Molecular scissors
Read the following passage carefully and answer the given questions.
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 recognises different recognition sequences. The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species of the prokaryotic cell from which they were isolated, e.g., EcoRI comes from Escherichia coli RY 13. Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds: exonucleases and endonucleases. 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.
Bacteria restrict the growth of bacteriophages through:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cutting of foreign DNA
Bacteriophages are viruses that attack bacteria by putting their own DNA into the bacterial cell. Bacteria protect themselves with restriction enzymes. These enzymes cut the foreign (phage) DNA, so the phage cannot multiply. This is how the enzymes got their name: they “restrict” the growth of bacteriophages. In E. coli, one enzyme adds methyl groups to the bacterium’s own DNA, while the other, the restriction endonuclease, cuts DNA.
(1) Joining of foreign DNA — Incorrect. Joining the foreign DNA would help the phage, not stop it.
(2) Mucilaginous Sheath — Incorrect. A mucilage sheath is a slimy outer covering. It is not how bacteria restrict phages.
(3) Cutting of foreign DNA — Correct. The restriction endonuclease cuts the phage DNA.
(4) Transcription of foreign DNA — Incorrect. Transcription of the phage DNA would let the phage make its proteins and multiply.
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.
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.”