Cloning Vectors
Read the following passage carefully 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. The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes. For example, you can ligate a foreign DNA at the BamH I site of tetracycline resistance gene in the vector pBR322. 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 ampicillin containing medium. The transformants growing on ampicillin containing medium are then transferred on a medium containing tetracycline. 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. Therefore, alternative selectable markers have been developed which differentiate recombinants from non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate.
E. coli cloning vector pBR322 does not have ______restriction site -
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Hind II
The E. coli cloning vector pBR322 has restriction sites for Hind III, EcoR I, BamH I, Sal I, Pvu II, Pst I and Cla I. It also has the ori sequence and two antibiotic resistance genes.
(1) Hind III — A restriction site of pBR322. It is shown on the map of the plasmid.
(2) Sal I — A restriction site of pBR322. It lies in the tetracycline resistance gene region of the map.
(3) Pst I — A restriction site of pBR322. It lies in the ampicillin resistance gene region of the map.
(4) Hind II — Not a restriction site of pBR322. It is not on the map. Hind II is a different enzyme from Hind III. It was the first restriction endonuclease to be isolated.
Note 1: 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.
Note 2: A similar question was also asked in CUET (UG) 2026 on 15-05-2026 (Slot 1).
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). The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes. For example, you can ligate a foreign DNA at the BamH I site of tetracycline resistance gene in the vector pBR322.”
