Which of the following statements about restriction enzymes is incorr…

The correct answer is Option (3) → They are used to glue together short segments of DNA.

Restriction enzymes cut DNA at specific sequences. They belong to the nucleases, the enzymes that break the bonds in nucleic acids. Joining DNA pieces is the job of another enzyme, DNA ligase. Cutting and joining are both needed to make recombinant DNA, but different enzymes do them.

Option (1) These are known as molecular scissors. — A correct statement. The book calls restriction enzymes ‘molecular scissors’ because they cut DNA at chosen places.

Option (2) Restriction enzymes belong to a larger class of enzymes called nucleases. — A correct statement. Restriction enzymes are part of the larger class of nucleases, which has two kinds, exonucleases and endonucleases.

Option (3) They are used to glue together short segments of DNA. — An incorrect statement. Gluing DNA pieces together is done by DNA ligase, which joins the ends of cut DNA. Restriction enzymes only cut.

Option (4) Their primary function is to cut DNA. — A correct statement. Their main role is to cut DNA at a specific recognition sequence.

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 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.”
Restriction Enzymes – “… Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds; exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of the DNA whereas, endonucleases make cuts at specific positions within the DNA.”