Statement I: DNA ligase acts on cut DNA molecules and joins their ends.
Statement II: Restriction endonucleases cut DNA at random sites within the molecule.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Only statement I is correct.
Recombinant DNA is made with two kinds of enzymes. Restriction enzymes cut DNA, and DNA ligase joins the cut pieces, for example a gene and a plasmid vector.
Statement I: DNA ligase acts on cut DNA molecules and joins their ends. — True. After a gene and a vector are cut with the same restriction enzyme, ligase seals their ends together. The result is a recombinant DNA molecule.
Statement II: Restriction endonucleases cut DNA at random sites within the molecule. — False. Each restriction endonuclease first finds its own specific recognition sequence, a palindrome. It cuts only there, at specific points. That is what makes these enzymes useful in genetic engineering.
NCERT Extract:
Principles of Biotechnology – “… 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. This makes a new combination of circular autonomously replicating DNA created in vitro and is known as recombinant DNA.”
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). Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA.”