Restriction endonucleases
Read the following passage carefully and answer the given question.
The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’– (restriction enzymes). They cut DNA molecule at particular points by recognition a specific sequence of six base pairs. The piece of DNA was then linked with the plasmid DNA. These plasmid DNA act as vectors to transfer the piece of DNA attached to it. 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. Each restriction endonuclease recognizes a specific palindromic nucleotide sequences in the DNA. Palindromes are groups of letters that form the same words when read both forward and backward, e.g., “MALAYALAM”. As against a word-palindrome where the same word is read in both directions.
Which one of the following enzymes is also called as molecular scissors?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Restriction endonuclease
The passage calls restriction enzymes the ‘molecular scissors’. A restriction endonuclease checks along the DNA until it finds its specific recognition sequence. It then cuts both strands of the double helix at specific points in their sugar-phosphate backbones, just as scissors cut at a chosen place.
(1) DNA polymerase — Incorrect. It makes new DNA; it does not cut it.
(2) DNA Ligase — Incorrect. It does the opposite job. It joins cut pieces of DNA together.
(3) Restriction endonuclease — Correct. It cuts DNA at specific places, like scissors.
(4) Taq polymerase — Incorrect. It is a heat-stable DNA polymerase from the bacterium Thermus aquaticus. It makes DNA in PCR.
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.