If a restriction enzyme is used to digest both a closed circular DNA plasmid and a linear DNA molecule with five recognition sites each, how many fragments will be produced in total?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 11
A restriction enzyme finds its recognition sequence and cuts both strands of the DNA there. The number of pieces depends on the shape of the DNA:
- Circular DNA (plasmid): it has no free ends. One cut only opens the circle into one linear piece. So the number of fragments = number of cuts. With 5 sites: 5 fragments.
- Linear DNA: it already has two ends. One cut gives 2 pieces, two cuts give 3 pieces. So the number of fragments = number of cuts + 1. With 5 sites: 5 + 1 = 6 fragments.
- Total: 5 + 6 = 11 fragments.
The answers 10 and 12 come from treating both molecules alike (5 + 5, or 6 + 6); 13 has no basis.
NCERT Extract:
Restriction Enzymes – “… 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).”