Read the following paragraph carefully and answer the given questions.
In the year 1963 the two enzymes responsible for restricting the growth of bacteriophage in E. coli were isolated. The first restriction endonuclease - Hind II, whose functioning depended on a specific DNA nucleotide sequence was characterized five years later. It was found that Hind II always cut DNA molecules at a particular point by recognizing 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 recognize different recognition sequences.
DNA fragments can be separated by a technique known as:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Gel electrophoresis
In gel electrophoresis, the fragments are made to move through an agarose gel under an electric field. DNA is negatively charged, so the fragments move towards the anode. The gel acts like a sieve, so smaller fragments move farther. In this way the fragments are separated according to their size.
Option (1) Autoradiography — Incorrect. Autoradiography detects radioactively labelled DNA bands on an X-ray film. In DNA fingerprinting it is used after the fragments have already been separated. It shows the bands; it does not separate them.
Option (2) ELISA — Incorrect. ELISA (Enzyme Linked Immuno-sorbent Assay) is a diagnostic test based on antigen-antibody interaction. It is used to detect an infection such as HIV. It does not work on DNA fragments.
Option (3) Ultracentrifugation — Incorrect. Ultracentrifugation spins a sample at very high speed and separates molecules according to their density. Meselson and Stahl used this kind of spinning in a caesium chloride gradient to separate heavy and light DNA. It is not the method used to separate DNA fragments by size.
Option (4) Gel electrophoresis — Correct. It separates DNA fragments by size. The separated bands are seen after staining with ethidium bromide under UV light, and are then cut out of the gel.
Note 1: This passage is taken from NCERT as it is. 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 07-06-2026 (Slot 2), on 29-05-2026 (Slot 2) and on 14-05-2026 (Slot 1).
NCERT Extract:
Restriction Enzymes – …
