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.

If Shigella is normally lactose negative, from where did the lac operon originate?

• A. The bacteria genome was removed, the lac operon was added, and then the genome was inserted back into the original cell.
• B. The lac operon was present on a plasmid which was inserted into the cell by transformation.
• C. The bacterial genome was removed and replaced by a plasmid containing the lac operon.
• D. The lac operon developed in the cell by random mutation and strong selection pressures.
• E. The lac operon was derived from mutations of the tryptophan operon which is normally present in Shigella.

Answer 1

Answer: (B)

Shigella is lactose negative but lac operon was present on the plasmid used for transformation of the bacteria as mentioned ampicillin resistant gene was inserted between the structural genes of lac operon only. The plasmid is extrachromosomal DNA which is introduced into the host cell while maintaining its original genome. Thus, the correct answer is option B.