Assertion (A): Restriction endonucleases are also called ‘molecular scissors’.
Reason (R): Restriction endonucleases join the ends of cut DNA molecules to form recombinant DNA.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is correct but (R) is not correct.
Assertion (A): Restriction endonucleases are also called ‘molecular scissors’. — True. These enzymes cut DNA at specific locations, just as scissors cut at a chosen point. Their discovery made it possible to cut a piece of DNA out and link it to a plasmid.
Reason (R): Restriction endonucleases join the ends of cut DNA molecules to form recombinant DNA. — False. Restriction endonucleases only cut DNA. The cut ends are joined by another enzyme, DNA ligase, which acts on cut DNA molecules and joins their ends. Both enzymes are needed to make recombinant DNA, but they do opposite jobs.
So A is correct but R is not correct.
NCERT Extract:
Principles of Biotechnology – “… The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’– restriction enzymes. The cut piece of DNA was then linked with the plasmid DNA. These plasmid DNA act as vectors to transfer the piece of DNA attached to it. … The linking of antibiotic resistance gene with the plasmid vector became possible with the enzyme DNA ligase, which acts on cut DNA molecules and joins their ends.”