Question

Due to difficulty in inactivation of antibiotics, alternate selectable markers developed to differentiate recombinants from non-recombinants on the basis of ability to

• A. All are correct
• B. Produce blue colour colonies due to insertional
  inactivation
  
• C. Produce colour in the presence of a chromogenic
  substrate
  
• D. Produce bright orange colour in the presence of a
  chromogenic substrate
  

Answer 1

The correct option is C Produce colour in the presence of a chromogenic

substrate

Insertional inactivation
Insertional inactivation is the process of inactivation of a marker gene by the insertion of foreign gene in its coding sequence.
For example, insertion of a foreign gene in the coding sequence of 𝛽-galactosidase gene.

Selectable marker
Selectable marker is a gene in a vector that helps in the selection of transformants (host cells which have successfully taken up the foreign DNA) and non-transformants (cells which have not taken up the vector DNA).
For example, antibiotic resistance genes in the vector act as selectable markers because they confer antibiotic resistance to the transformants while the non-transformants remain susceptible to antibiotics.

Insertional inactivation of antibiotic markers
Selection of recombinants using insertional inactivation of antibiotic markers is a tedious process. This is because it requires careful plating of transformants on two different antibiotics containing media in consecutive days.

Alternative selectable markers
Alternative selectable markers differentiate recombinants from non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate.
For example, insertion of a foreign gene in the coding sequence of 𝛽-galactosidase gene, synthesize a non-functional enzyme, which does not convert chromogenic substrate into a product. Hence only the non-recombinant colonies produce blue colour due to the presence of chromogenic substrate and absence of insert. In this way, it helps in the selection of recombinants.

Advantages of alternate selectable markers
Advantages of alternate selectable markers are as follows:
● It is an easy method
● It requires plating on one medium with the chromogenic substrates.
● The recombinants will produce white colonies while the non-recombinants will produce colored colonies in the presence of chromogenic substrate.
● It is easy to identify recombinants and non-recombinants on the basis of colour.

Final answer
(B) Produce colour in the presence of a chromogenic substrate