What is the result of having more than one recognition site for the same restriction enzyme within a vector?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Several fragments are generated, which complicates gene cloning.
To link alien DNA, a vector should have very few, preferably single, recognition sites for the commonly used restriction enzymes. If the same enzyme finds more than one site in the vector, it cuts the vector at each of them, and the vector falls into several fragments. Putting the alien DNA in the right place then becomes difficult.
Option (1) Several fragments are generated, which complicates gene cloning. — Correct. More than one recognition site gives several fragments, and this complicates gene cloning.
Option (2) A single linear vector molecule is produced, which makes ligation easier. — Incorrect. A single linear vector molecule forms only when the vector has a single site for that enzyme.
Option (3) Blue colonies appear even when the insert is present. — Incorrect. Blue colonies are about insertional inactivation of the β-galactosidase gene; colonies with an insert do not produce colour. This has nothing to do with the number of recognition sites.
Option (4) The alien DNA can be ligated at any of these sites, which makes cloning easier. — Incorrect. Many sites do not make cloning easier. The vector is broken into pieces, which is why vectors with single sites are preferred.
NCERT Extract:
Cloning Vectors – “… (iii) Cloning sites: In order to link the alien DNA, the vector needs to have very few, preferably single, recognition sites for the commonly used restriction enzymes. Presence of more than one recognition sites within the vector will generate several fragments, which will complicate the gene cloning (Figure 9.4).”