Question

The pBR322 cloning vector has genes coding for tetracycline and ampicillin resistance. A foreign DNA to be cloned is inserted into the tetracycline resistance gene and the recombinant plasmid is then transformed into E. coli cells. Which one of the following choices is the most likely outcome of this cloning reaction?

• A. The cells with the recombinant plasmid can grow in the presence of ampicillin but not tetracycline
• B. The cells with the recombinant plasmid can grow in the presence of both ampicillin and tetracycline
• C. The cells with the non-recombinant plasmid can grow in the presence of ampicillin but not tetracycline
• D. The cells with the non-recombinant plasmid can grow in the presence of tetracycline but not ampicillin

Hint:

Insertional Inactivation Rule:
Whichever marker gene you "insert" into is "inactivated" (destroyed).
Inserted in $tet^R$ $\rightarrow$ Tetracycline resistance is lost.
Inserted in $amp^R$ $\rightarrow$ Ampicillin resistance is lost.
The other marker gene remains perfectly intact and serves for primary selection.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question deals with insertional inactivation. When a foreign DNA fragment is inserted into the coding sequence of a selectable marker gene (like an antibiotic resistance gene), that gene is disrupted and becomes non-functional.

Step 2: Detailed Explanation:
$\bullet$ The pBR322 plasmid contains two resistance genes: $amp^{R}$ (ampicillin) and $tet^{R}$ (tetracycline).
$\bullet$ In this experiment, foreign DNA is inserted into the $tet^{R}$ gene.
$\bullet$ As a result, the $tet^{R}$ gene is inactivated. The transformed cell loses its resistance to tetracycline.
$\bullet$ However, the $amp^{R}$ gene remains intact because no DNA was inserted there. Therefore, the cell remains resistant to ampicillin.
$\bullet$ Non-recombinant plasmids (those that didn't take the insert) would retain both resistances.

Step 3: Final Answer:
Recombinant cells will be ampicillin-resistant ($Amp^{R}$) but tetracycline-sensitive ($Tet^{S}$).
This corresponds to option (A).