Statement I: Exonucleases make cuts at specific positions within the DNA.
Statement II: A restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence and binds to the DNA once it finds its specific recognition sequence.
In the light of the above statements, choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Statement I is incorrect but Statement II is correct.
Restriction enzymes belong to a larger group of enzymes called nucleases. Nucleases are of two kinds. Exonucleases remove nucleotides from the ends of DNA. Endonucleases make cuts at specific positions within the DNA.
Statement I: Exonucleases make cuts at specific positions within the DNA. — False. Cutting within the DNA is the work of endonucleases. Exonucleases work only at the ends, removing nucleotides one after another.
Statement II: A restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence and binds to the DNA once it finds its specific recognition sequence. — True. The enzyme moves along the DNA until it finds its recognition sequence, a palindrome. It then binds and cuts both strands at specific points in the sugar-phosphate backbones.
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).”