Which of the following statements about restriction endonucleases is…

The correct answer is Option (2) → They recognise a specific palindromic sequence and cut both DNA strands at specific points.

Restriction endonucleases are the ‘molecular scissors’ of genetic engineering. Each one inspects the DNA until it finds its own palindromic recognition sequence, binds there and cuts both strands of the double helix at specific points in their sugar-phosphate backbones.

Option (1) They remove nucleotides one by one from the ends of the DNA molecule. — Incorrect. Removing nucleotides from the ends of DNA is the work of exonucleases. Endonucleases cut at specific positions within the DNA.

Option (2) They recognise a specific palindromic sequence and cut both DNA strands at specific points. — Correct. Recognising a specific palindromic sequence and cutting both strands at specific points describes a restriction endonuclease.

Option (3) They act on cut DNA molecules and join their ends to form recombinant DNA. — Incorrect. Joining the ends of cut DNA molecules is the work of DNA ligase.

Option (4) They add methyl groups to the DNA at a specific recognition sequence. — Incorrect. Adding methyl groups to DNA was done by the other enzyme isolated along with the restriction endonuclease from E. coli, not by the endonuclease itself.

NCERT Extract:
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. Each restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence. Once it finds its specific recognition sequence, it will bind to the DNA and cut each of the two strands of the double helix at specific points in their sugar-phosphate backbones (Figure 9.1). Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA.”