Genetics of Bacteria Practice Test (page 3)

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

Mapping The Bacterial Chromosome Questions

  1. A cross is made between the streptomycin-resistant (strr) F strain of genotype gal thr azir lacTonr mal xylleu and the prototrophic Hfr strain that has opposite characters. After 60 min of contact, samples are transferred to plates with minimal medium plus streptomycin. The original mixture is in the ratio 2 × 107 Hfr to 4 × 108 F . The percentages of each Hfr gene transferred are: 72% Tons, 0% mal+, 27% gal+, 91% azis, 0% xyl +, 48% lac+.     (a) How many F cells exist in the original mixture for every Hfr cell?   (b) What is the counterselective agent that prevents Hfr individuals from obscuring the detection of recombinants?   (c) In what order are these genes probably being transferred by the Hfr strain?
  2. Four Hfr strains of E. coli are known to transfer their genetic material during conjugation in different sequences. Given the time of entry of the markers into the F recipient, construct a genetic map that includes all of these markers and label the time distance between adjacent gene pairs.
  3. Two mutants at the tryptophan locus, trp A and trpB, are known to be close to a cysteine locus (cys). A bacterial strain of genotype cys+ trpA is transduced by phage from a bacterial strain that is cys trpB. The reciprocal cross is also made wherein the strain cystrpB is transduced by phage from a strain that is cys+ trpA. In both cases, the numbers of prototrophic recombinants are equivalent. Determine the order of the tryptophan mutants relative to the cysteine marker.
  4. Five point mutations (ae) were tested for wild-type recombinants with each of the five deletions shown in the topological map below. The results are listed in the table below (+ = recombination, 0 = no recombination). Determine the order of the point mutations.



  1. auxotrophic
  2. clone
  3. theta (θ)
  4. lawn
  5. conjugation
  6. transduction
  7. merozygote
  8. plasmid
  9. DNA gyrase
  10. helicase


  1. c
  2. d
  3. c
  4. a
  5. d
  6. c
  7. d
  8. a
  9. e
  10. c

Isolation of Bacterial Mutants

  1. Plate 1 contains a mutation to ser+; plate 3 contains a mutation to arg+.
  3. 20% recombination
  4. (a) Back mutations (reverse mutations) from his to his+.   (b) It supplies the mammalian metabolic functions that are usually required to convert a chemical into its carcinogenic metabolites.  (c) After 2 days, most of the his- bacteria have died for lack of histidine. Back-mutation rates are expected to be proportional to concentration of the chemical that forms a radially diminishing concentration gradient around the paper disk. Close to the disk there is a zone in which no cells grow because of toxic levels of the chemical. Beyond this zone there may be so many his+ revertants that the cells almost form a continuous lawn. At the periphery are a few larger clones (because they are isolated) representing spontaneous his+ mutants that have not been exposed to the chemical.

Bacterial DNA Replication and Cell Division


Regulation of Bacterial Gene Activity

  1. (a) Is, I+, I  (b) ps, p+, pi cr, p
  2. (a),   (d) = inductive;   (b),   (c),   (e) = constitutive
  3. (a)Yes   (b) Constitutively   (c)Yes   (d) Inductively   (e) Note that in genotypes (2) and (3) the lacI allele fails to produce enzymes in the absence of the external inducer (lactose); it fails to exhibit dominance. Therefore, the first hypothesis is incorrect. Under the second hypothesis, lacI+ is dominant and produces a repressor that (in the absence of lactose inducer) blocks enzyme synthesis as seen in genotypes (2) and (3). (f) Genotype (3) has lacY+ and lacZ+ on one DNA molecule and lacI+ on a different DNA molecule. Yet in the absence of lactose inducer, the repressor made by lacI+ still prevents the production of enzymes by lacY+ and lacZ+. Therefore, the repressor must be able to act at a distance (it behaves as a diffusible substance) on genes that are either on the same DNA molecule (cis position) or on a different DNA molecule (trans position).
  4. (1)A change that prevents repressor binding. (2) Modifications that increase repressor binding so that operons cannot be derepressed even when inducer is bound.
  5. The mutation could be in the gene for adenyl cyclase or in the gene for catabolite activator protein (CAP).
  6. I inhibits 7; J inhibits 9; G alone or (I, J) together inhibits 5; E inhibits 3; enzyme 1 could be inhibited by (I, J, E), (G, E), (I, J, C), or (C, G).

Mapping the Baterial Chromosome

  1. (a) 20   (b) Streptomycin; Hfr is streptomycin-sensitive (strs).   (c) Origin-(thr+ Leu+)-azis-Tons-lac+-gal+-strs-(mal+ xyl+). The genes for synthesizing the amino acids threonine and leucine must have entered first, otherwise none of the other recombinants could survive on unsupplemented medium. Note: The order of markers within parentheses has not been determined.
  2. cys-trpB-trpA
  3. a-c-d-e-b
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