Read the passage and answer the following question :
In 1963, enzymes that restricted the growth of bacteriophage in Escherichia coli were discovered. Among them, restriction endonucleases played a vital role. Five years later, Hind II, the first restriction endonuclease, was isolated and characterized. Hind II was known for its ability to cut DNA molecules at a specific point by recognizing a unique six-base pair sequence, which was termed the "recognition sequence" for Hind II. Currently, over 900 restriction enzymes have been identified from more than 230 bacterial strains, each recognizing different recognition sequences. For example, EcoRI is derived from Escherichia coli RY 13, where the letter 'R' represents the name of the strain, and the Roman numeral indicates the order of enzyme isolation from that strain. Restriction enzymes belong to a larger class of enzymes that can be divided into two types: exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of DNA, while endonucleases make cuts at specific positions within the DNA. Each restriction endonuclease "inspects" the length of a DNA sequence until it finds its specific recognition sequence. Once located, it binds to the DNA and cuts both strands of the double helix at specific points in their sugar-phosphate backbones. Notably, each restriction endonuclease recognizes a specific palindromic nucleotide sequence in the DNA.
Two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated. One of them cut DNA. What did the other enzyme do?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → It added methyl groups to DNA.
Two enzymes that restricted the growth of bacteriophage in E. coli were isolated together. One of them added methyl groups to DNA. The other cut DNA, and this one was named restriction endonuclease. A few years later, Hind II became the first restriction endonuclease to be isolated and characterised.
Option (1) It added methyl groups to DNA. — Correct. One of the two enzymes methylated DNA, that is, it added methyl groups to it. The book contrasts it with its partner, which cut DNA.
Option (2) It joined the cut ends of DNA. — Incorrect. Joining the cut ends of DNA is the work of DNA ligase, which seals the ends of cut DNA molecules during the making of recombinant DNA.
Option (3) It removed nucleotides from the ends of DNA. — Incorrect. Removing nucleotides from the ends of DNA is the work of exonucleases. They are nucleases, and nucleases break DNA rather than add anything to it.
Option (4) It extended primers on the DNA template. — Incorrect. Extending primers on a DNA template is the job of DNA polymerase, as in the polymerase chain reaction (PCR).
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.”