Question

The construction of the first recombinant DNA was done by using the native plasmid of :

• A. E. coli
• B. Salmonella typhimurium
• C. B. thuringiensis
• D. Agrobacterium

Hint:

Cohen and Boyer in 1972 linked an antibiotic resistance gene to a native plasmid of Salmonella typhimurium.

Answer 1

The Correct Option is B

Step 1: Recall the history:
In 1972 Stanley Cohen and Herbert Boyer made the first recombinant DNA. They joined a gene for antibiotic resistance to a native plasmid.
The NCERT text says this plasmid came from the bacterium Salmonella typhimurium.

Step 2: Meaning of the experiment:
They cut the plasmid and the foreign gene with the same restriction enzyme. DNA ligase then joined them.
The new molecule was put into E. coli and it copied itself there.

Step 3: Check option (A):
E. coli was the host in which the recombinant DNA was copied. It was not the source of the native plasmid. So (A) is wrong.

Step 4: Check option (B):
Salmonella typhimurium gave the native plasmid. So (B) is right.

Step 5: Check option (C):
B. thuringiensis gives the Bt toxin gene used in crop plants. It is not linked to the first recombinant DNA. So (C) is wrong.

Step 6: Check option (D):
Agrobacterium has the Ti plasmid. It is used as a vector for plants, and it came later. So (D) is wrong.

Final Answer:
The first recombinant DNA used the native plasmid of Salmonella typhimurium, option (B). \[ \boxed{\text{(B)}} \]