Question

In pBR 322 ampR and tetR genes are present. When we ligate a foreign DNA at Pst I site, then recombinant plasmids will lose the resistance to the

• A. Both (A) and (B)
• B. Ampicillin
• C. None of the above
• D. Tetracycline

Answer 1

The correct option is B Ampicillin

pBR322
pBR322 is the most common plasmid vector. It is named after Bolivar and Rodriguez.

Features of pBR 322
Common features of pBR322 are as follows:
● Restriction sites - It has multiple restriction sites for different restriction enzymes such as Pvu II, Pvu I, BamH I, EcoR I, Hind III, Pst I, Sal I, Cla I, etc.
● Rop - It is a specific base sequence that codes for the rop protein involved in the replication of the plasmid.
● Selectable markers - It has two selectable markers, one for ampicillin resistance ampR and one for tetracycline resistance tetR.
● Ori - It has an ori site or origin of replication site. Any piece of DNA when linked to this sequence can be made to replicate within the host cells.

Pictorial representation of pBR322

Figure : pBR322

Role of ampR and tetR gene in pBR322
ampR and tetR gene present on pBR 322 provides ampicillin and tetracycline resistance to the host cell respectively. Therefore, host cells containing pBR322 can grow in nutrient media with ampicillin and tetracycline. These two genes act as selectable markers that are used to identify and differentiate transformed and non-transformed cells.

Gene insertion at Pst I restriction site
Pst I restriction site is located at the ampicillin resistance gene. The insertion and ligation of the gene of interest in the ampR gene would result in the loss of resistance to ampicillin. This is called insertional inactivation. Therefore a host cell transformed with such a plasmid will not be able to grow in a medium with ampicillin.

Final answer
(A) Ampicillin