Genetics: Analysis and Principles 5th Edition by Robert Brooker

Exercise 10

In a cotransformation experiment (see solved problem S4), DNA was isolated from a donor strain that was proA + and strC + and sensitive to tetracycline. (The proA and strC genes confer the ability to synthesize proline and confer streptomycin resistance, respectively.) A recipient strain is proA and strC and is resistant to tetracycline. After transformation, the bacteria were first streaked on a medium containing proline, streptomycin, and tetracycline. Colonies were then restreaked on a medium containing streptomycin and tetracycline. (Note: Each type of medium had carbon and nitrogen sources for growth.) The following results were obtained:
70 colonies grew on the medium containing proline, streptomycin, and tetracycline, but only 2 of these 70 colonies grew when restreaked on the medium containing streptomycin and tetracycline but lacking proline
a. If we assume the average size of the DNA fragments is 2 minutes, how far apart are these two genes
B. What would you expect the cotransformation frequency to be if the average size of the DNA fragments was 4 minutes and the two genes are 1.4 minutes apart
Solved Problem S4
Genetic transfer via transformation can also be used to map genes along the bacterial chromosome. In this approach, fragments of chromosomal DNA are isolated from one bacterial strain and used to transform another strain. The experimenter examines the transformed bacteria to see if they have incorporated two or more different genes. For example, the DNA may be isolated from a donor E. coli bacterium that has functional copies of the araB and leuD genes. Let's call these genes araB + and leuD + to indicate the genes are functional. These two genes are required for arabinose metabolism and leucine synthesis, respectively. To map the distance between these two genes via transformation, a recipient bacterium is used that is araB and leuD . Following transformation, the recipient bacterium may become araB + and leuD +. This phenomenon is called cotransformation because two genes from the donor bacterium have been transferred to the recipient via transformation. In this type of experiment, the recipient cell is exposed to a fairly low concentration of donor DNA, making it unlikely that the recipient bacterium will take up more than one fragment of DNA. Therefore, under these conditions, cotransformation is likely only when two genes are fairly close together and are found on one fragment of DNA.In a cotransformation experiment, a researcher has isolated DNA from an araB + and leuD + donor strain. This DNA was transformed into a recipient strain that was araB and leuD . Following transformation, the cells were plated on a medium containing arabinose and leucine. On this medium, only bacteria that are araB + can grow. The bacteria can be either leuD + or leuD because leucine is provided in the medium. Colonies that grew on this medium were then restreaked on a medium that contained arabinose but lacked leucine. Only araB + and leuD + cells could grow on these secondary plates. Following this protocol, a researcher obtained the following results:
Number of colonies growing on a medium containing arabinose plus leucine: 57
Number of colonies that grew when restreaked on a medium containing arabinose medium without leucine: 42
What is the map distance between these two genes Note: This problem can be solved using the strategy of a cotransduction experiment except that the researcher must determine the average size of DNA fragments that are taken up by the bacterial cells. This would correspond to the value of L in a cotransduction experiment.