Q: If a plate is spread with 1 ml of a 1/10,000 dilution of a bacterial suspension and 15 colonies are counted, what is the concentration of bacteria in the original suspension?

$$15 \times 10,000 = 150,000$$ colonies/ml

Question 1

Which of the following best describes the main difference between conjugation, transduction, and transformation in bacterial genetic transfer?

Accepted Answer

Conjugation requires direct cell-to-cell contact, transduction involves viruses, and transformation involves uptake of naked DNA from the environment.

Question 2

In an experiment, equal numbers of F+ and F- E. coli cells are mixed together. What would you expect to happen to the ratio of F+ to F- cells over time?

Accepted Answer

The ratio of F+ to F- cells will increase as more F- cells become F+ through conjugation.

Question 3

Which of the following statements about Hfr strains is correct?

Accepted Answer

Hfr strains have the F factor integrated into their chromosome and can transfer many bacterial genes to recipient cells.

Question 4

What is the role of the origin of transfer (oriT) during F+ and Hfr-mediated conjugation?

Accepted Answer

It is the site where DNA transfer begins, determining the direction and order of gene transfer.

Question 5

In the Lederberg and Tatum experiment, how did they rule out the possibility that prototrophic colonies arose from rare back mutations rather than genetic recombination?

Accepted Answer

By showing that neither strain alone produced prototrophs and that physical contact was required for recombination.

Question 6

Which experimental technique is used to determine the order of genes on a bacterial chromosome by interrupting conjugation at different time points?

Accepted Answer

Interrupted mating experiment

Question 7

A researcher performs an interrupted mating experiment with an Hfr strain and an F- strain. The order of gene transfer is observed as thr+, leu+, aziS, tonS, lac+, gal+. Which gene is likely closest to the origin of transfer?

Accepted Answer

thr+

Question 8

If a plate is spread with 1 ml of a 1/10,000 dilution of a bacterial suspension and 15 colonies are counted, what is the concentration of bacteria in the original suspension?

Accepted Answer

$$15 	imes 10,000 = 150,000$$ colonies/ml

Question 9

Which of the following best describes an auxotroph?

Accepted Answer

An organism that cannot synthesize a required metabolite and must obtain it from the environment.

Question 10

In the U-tube experiment by Bernard Davis, what was the key finding regarding genetic recombination in bacteria?

Accepted Answer

Physical contact between bacterial cells is required for genetic recombination.

Question 11

Which of the following statements about the F factor is correct?

Accepted Answer

The F factor is a plasmid that can integrate into the bacterial chromosome to form an Hfr strain.

Question 12

During conjugation between an Hfr cell and an F- cell, why does the recipient cell usually remain F- after the process?

Accepted Answer

Because the entire F factor is rarely transferred before the mating is interrupted.

Question 13

If the genetic distance between two genes on the E. coli chromosome is 30 minutes, what does this mean in the context of an interrupted mating experiment?

Accepted Answer

It takes 30 minutes for the second gene to be transferred after the first gene during conjugation.

Question 14

Which of the following is a real-world application of replica plating in bacterial genetics?

Accepted Answer

Screening for auxotrophic mutants by comparing growth on minimal and supplemented media.

Question 15

A prototrophic E. coli strain is plated on minimal medium. What is the expected result?

Accepted Answer

Growth of colonies, because prototrophs can synthesize all required metabolites.

Question 16

Which of the following best explains why interrupted mating experiments are useful for mapping bacterial genes?

Accepted Answer

They allow determination of the order and relative distances of genes based on the timing of their transfer.

Question 17

In a dilution plating experiment, why is it important to use plates with a countable number of colonies (e.g., 30-300)?

Accepted Answer

To ensure accurate counting and avoid merged colonies that form a bacterial lawn.

Question 18

Which of the following is NOT a mechanism of horizontal gene transfer in bacteria?

Accepted Answer

Binary fission

Question 19

A student observes that an E. coli strain can grow on minimal medium only when methionine is added. How should this strain be classified?

Accepted Answer

Methionine auxotroph

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