(a) Name the selectable markers in the cloning vector pBR322 ? Mention the role they play.

(b) Why is the coding sequence of an enzyme -galactosidase a preferred selectable marker in comparison to the ones named above ?

(a) Selectable markers in the coming vectors pBR322 :

i. Ampicillin resistance antibiotics
ii.Tetracycline
(b) The selection of recombinants due to inactivation of antibiotics is a difficult process and requires simultaneous plating on two plates having different antibiotics. Thus, to differentiate between the non-recombinants and recombinants The enzyme  - galactosidase is preferred as a selectable marker as it allows the person to differentiate the non-recombinants from recombinants. in this case when the recombinant DNA is inserted into the coding sequence of the enzyme -galactosidase. The insertion leads to the inactivation of the enzyme and hence in the presence of a chromogenic substance the recombinants do not give coloured colonies, while the non-recombinants produce blue colonies. Thus no separate plating is required.