A culture of Lactobacillus is showing blue colonies and colourless colonies. The appearance of blue colour is due to:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Chromogenic substrate
In insertional inactivation, the new DNA is inserted into the gene for the enzyme β-galactosidase. Bacteria whose plasmid has no insert still make the enzyme. It acts on a chromogenic substrate (one that gives a colour) and turns the colonies blue. Bacteria with an insert lose the enzyme, so their colonies stay colourless. These are the recombinants.
(1) Chitinase — Incorrect. This enzyme breaks down chitin, the material of fungal cell walls. It gives no colour.
(2) Chromogenic substrate — Correct. It gives blue colonies where the enzyme is active.
(3) Cellulase — Incorrect. This enzyme breaks down cellulose, the material of plant cell walls. It is not a colour marker.
(4) Permease — Incorrect. This is a protein of the lac operon that carries lactose into the cell. It does not give a colour.
NCERT Extract:
Cloning Vectors – “… The presence of a chromogenic substrate gives blue coloured colonies if the plasmid in the bacteria does not have an insert. Presence of insert results into insertional inactivation of the β-galactosidase gene and the colonies do not produce any colour, these are identified as recombinant colonies.”