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.
In order to link the foreign DNA, the vector needs to have preferably ______ recognition site.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Single
A vector is cut by a restriction enzyme so that the alien DNA can be joined into it. For this, the vector should have very few recognition sites, and preferably only one.
(1) More than four — Incorrect. Many sites would cut the vector into many fragments.
(2) Single — Correct. One site opens the circular vector at one point only. The alien DNA is joined at that point and the vector stays in one piece.
(3) Two — Incorrect. Two sites would cut the vector into two fragments, which is more than is wanted.
(4) More than two — Incorrect. More sites mean more fragments, and the cloning gets complicated.
Figure 9.4 E. coli cloning vector pBR322 showing restriction sites (Hind III, EcoR I, BamH I, Sal I, Pvu II, Pst I, Cla I), ori and antibiotic resistance genes (ampR and tetR). rop codes for the proteins involved in the replication of the plasmid
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.
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.”
