Question

(a) Name the selectable markers in the cloning vector pBR322? Mention the role they play.
(b) Why is the coding sequence of an enzyme $\beta$-galactosidase, a preferred selectable marker in comparison to the ones named above?

Answer 1

a) In the cloning vector pBR322, the genes encoding for resistance to antibiotics such as ampicillin, tetracycline and chloramphenicol are considered as selectable markers. These markers are used to identify and eliminate non-transformants and permit the growth of transformants in media containing antibiotic like ampicillin.

b) The coding sequence of beta-galactosidase is considered as a selectable marker because the marker is present in the antibiotic resistance gene, due to which the antibiotics' inactivation occurs. This helps in the selection of recombinants. Such enzymatic markers can be used to differentiate between recombinants and non-recombinants on the basis of colour production in the presence of the chromogenic substance (X-gal) in the media. Either the colonies are blue or white due to the conversion of the chromogenic substance and the colour of the colonies helps to identify the recombinant organisms from the non-recombinant ones.