A thermostable DNA polymerase is isolated from

The correct answer is Option (1) → Thermus aquaticus

Option (1) Thermus aquaticus — Correct. This bacterium lives in hot springs, so its DNA polymerase stays active at the high temperature used in PCR. The enzyme is called Taq polymerase after it.

Option (2) Agrobacterium tumefaciens — Incorrect. It is a pathogen of dicot plants. Its Ti plasmid is used as a cloning vector to carry genes into plants.

Option (3) E. coli — Incorrect. E. coli is the usual host bacterium for cloning, with vectors such as pBR322. Its DNA polymerase is destroyed by the heat of PCR.

Option (4) Salmonella typhimurium — Incorrect. The first recombinant DNA was made by joining an antibiotic resistance gene to a plasmid of this bacterium. It is not the source of a heat-stable polymerase.

Note: A similar question was also asked in CUET (UG) 2026 on 06-06-2026 (Slot 1), in CUET (UG) 2025 on 29-05-2025 (Slot 1), on 20-05-2025 (Slot 2) and on 16-05-2025 (Slot 1), and in CUET (UG) 2023 on 30-05-2023 (Slot 1).

NCERT Extract:
Amplification of Gene of Interest Using PCR – “… PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene (or DNA) of interest is synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase. The enzyme extends the primers using the nucleotides provided in the reaction and the genomic DNA as template. If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times, i.e., 1 billion copies are made. Such repeated amplification is achieved by the use of a thermostable DNA polymerase (isolated from a bacterium, Thermus aquaticus), which remain active during the high temperature induced denaturation of double stranded DNA.”