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.
In pBR322 'rop' codes for -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Replication of the plasmid
Option (1) Translation of the plasmid — Incorrect. Translation makes proteins from mRNA; a plasmid is DNA and is not translated.
Option (2) Transcription of the plasmid — Incorrect. Transcription is done by the host’s RNA polymerase; rop has no part in it.
Option (3) Replication of the plasmid — Correct. rop codes for the proteins involved in the replication of the plasmid.
Option (4) Recombination of plasmid — Incorrect. Joining alien DNA into the plasmid is done by the scientist with restriction enzymes and ligase, not by the rop gene.
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 –
