Question

Read the passage and answer the following question.

Shigella is a bacterium that is very closely related to E. coli but is incapable of fermenting lactose (lactose negative). A gene for resistance to the antibiotic ampicillin (amR) was inserted in between two of the structural genes of the lac operon and then inserted into Shigella.

The effect of this insertion would be

• A. The cell would immediately be resistant to ampicillin.
• B. The cell would be resistant to ampicillin only in the presence of tryptophan.
• C. The cell would produce the enzyme $\beta$-lactamase as soon as it started to grow exponentially.
• D. The cell would spontaneously utilize DNA-editing enzymes to restore the original genotye.
• E. The cell would be resistant to ampicillin only in the presence of lactose.

Answer 1

Answer: (E)

The cell would be resistant to ampicillin only in the presence of lactose.

The lac operon is an operon responsible for transport and metabolism of lactose in bacteria and carries the genes that code for enzymes involved in lactose metabolism only. It is expressed in the presence of lactose (co inducer; not tryptophan) in the medium and otherwise its expression is repressed by binding of repressor protein to operator region. Therefore, the ampicillin-resistant gene will be expressed in the presence of lactose only making the cells ampicillin resistant. The lac z, lac y, and lac a genes in lac operon are the structural genes that code for beta-galactosidase, beta-galactoside permease, and beta-galactoside transacetylase respectively. Thus, the correct answer is option E.