or minimal medium containing leucine. The first culture condition (minimal medium) allows the growth of only leu⁺ cells, while the second culture condition (minimal medium with leucine added) allows growth of all cells. The results of the experiment are as follows:Culture Condition Dilution ColoniesMinimal medium 10⁻¹ 18Minimal medium + leucine 10⁻⁷ 9What is the rate of mutation at the locus associated with leucine biosynthesis?
Open Question
In this chapter, three features of genes or of DNA sequence that contribute to the occurrence of mutational hotspots were described. Identify those three features and briefly describe why they are associated with mutational hotspots.
Open Question
Briefly compare the production of DNA double-strand breaks in bacteria versus the double-strand breaks that precede homologous recombination.
Open Question
Imagine yourself as one of the team of geneticists who launches a study of the genetic effects of high-energy radiation on the surviving Japanese population immediately following the atom bomb attacks at Hiroshima and Nagasaki in 1945. Demonstrate your insights into both chromosomal and gene mutation by outlining a short-term and long-term study that addresses these radiation effects. Be sure to include strategies for considering the effects on both somatic and germ-line tissues.
Open Question
During mismatch repair, why is it necessary to distinguish between the template strand and the newly made daughter strand? Describe how this is accomplished.
Open Question
With the knowledge that radiation causes mutations, many assume that human-made forms of radiation are the major contributors to the mutational load in humans. What evidence suggests otherwise?
Open Question
Yeast are single-celled eukaryotic organisms that grow in culture as either haploids or diploids. Diploid yeast are generated when two haploid strains fuse together. Seven haploid mutant strains of yeast exhibit similar normal growth habit at 25°C, but at 37°C, they show different growth capabilities. The table below displays the growth patternResearchers induce fusion in pairs of haploid yeast strains (all possible combinations), and the resulting diploids are tested for their ability to grow at 37°C. The results of the growth experiment are shown below. How many different genes are mutated among these seven yeast strains? Identify the strains that represent each gene mutation.
Open Question
Yeast are single-celled eukaryotic organisms that grow in culture as either haploids or diploids. Diploid yeast are generated when two haploid strains fuse together. Seven haploid mutant strains of yeast exhibit similar normal growth habit at 25°C, but at 37°C, they show different growth capabilities. The table below displays the growth patternHypothesize about the nature of the mutation affecting each of these mutant yeast strains, including why strains B and G display different growth habit at C37°C than the other strains.
Open Question
Most organisms display a circadian rhythm, a cycling of biological processes that is roughly synchronized with day length (e.g., jet lag occurs in humans when rapid movement between time zones causes established circadian rhythms to be out of synch with daylight hours). In Drosophila, pupae eclose (emerge as adults after metamorphosis) at dawn.

In most plants, such as Arabidopsis, genes whose encoded products have roles related to photosynthesis have expression patterns that vary in a circadian manner. Using this knowledge, how would you screen for Arabidopsis mutants that have an impaired circadian rhythm?