How many fragments will be generated, if a linear DNA molecule is digested with a restriction enzyme having 4 recognition sites on the DNA?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5
A restriction enzyme cuts DNA at each place where it finds its recognition sequence. Every cut splits one piece into two. In a linear DNA molecule, the number of pieces is always one more than the number of cuts.
- Recognition sites on the DNA = 4, so the number of cuts = 4.
- Fragments from a linear DNA = number of cuts + 1.
- Fragments = 4 + 1 = 5.
In a circular DNA, such as a plasmid, the number of fragments equals the number of cuts, so 4 sites would give 4 fragments. Here the DNA is linear, so the answer is 5.
NCERT Extract:
Restriction Enzymes – “… 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).”