Hind II always cut DNA molecules at a particular point. This was because Hind II:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → recognised a specific sequence of base pairs, its recognition sequence
Hind II was the first restriction endonuclease to be isolated and characterised. It always cut DNA at the same point because it recognised a specific sequence of base pairs, called its recognition sequence. Wherever that sequence occurs, Hind II binds and cuts.
Option (1) added methyl groups to the DNA at that point — Incorrect. Adding methyl groups to DNA was the work of the other enzyme isolated along with the restriction endonuclease from E. coli. It does not cut DNA.
Option (2) recognised a specific sequence of base pairs, its recognition sequence — Correct. The functioning of Hind II depended on a specific DNA nucleotide sequence. Recognising this sequence is what fixed the point of the cut.
Option (3) removed nucleotides one by one from the ends of the DNA — Incorrect. Removing nucleotides from the ends of DNA is the work of exonucleases. Hind II is an endonuclease and cuts within the DNA.
Option (4) bound to the origin of replication of the DNA — Incorrect. The origin of replication is where DNA replication starts. A restriction enzyme does not look for it; it looks for its own recognition sequence.
NCERT Extract:
Restriction Enzymes – “In the year 1963, the two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated. One of these added methyl groups to DNA, while the other cut DNA. The later was called restriction endonuclease. The first restriction endonuclease–Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years later. It was found that Hind II always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind II.”