Concept explainers
One issue with interrupted-mating experiments such as that in Problem 19 is that gene order may be ambiguous if the genes are close together. Another shortcoming is that such experiments do not provide accurate map distances. The reason is that researchers select for the first Hfr marker transferred into the recipient, but the recovery of F exconjugants with a later Hfr marker is complex, depending both on transfer of the marker into the cell and on crossovers that transfer the marker into the recipient chromosome.
To make more accurate maps, bacterial geneticists often do Hfr × F- crosses in a different way: They select for exconjugants that contain a late Hfr marker, and then screen for the presence of the earlier markers. This method ensures that all of the markers have entered the F- cell, so relative gene distances are now accounted for solely by crossover frequencies. Furthermore, gene order is clarified by considering the crossovers responsible for each class of exconjugants.
As an example, suppose you performed the same cross as in Problem 19, but you selected for Arg+ exconjugants, and then screened them for the earlier Hfr markers Mal Xyl and Pyr . You obtained the following data:
Exconjugate type | Number of exconjugates |
Arg+ Mal+ Xyl+ Pyr+ |
80 |
Arg+ Mal+ Xy+l Pyr- |
40 |
Arg+ Mal+ Xyl- Pyr- |
20 |
Arg+ Mal- Xyl- Pyr- |
20 |
Arg+ Mal- Xyl+ Pyr- |
1 |
Arg+ Mal- Xyl- Pyr+ | 1 |
a. | Explain why four of the exconjugant types are much more frequent than the other two. |
b. | What can you conclude about the relative distances between the four genes? |
c. | The data allow you to estimate one other relevant genetic distance. Explain. |
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Genetics: From Genes to Genomes
- In five Hfr strains, each of which was used to build a time-of-transfer map, the genes entered the recipient cells as follows: Strain 1: S L A C T F Strain 2: N P F T C A Strain 3: T F P N U Y Strain 4: S H Y U N P Strain 5: U N P F T C Which of the following represents a correct gene map of these results? N P F T S L A C H U Y S L A C T F P N H Y U C T F P N U Y H S L A T C A L S P N U Y H F U N P C A L S F T H Yarrow_forwardFour Hfr strains are derived from an F+ strain of E. Coli to serve as donors for an interrupted-mating experiment. Use the time-table and partial map of the F+ strain (shown below) to determine the genes’ respective positions. Keep in mind that the map distances are NOT proportional, only the FIRST 5 markers are indicated per strain, and the entry times, recorded in minutes, are in parentheses. Transferred genes represent wild-type alleles. Based on the data, which gene can be located at position 5 on the map?arrow_forwardA wild-type mouse that is heterozygous for two immunoglobulin heavy chain alleles (IgHa/b) generates the population of B cells shown on the left of the figure below. A mouse strain, also IgHa/b, carries an inactivating mutation in the VpreB gene. In addition to producing fewer mature B cells than the wild-type mice, the VpreB-deficient mice generate B cells as shown on the right. What is the explanation of the difference seen between the wild-type and the VpreB-mutant B cells?arrow_forward
- Use the sequence provided here to identify the tag and tag location for the encoded DHFR fusion protein: ATATGCATCACCATCACCATCACATGGTTGGTTCGCTAAACTGCATCGTCGCTGTGTCCCAGAACATGGGCATCGGCAAGAACGGGGACCTGCCCTGGCCACCGCTCAGGAATGAATTCAGATATTTCCAGAGAATGACCACAACCTCTTCAGTAGAAGGTAAACAGAATCTGGTGATTATGGGTAAGAAGACCTGGTTCTCCATTCCTGAGAAGAATCGACCTTTAAAGGGTAGAATTAATTTAGTTCTCAGCAGAGAACTCAAGGAACCTCCACAAGGAGCTCATTTTCTTTCCAGAAGTCTAGATGATGCCTTAAAACTTACTGAACAACCAGAATTAGCAAATAAAGTAGACATGGTCTGGATAGTTGGTGGCAGTTCTGTTTATAAGGAAGCCATGAATCACCCAGGCCATCTTAAACTATTTGTGACAAGGATCATGCAAGACTTTGAAAGTGACACGTTTTTTCCAGAAATTGATTTGGAGAAATATAAACTTCTGCCAGAATACCCAGGTGTTCTCTCTGATGTCCAGGAGGAGAAAGGCATTAAGTACAAATTTGAAGTATATGAGAAGAATGATTAAarrow_forwardA donor strain of bacteria with genotype leu+ gal− pro+ is infected with phages. The phage lysate from the bacterial cells is collected and used to infect a second strain of bacteria that are leu− gal+ pro−. The second strain is selected for leu+, and the following cotransduction data are obtained: Donor Recipient Selected allele Cells with cotransduced allele (%) leu+ gal− pro+ leu− gal+ pro− leu+ 47 pro+ leu+ 26 gal− Which genes are closest, leu and gal or leu and pro?arrow_forwardIf you were to engineer some non-adhesive cells to express E-cadherin at either high levels or low levels and then mixed the E-cadherin high cells with the E-cadherin low cells, what do you predict will happen?arrow_forward
- T. Miyake and M. Demerec examined proline-requiring mutations in the bacterium Salmonella typhimurium (). On the basis of complementation testing, they found four proline auxotrophs: proA, proB, proC, and proD. To determine whether proA, proB, proC, and proD loci were located close together on the bacterial chromosome, they conducted a transduction experiment. Bacterial strains that were proC+ and had mutations at proA, proB, or proD were used as donors. The donors were infected with bacteriophages, and progeny phages were allowed to infect recipient bacteria with genotype proC− proA+ proB+ proD+. The recipient bacteria werethen plated on a selective medium that allowed only proC+ bacteria to grow. After this, the proC+ transductants were plated on selective media to reveal their genotypes at the other three pro loci. The following results were obtained: Q.Which genotypes represent single transductants and which represent cotransductants?arrow_forwardT. Miyake and M. Demerec examined proline-requiring mutations in the bacterium Salmonella typhimurium (). On the basis of complementation testing, they found four proline auxotrophs: proA, proB, proC, and proD. To determine whether proA, proB, proC, and proD loci were located close together on the bacterial chromosome, they conducted a transduction experiment. Bacterial strains that were proC+ and had mutations at proA, proB, or proD were used as donors. The donors were infected with bacteriophages, and progeny phages were allowed to infect recipient bacteria with genotype proC− proA+ proB+ proD+. The recipient bacteria werethen plated on a selective medium that allowed only proC+ bacteria to grow. After this, the proC+ transductants were plated on selective media to reveal their genotypes at the other three pro loci. The following results were obtained: Q.Is there evidence that proA, proB, and proD are located close to proC? Explain your answer.arrow_forwardT. Miyake and M. Demerec examined proline-requiring mutations in the bacterium Salmonella typhimurium (). On the basis of complementation testing, they found four proline auxotrophs: proA, proB, proC, and proD. To determine whether proA, proB, proC, and proD loci were located close together on the bacterial chromosome, they conducted a transduction experiment. Bacterial strains that were proC+ and had mutations at proA, proB, or proD were used as donors. The donors were infected with bacteriophages, and progeny phages were allowed to infect recipient bacteria with genotype proC− proA+ proB+ proD+. The recipient bacteria werethen plated on a selective medium that allowed only proC+ bacteria to grow. After this, the proC+ transductants were plated on selective media to reveal their genotypes at the other three pro loci. The following results were obtained: Q.Why are there no proC− genotypes among the transductants?arrow_forward
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