Question 12
An Hfr strain with the genotype cys+ leu+ met+ strS is mated with an F- strain carrying the genotype cys- leu -met- strR .
In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:
Medium 1: Minimal medium plus leucine, methionine, and streptomycin
Medium 2: Minimal medium plus cysteine, methionine, and streptomycin
Medium 3: Minimal medium plus cysteine, leucine, and streptomycin
Question: What donor gene is the selected marker in Medium 1?
A. leucine
B. streptomycin
C. methionine
D. cysteine
E. pleiotropy
Question 2: Select the genotypes of the bacteria that are able to grow on Medium 1:
A.cys- leu- met+ strR
B.cys+ leu+ met+ strR
C. cys+ leu+ met- strR
D.cys- leu+ met- strR
E. cys- leu+ met+ strR
F. cys- leu- met- strR
G. cys+ leu- met+ strR
H. cys+ leu- met- strR
Question 3: Select the genotypes of the bacteria that are able to grow on Medium 3:
A.cys- leu- met+ strR
B.cys+ leu+ met+ strR
C.cys+ leu+ met- strR
D.cys- leu+ met- strR
E. cys- leu+ met+ strR
F.cys- leu- met- strR
G.cys+ leu- met+ strR
H.cys+ leu- met- strR
Question4: What is the purpose of adding streptomycin to each selection medium?
A.to ensure that no accident contamination will germinate on the media
B.to ensure that only exconjugate bacteria and not donor bacteria will grow on the media
C.to prevent the exconjugate bacteria from overgrowing and running out of supplements
D.to interrupt the mating experiment at the appropriate time to collect all data needed
E.all bacteria need streptomycin to grow
2. H
To answer the questions, we need to understand the concept of interrupted mating experiment and how selection media works.
Interrupted mating experiment involves the interruption of conjugation at different time intervals to study the transfer of genetic material between donor and recipient cells. The cells are withdrawn, shaken to stop conjugation, and then placed on selection media to assess the growth of different genotypes.
Selection media are designed to allow the growth of certain genotypes while inhibiting the growth of others by including specific nutrients or antibiotics. In this case, we have three different selection media with different combinations of nutrients and streptomycin.
Now let's answer each question:
Question 12: What donor gene is the selected marker in Medium 1?
To determine the selected marker in Medium 1, we need to understand that the donor genotype carries all the genes, including the selected marker, which will be transferred to the recipient cells. By comparing the genotypes of the donor and recipient, we can identify the gene that is present in the donor but absent in the recipient.
In this case, the donor genotype is cys+ leu+ met+ strS and the recipient genotype is cys- leu- met- strR. By comparing the genotypes, we can see that the donor has the strS gene, while the recipient has the strR gene. Therefore, the selected marker in Medium 1 is streptomycin. So the answer is B. streptomycin.
Question 2: Select the genotypes of the bacteria that are able to grow on Medium 1:
To determine the genotypes that can grow on Medium 1, we need to consider the nutrients included in this medium. Medium 1 contains leucine, methionine, and streptomycin. Only the genotypes that can synthesize their own leucine and methionine will be able to grow on this medium.
By analyzing the genotypes given, we can see that only the genotype cys+ leu+ met+ strR can grow on Medium 1 because it has all the necessary genes to synthesize leucine and methionine. So the answer is B. cys+ leu+ met+ strR.
Question 3: Select the genotypes of the bacteria that are able to grow on Medium 3:
Similar to the previous question, we need to consider the nutrients included in Medium 3 to determine the genotypes that can grow on it. Medium 3 contains cysteine, leucine, and streptomycin. Only the genotypes that can synthesize their own cysteine and leucine, and are resistant to streptomycin, will be able to grow on this medium.
By analyzing the genotypes given, we can see that the genotypes cys+ leu+ met+ strR and cys- leu+ met+ strR both have the necessary genes to synthesize cysteine and leucine, and are resistant to streptomycin. So the answer is B. cys+ leu+ met+ strR and G. cys- leu+ met+ strR.
Question 4: What is the purpose of adding streptomycin to each selection medium?
To answer this question, we need to understand the role of streptomycin in the experiment. Streptomycin is an antibiotic that inhibits the growth of bacteria that do not possess a specific resistance gene (strR).
By including streptomycin in each selection medium, the purpose is to ensure that only the exconjugate bacteria (recipient cells that have acquired genetic material from the donor) which possess the strR gene can grow on the media. This allows for the selective growth of the desired genotypes and prevents the growth of donor bacteria that do not carry the strR gene.
Therefore, the answer is B. to ensure that only exconjugate bacteria and not donor bacteria will grow on the media.