Read the following passage carefully.
Restriction enzyme
Restriction enzymes cut the strand of DNA a little away from the center of the palindrome sites, but between the same two bases on the opposite strands. This leaves single stranded portions at the ends. There are overhanging stretches called sticky ends on each strand. These are named so because they form hydrogen bonds with their complementary cut counterparts. This stickiness of the ends facilitates the action of the enzyme DNA ligase. The word palindromes refer to group of letters that forms the same words when read both forward and backward. Restriction endonucleases are used in genetic engineering as molecular scissors.
Which one of the following enzyme helps to join DNA?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → DNA ligase
DNA ligase joins two DNA pieces by forming phosphodiester bonds between them. In recombinant DNA work, the sticky ends of the vector and the alien DNA pair up first, and ligase then seals the joints.
Option (1) DNA polymerase — Incorrect. It builds a new DNA strand by adding nucleotides on a template, during replication and in PCR. It does not join two separate pieces.
Option (2) RNA polymerase — Incorrect. It makes RNA from a DNA template during transcription.
Option (3) DNA ligase — Correct. The passage says the stickiness of the ends helps the action of DNA ligase, the enzyme that joins DNA.
Option (4) Endonuclease — Incorrect. An endonuclease cuts DNA at specific positions inside it. Restriction enzymes are endonucleases; they cut, while ligase joins.
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.