Cloning Sites & Antibiotic Resistance — NEET Biology
✅ Asked in NEET 2026Cloning 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. 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
NTA tests understanding of how restriction sites in vectors enable selection of recombinant DNA. The core concept: inserting foreign DNA at a restriction site within an antibiotic resistance gene (like BamHI in tetracycline resistance) causes loss of that resistance, creating a selectable marker. The common mistake: students confuse which plasmid grows on which medium—recombinants grow on ampicillin only (lost tetracycline resistance due to insertion), while non-recombinants grow on both antibiotics. Remember: the disrupted gene fails, the intact gene works. This dual antibiotic system elegantly separates recombinants from non-recombinants, making it a favorite NEET concept since it combines vector design, restriction enzymes, and selection strategy.
This paragraph was tested 4 times in NEET.
Match List I with List II — A. Genetically modified organism, B. Thermostable DNA polymerase, C. Ti plasmid, D. pBR322 with I. Agrobacterium tumefaciens, II. Bt cotton, III. Thermus aquaticus, IV. Escherichia coli.
Select the restriction endonuclease enzymes whose restriction sites are present for the tetracycline resistance (tetR) gene in the pBR322 cloning vector: (NEET 2024)
Ligation of foreign DNA at which of the following site will result in loss of tetracycline resistance of pBR322? (NEET 2023)
Plasmid pBR322 has a PstI restriction enzyme site within gene ampR that confers ampicillin resistance. If this enzyme is used for inserting a gene for β-galactosidase production and the recombinant plasmid is inserted in an E. coli strain: (NEET 2021)
Through deep analysis of NEET and NTA, 88 of 90 questions from the NEET 2026 paper were matched straight from the MedicNEET Biology question bank.
88/90MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.