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.
The DNA fragments can be separated by a technique known as:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Gel electrophoresis
Restriction enzymes cut DNA into fragments of different sizes. In gel electrophoresis, the fragments are placed in an agarose gel under an electric field. DNA is negatively charged, so the fragments move towards the anode. The gel acts as a sieve: smaller fragments move farther. So the fragments get separated according to their size.
(1) Centrifugation — Incorrect. Centrifugation separates materials by spinning them at high speed, according to their density. It is not the method used to separate DNA fragments by size.
(2) Gel electrophoresis — Correct. It separates DNA fragments by size.
(3) DNA fingerprinting — Incorrect. DNA fingerprinting is a method of identifying a person. It uses electrophoresis as one of its steps, but it is not itself a separation technique.
(4) Autoradiography — Incorrect. Autoradiography detects DNA bands that carry a radioactive label on an X-ray film. It detects the fragments; it does not separate them.
Note 1: 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.
Note 2: A similar question was also asked in CUET (UG) 2026 on 29-05-2026 (Slot 2) and on 14-05-2026 (Slot 1), and in CUET (UG) 2025 on 29-05-2025 (Slot 1).
NCERT Extract:
Restriction Enzymes – “… Separation and isolation of DNA fragments : The cutting of DNA by restriction endonucleases results in the fragments of DNA. These fragments can be separated by a technique known as gel electrophoresis. Since DNA fragments are negatively charged molecules they can be separated by forcing them to move towards the anode under an electric field through a medium/matrix.”