Comprehension:
Restriction Enzymes
Read the following passage carefully and answer the given questions.
Genetic engineering is the combination of techniques to alter the chemistry of genetic material (DNA and RNA), to introduce these into host organisms and thus change the phenotype of the host organism. 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. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strains of bacteria each of which recognise different recognition sequences. Restriction enzymes belong to a larger class of enzymes called nucleases namely exonucleases and endonucleases. The DNA fragments can be separated according to their size through sieving effect by gel electrophoresis. Hence, the smaller the fragment size, the farther it moves.
Restriction endonucleases cut the DNA strand of bacteriophage in E. coli due to:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Bacteriophage DNA is unmethylated
In E. coli two enzymes work together. One adds methyl groups to the DNA. The other, the restriction endonuclease, cuts DNA. The methyl groups protect DNA from the cutting enzyme.
(1) E. coli DNA is unmethylated — Incorrect. If its own DNA had no methyl groups, the endonuclease of E. coli would cut it too.
(2) Bacteriophage DNA is methylated — Incorrect. Methylated DNA is protected. A methylated phage DNA would not be cut.
(3) Both E. coli and bacteriophage DNA are methylated — Incorrect. Both would then be protected, and nothing would be cut.
(4) Bacteriophage DNA is unmethylated — Correct. The phage DNA that enters E. coli has no methyl groups, so the endonuclease cuts it and the growth of the phage is restricted. The E. coli DNA is methylated, so it is safe.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.