Read the following paragraph and answer the given questions:
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. The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes. The recombinant plasmids will lose tetracycline resistance due to insertion of foreign DNA but can still be selected out from non-recombinant ones by plating the transformants on tetracycline containing medium. The recombinants will grow in ampicillin containing medium but not on that containing tetracycline. But, non- recombinants will grow on the medium containing both the antibiotics. In this case, one antibiotic resistance gene helps in selecting the transformants, whereas the other antibiotic resistance gene gets inactivated due to insertion of alien DNA, and helps in selection of recombinants. Origin of replication (ori) is a sequence from where replication starts and any piece of DNA when linked to this sequence can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA. So, if one wants to recover many copies of the target DNA it should be cloned in a vector whose origin support has high copy number.
Which one of the given sites is commonly used by the restriction enzyme?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Recognition site
A restriction enzyme first recognises a specific palindromic sequence in DNA, its recognition site, and then cuts there. The passage speaks of the “recognition sites for the commonly used restriction enzymes”.
Option (1) Selectable marker — Incorrect. A selectable marker is a gene, such as an antibiotic resistance gene, that helps pick out transformants; it is not a site an enzyme uses.
Option (2) Recognition site — Correct. Each restriction enzyme inspects the DNA and binds a specific sequence called the recognition sequence (recognition site). It then cuts each of the two strands at specific points.
Option (3) Restriction site — Incorrect. For one enzyme the two lie at the same place (see Q1 of this paper), but the passage keeps the words apart: ‘recognition sites for the commonly used restriction enzymes’, and ‘restriction site’ only for the place where the alien DNA is ligated.
Option (4) Vector site — Incorrect. There is no such site.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.