Application Questions And Problems

*28. Suppose that a geneticist discovers a new restriction enzyme in the bacterium Aeromonas ranidae. This restriction enzyme is the first to be isolated from this bacterial species. Using the standard convention for abbreviating restriction enzymes, give this new restriction enzyme a name (for help, see footnote to Table 18.2).

29. How often, on average, would you expect a type II restriction endonuclease to cut a DNA molecule if the recognition sequence for the enzyme had 5 bp? (Assume that the four types of bases are equally likely to be found in the DNA and that the bases in a recognition sequence are independent.) How often would the endonuclease cut the DNA if the recognition sequence had 8 bp?

*30. A microbiologist discovers a new type II restriction endonuclease. When DNA is digested by this enzyme, fragments that average 1,048,500 bp in length are produced. What is the most likely number of base pairs in the recognition sequence of this enzyme?

31. Will restriction sites for an enzyme that has 4 bp in its restriction site be closer together, farther apart, or similarly spaced, on average, compared with those of an enzyme that has 6 bp in its restriction site? Explain your reasoning.

*32. About 30% of the base pairs in a human DNA molecule are AT. If the human genome has 3 billion base pairs of DNA, about how many times will the following restriction sites be present?

*33. Restriction mapping of a linear piece reveals the following EcoRI restriction sites.

Eco RI site 1 Eco RI site 2

2 kb

4 kb

5 kb

(a) This piece of DNA is cut by EcoRI, the resulting fragments are separated by gel electrophoresis, and the gel is stained with ethidium bromide. Draw a picture of the bands that will appear on the gel.

(b) If a mutation that alters EcoRI site 1 occurs in this piece of DNA, how will the banding pattern on the gel differ from the one that you drew in part a?

(c) If mutations that alter EcoRI sites 1 and 2 occur in this piece of DNA, how will the banding pattern on the gel differ from the one that you drew in part a?

(d) If a 1000-bp insertion occurred between the two restriction sites, how would the banding pattern on the gel differ from the one that you drew in part a?

(e) If a 500-bp deletion occurred between the two restriction sites, how would the banding pattern on the gel differ from the one that you drew in part a?

*34. Which vectors (plasmid, phage X, cosmid) can be used to clone a continuous fragment of DNA with the following lengths?

35. A geneticist uses a plasmid for cloning that has a gene that confers resistance to penicillin and the lacZ gene. The geneticist inserts a piece of foreign DNA into a restriction site that is located within the lacZ gene and transforms bacteria with the plasmid. Explain how the geneticist can identify bacteria that contain a copy of a plasmid with the foreign DNA.

*36. Suppose that you have just graduated from college and have started working at a biotechnology firm. Your first job assignment is to clone the pig gene for the hormone prolactin. Assume that the pig gene for prolactin has not yet been isolated, sequenced, or mapped; however, the mouse gene for prolactin has been cloned and the amino acid sequence of mouse prolactin is known. Briefly explain two different strategies that you might use to find and clone the pig gene for prolactin.

37. A genetic engineer wants to isolate a gene from a scorpion that encodes the deadly toxin found in its stinger, with the ultimate purpose of transferring this gene to bacteria and producing the toxin for use as a commercial pesticide. Isolating the gene requires a DNA library. Should the genetic engineer create a genomic library or a cDNA library? Explain your reasoning.

*38. A protein has the following amino acid sequence:

Met-Tyr-Asn-Val-Arg-Val-Tyr-Lys-

Ala-Lys-Trp-Leu-Ile-His-Thr-Pro

You wish to make a set of probes to screen a cDNA library for the sequence that encodes this protein. Your probes should be at least 18 nucleotides in length.

(a) Which amino acids in the protein should be used to construct the probes so that the least degeneracy results? (Consult the genetic code in Table 15.12.)

(b) How many different probes must be synthesized to be certain that you will find the correct cDNA sequence that specifies the protein?

*39. A gene in mice is discovered that is similar to a gene in yeast. How might it be determined whether this gene is essential for development in mice?

*40. A hypothetical disorder called G syndrome is an autosomal dominant disease characterized by visual, skeletal, and cardiovascular defects. The disorder appears in middle age. Because the symptoms of the disorder are variable, the disorder is difficult to diagnose. Early diagnosis is important, however, because the cardiovascular symptoms can be treated if the disorder is recognized early. The gene for G syndrome is known to reside on chromosome 7, and it is closely linked to two RFLPs on the same chromosome, one at the A locus and one at the C locus. The genes at the G, A, and C loci are very close together, and there is little crossing over between them. The following RFLP alleles are found at the A and C loci:

Sally, shown in the following pedigree, is concerned that she might have G syndrome. Her deceased mother had G syndrome, and she has a brother with the disorder. A geneticist genotypes Sally and her immediate family for the A and C loci and obtains the genotypes shown on the pedigree.

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