Question

Recombinant DNA bearing ampicillin resistance gene is passed in $E.coli$. The latter are spread on agar plates containing ampicillin. Then

• A. both transformed and untransformed cells die
• B. transformed recipient cells grow and untransformed cells die
• C. both transformed and untransformed cells grow
• D. transformed recipient cells die and untransformed cells grow

Answer 1

The correct option is B transformed recipient cells grow and untransformed cells die

Selectable marker
Selectable marker is a gene in a vector that helps in the selection of transformants (host cells which have successfully taken up the foreign DNA) and non-transformants (cells which have not taken up the vector DNA).

For example, antibiotic resistance genes in the vector act as selectable markers because they confer antibiotic resistance to the transformants while the non-transformants remain susceptible to antibiotics.

Role of the ampicillin resistance gene
Ampicillin resistance gene acts as a selectable marker in host $Escherichiacolicells$ conferring resistance to ampicillin. Recombinant DNA contains the ampicillin resistance gene and hence the transformed cells will be resistant to the ampicillin causing them to grow on the ampicillin containing agar plates, whereas the non-transformants will die.

Final answer:
c. transformed recipient cells grow and untransformed cells die