The blue and white selectable markers have been developed which differentiate recombinant colonies from non-recombinant colonies on the basis of their ability to produce colour in the presence of a chromogenic substrate. Given below are two statements about this method:

Statement I: The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies.

Statement II: The colonies without blue colour have DNA insert in the plasmid and are identified as recombinant colonies.

In the light of the above statements, choose the most appropriate answer from the options given below:

The correct answer is: Statement I is incorrect but Statement II is correct

Explanation:

Blue-white screening is a molecular biology technique that allows for the rapid and convenient detection of recombinant bacteria in cloning experiments.

This method relies on the principle of insertional inactivation of the lacZ gene, which codes for the enzyme β-galactosidase. The process works as follows:

1. Non-recombinant plasmid (No insert):
When a plasmid does not have a DNA insert in its cloning site (which lies within the lacZ gene sequence), the lacZ gene remains intact and functional. The host bacteria carrying this plasmid produce active β-galactosidase. In the presence of a chromogenic substrate like X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside), β-galactosidase cleaves X-gal, yielding a blue product. Consequently, colonies formed by non-recombinant cells turn blue.

2. Recombinant plasmid (DNA insert present):
When a foreign DNA insert is successfully ligated into the cloning site, it disrupts the coding sequence of the lacZ gene. This is called insertional inactivation. Because the gene is disrupted, no functional β-galactosidase is produced. Therefore, these bacteria cannot cleave X-gal, and the colonies remain white (without blue colour).

Now, let's analyze the given statements:

• Statement I: "The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies."
  This statement is incorrect because blue colonies are non-recombinant (they do not contain the DNA insert).
• Statement II: "The colonies without blue colour have DNA insert in the plasmid and are identified as recombinant colonies."
  This statement is correct because white colonies (without blue colour) represent recombinant clones containing the DNA insert.

Thus, Statement I is incorrect but Statement II is correct.