Question

How does the β-galactosidase coding sequence act as a selectable marker? Why is it a preferred selectable marker to antibiotic resistance genes? Explain.

Answer 1

Selective marker is used in the selection of recombinants on the basis of the ability to produce color in the presence of chromogenic substrate. β-galactosidase is an enzyme that converts galactose into lactose. In this, a  recombinant DNA is inserted within the coding sequence of enzyme, β-galactosidase, which results in inactivation of an enzyme referred to as "insertional inactivation". As a result of this, non-recombinants will produce blue-colored colonies while the recombinants will produce color-less colonies.
The coding sequence for the enzyme ​β-galactosidase is preferred over antibiotic resistance genes because recombinants can be easily visualized and the process is less cumbersome.