17.For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator.

*18.A mutation at the operator prevents the regulator protein from binding. What effect will this mutation have in the following types of operons?

19.The blob operon produces enzymes that convert compound A into compound B. The operon is controlled by a regulatory gene S. Normally, the enzymes are synthesized only in the absence of compound B. If gene S is mutated, the enzymes are synthesized in the presence and in the absence of compound B. Does gene S produce a repressor or an activator? Is this operon inducible or repressible?

*20.A mutation prevents the catabolite activator protein (CAP) from binding to the promoter in the lac operon. What will the effect of this mutation be on the transcription of the operon?

21.Under which of the following conditions would a lac operon produce the greatest amount of β-galactosidase? The least? Explain your reasoning.

22.A mutant strain of E. coli produces β-galactosidase in the presence and in the absence of lactose. Where in the operon might the mutation in this strain be located?

23.Examine Figure 12.7. What would be the effect of a drug that altered the structure of allolactose so that it was unable to bind to the regulator protein?

24.For E. coli strains with the lac genotypes given below, use a plus sign (+) to indicate the synthesis of β-galactosidase and permease and a minus sign (−) to indicate no synthesis of the proteins.

25.Give all possible genotypes of a lac operon that produces β-galactosidase and permease under the following conditions. Do not give partial-diploid genotypes.

*26.Explain why mutations in the lacI gene are trans in their effects, but mutations in the lacO gene are cis in their effects.

27.The mmm operon, which has sequences A, B, C, and D (which may be structural genes or regulatory sequences), encodes enzymes 1 and 2. Mutations in sequences A, B, C, and D have the following effects, where a plus sign (+) indicates that the enzyme is synthesized and a minus sign (−) indicates that the enzyme is not synthesized.

28. Ellis Engelsberg and his coworkers examined the regulation of genes taking part in the metabolism of arabinose, a sugar (E. Engelsberg et al. 1965. Journal of Bacteriology 90:946–957). Four structural genes encode enzymes that help metabolize arabinose (genes A, B, D, and E). An additional gene C is linked to genes A, B, and D. These genes are in the order D-A-B-C. Gene E is distant from the other genes. Engelsberg and his colleagues isolated mutations at the C gene that affected the expression of structural genes A, B, D, and E. In one set of experiments, they created various genotypes at the A and C loci and determined whether arabinose isomerase (the enzyme encoded by gene A) was produced in the presence or absence of arabinose (the substrate of arabinose isomerase). Results from this experiment are shown in the following table, where a plus sign (+) indicates that the arabinose isomerase was synthesized and a minus sign (−) indicates that the enzyme was not synthesized.

29. In E. coli, three structural genes (A, D, and E) encode enzymes A, D, and E, respectively. Gene O is an operator. The genes are in the order O-A-D-E on the chromosome. These enzymes catalyze the biosynthesis of valine. Mutations were isolated at the A, D, E, and O genes to study the production of enzymes A, D, and E (T. Ramakrishnan and E. A. Adelberg. 1965. Journal of Bacteriology 89:654–660). Levels of the enzymes produced by partial-diploid E. coli with various combinations of mutations are shown in the following table.

30.A geneticist is trying to determine how many genes are found in a 300,000-bp region of DNA. Analysis shows that four H3K4me3 modifications are found in this piece of DNA. What might their presence suggest about the number of genes located there?

31.In a line of human cells grown in culture, a geneticist isolates a temperature-sensitive mutation at a locus that encodes an acetyltransferase enzyme; at temperatures above 38°C, the mutant cells produce a nonfunctional form of the enzyme. What would be the most likely effect of this mutation when the cells are grown at 40°C?

32.X31b is an experimental compound that is taken up by rapidly dividing cells. Research has shown that X31b stimulates the methylation of DNA. Some cancer researchers are interested in testing X31b as a possible drug for treating prostate cancer. Offer a possible explanation for why X31b might be an effective anticancer drug.

33.What would be the effect of moving the insulator shown in Figure 12.18 to a position between enhancer II and the promoter for gene B?

*34.An enhancer is surrounded by four genes (A, B, C, and D), as shown in the diagram below. An insulator lies between gene C and gene D. On the basis of the positions of the genes, the enhancer, and the insulator, the transcription of which genes is most likely to be stimulated by the enhancer? Explain your reasoning.

35.Some eukaryotic mRNAs have an AU-rich element in the 3′ untranslated region. What would be the effect on gene expression if this element were mutated or deleted?

36.A strain of Arabidopsis thaliana possesses a mutation in the APETALA2 gene, in which much of the 3′ untranslated region of mRNA transcribed from the gene is deleted. What is the most likely effect of this mutation on the expression of the APETALA2 gene?

*37.How do epigenetic traits differ from traditional genetic traits, such as the differences in color and shape of peas that Mendel studied?

*38.A scientist does an experiment in which she removes the offspring of rats from their mother at birth and has her genetics students feed and rear the offspring. Assuming that the students do not lick and groom the baby rats like their mothers normally do, what long-term behavioral and epigenetic effects would you expect to see in the rats when they grow up?

39. Pregnant female rats were exposed to a daily dose of 100 or 200 mg/kg of vinclozolin, a fungicide commonly used in the wine industry (M. D. Anway et al. 2005, Science 308:1466–1469). The F₁ offspring of the exposed females were interbred, producing F₂, F₃, and F₄ rats. None of the F₂, F₃, or F₄ rats were exposed to vinclozolin. Testes from the F₁–F₄ male rats were examined and compared with those of control rats descended from females that had not been exposed to vinclozolin. There were higher percentages of apoptotic cells (cells that underwent controlled cell death) in the testes of F₁–F₄ male descendants of females who were exposed to vinclozolin than in descendants of control females (graph a). Furthermore, sperm numbers (graph b) and motilities (graph c) were lower in the F₁–F₄ descendants of vinclozolin-exposed females than in those of control females. In addition, 8% of the F₁–F₄ males descended from vinclozolin-exposed females developed complete infertility, compared with 0% of the F₁–F₄ males descended from control females. Molecular analysis of the testes demonstrated that DNA methylation patterns differed between descendants of vinclozolin-exposed females and descendants of control females. Explain the transgenerational effects of vinclozolin on male fertility.