How does b-galactosidase coding sequence act as a selectable marker? Why is it a preferred selectable marker to antibiotic resistance genes? Explain.
Answer:- genes called selectable markers help in selecting those host cells which contain the vectors and eliminating the non-transformants. b-galactosidase is an alternative selectable marker developed to differentiate recombinants and non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substance. A recombinant DNA is inserted in the coding sequence of an enzyme b-galactosidase. This causes inactivation of the enzyme which is called insertional inactivation. If the plasmid in the bacterium does not have an insert, the presence of a chromogenic substrate gives blue coloured colonies.
Presence of insert results into insertional inactivation of the b-galactosidase and, therefore, the colonies do not produce any colour, these colonies are marked as recombinant colonies. b-galactosidase is a preferred selectable marker to antibiotic resistance genes because due to inactivation of antibiotics, selection of recombinants becomes burdensome process as it requires simultaneous plating on two plates having different antibiotics. But by using b-galactosidase as selectable marker, we can select recombinants and non-recombinants on a single plate.
