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Viruses, Transposable Elements, and Cancer Practice Test (page 2)

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By — McGraw-Hill Professional
Updated on Aug 23, 2011

Bacteriophages Questions

  1. Six deletion mutants within the A gene of the rII region of phage T4 were tested in all pairwise combinations for wild-type recombinants. In the following table, + = recombination, 0 = no recombination. Construct a topological map for these deletions. (Hint: see Solved Problem 11.3.)
  2. Supplementary Problems

  3. Phage MS2 is a single-stranded RNA virus of E. coli. After infecting a cell, the phage RNA (the "plus" strand) is made into a double-stranded replicative intermediate form ("plus-minus") from which "plus" RNA is synthesized. The "minus" strands when isolated are not infective. Phage X174 is a single-stranded DNA virus of E. coli. When injected into a bacterium, the same events as described for MS2 occur, but the "minus" strands when isolated are infective. Devise a reasonable hypothesis to account for these observations.
  4. The DNA of bacteriophage T4 contains approximately 200,000 nucleotide pairs. The rII region of the T4 genome occupies about 1%of its total genetic length. Benzer has found that about 300 sites are separable by recombination within the rII region. Determine the average number of nucleotides in each recon.
  5. The molecular weight of DNA in phage T4 is estimated to be 160 × 106. The average molecular weight of each nucleotide is approximately 400. The total genetic map of T4 is calculated to be approximately 2500 recombination units long. With what frequency are r+ recombinants expected to be formed when two different r mutants (with mutations at adjacent nucleotides) are crossed?
  6. A number of mutations were found in the rII region of phage T4. From the recombination data shown in the table below, determine whether each mutant is a point defect or a deletion (+ = recombination, 0 = no recombination). Two of the four mutants have been known to undergo backmutation; the other two have never been observed to backmutate.
  7. Supplementary Problems

  8. Escherichia coli strain B is doubly infected with two rII mutants of phage T4. A 6 × 107 dilution of the lysate is plated on E. coli B. A 2 × 105 dilution is plated on strain K. Twelve plaques appeared on strain K, 16 on strain B. Calculate the amount of recombination between these two mutants.
  9. Anonlytic response usually is observed in lysogenic (λ) E. coli cells when conjugated with nonlysogenic Hfr donors or in crosses of Hfr (λ) × F (λ). The donated prophage is almost never inherited by the recombinants. Lysis is very anomalous in crosses of Hfr (λ) × F. Explain these observations.
  10. Temperate phages such as lambda sometimes produce turbid plaques on lambda-sensitive indicator cells; virulent phages that cannot lysogenize always produce clear plaques on cells of their host range.     (a)  Offer an explanation for the turbid plaques.     (b)  Some lambda mutants produce only clear plaques. What genetic locus is most likely mutant in these cases?
  11. When bacterial DNA is damaged by a mutagenic agent, excision repair normally operates to repair the lesion. This process is less than 100% efficient, however, so that some residual lesions remain unrepaired. If these lesions delay replication of DNA, an error-prone "SOS repair" system becomes operative, involving activation and increased production of a multifunctional protein called RecA protein (for "recombination"). RecA protein interferes with cell partition, resulting in elongation of cells into filaments. RecA protein also cleaves lambda repressor; this repressor must remain intact for the virus to remain dormant as a prophage. E. coli strain B is lysogenic for lambda; strain A is not lysogenic for lambda. This knowledge led Moreau, Bailone, and Devoret to devise a "prophage induction test" or "inductest" for potential carcinogens. Lysogenic strain B of E. coli is made defective in its excision repair system and genetically modified to make the cell envelopes permeable to a wide variety of test chemicals. This special strain is mixed with indicator strain A and rat liver extract; the mixture is then plated; the medium is covered with a thin layer of indicator bacteria interspersed with a few lysogenic bacteria. The test chemical is applied to a filter paper disk and placed in the center of the plate for a "spot test."     (a)  After incubation, how is DNA damage assayed?     (b)  Why is strain A required as an indicator?     (c)  What advantage does an inductest have over an Ames test?     (d)  Explain the selective advantage of lysogenic induction.     (e)  Genetic engineers have spliced the gene for galacto-kinase into a bacterial chromosome, thereby creating an organism for assaying mutagens by an enzymatic activity test. Where was this gene inserted into the chromosome and how does the system work?
  12. The single-stranded phage X174 of E. coli contains 5386 nucleotides coding for 11 proteins with a combined molecular weight of 262,000 bp.     (a)  If an average amino acid has a molecular weight of 110, by how many amino acids is the coding capacity of the phage exceeded?     (b)  How can X174 code for more proteins than it has coding triplets?     (c)  Several animal viruses make more proteins than for which they seem to have coding triplets. Suggest some ways by which they might accomplish this feat if a single reading frame is used.

Transposable Elements Questions

  1. A given transposable element becomes duplicated at a fairly constant (although usually low) rate. Therefore, over evolutionary time, the descendants of a bacterial cell might be expected to contain thousands of copies of such a transposon. However, the number of copies of bacterial transposons is very low(usually only one or two per cell).     (a)  Offer an explanation for this low copy number.     (b)  Why have most bacterial transposons been isolated from plasmids rather than from the bacterial chromosome?
  2. Transposition of a particular transposable element is found to be dependent on reverse transcriptase activity. Propose a mechanism for its transposition.
  3. How might a transposition event result in oncogenesis?

Eukaryotic Viruses Questions

  1. Give at least two mechanisms whereby RNA viruses produce mRNA.
  2. With regard to retroviruses:     (a)  specify their defining characteristic,     (b)  name the enzyme contained in their virions and list three biochemical activities of that enzyme,     (c)  identify the template for synthesis of retroviral mRNA,     (d)  identify the cellular location of their replication,     (e)  specify those attributes suggesting that their DNA-insertion mechanism is related to transposition.
  3. The life cycles of eukaryotic viruses and bacteriophages have many similarities, including the establishment of new replication and transcription systems, regulation of gene action (e.g., early vs. late transcription), and synthesis of large quantities of structural proteins. There are certain aspects of viral life cycles, however, that are not (or only rarely) found in the life cycles of phage. Specify some of these unique aspects.
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