BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
Read the following passage carefully and answer the given questions.
The first constructed recombinant DNA emerged from the possibility of linking a gene encoding antibiotic resistance with a native autonomously replicating circular extra-chromosomal DNA of Salmonella typhimurium. Stanley Cohen and Herbert Boyer accomplished this by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance. The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’. 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. The cut 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.
DNA fragments can be separated by a technique known as:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Gel electrophoresis
Restriction enzymes cut DNA into fragments. 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.
(1) Autoradiography — Incorrect. Autoradiography detects radioactively labelled DNA bands on an X-ray film. It does not separate them.
(2) ELISA — Incorrect. ELISA is a diagnostic test based on antigen-antibody interaction.
(3) Gel electrophoresis — Correct. It separates DNA fragments by size.
(4) Centrifugation — Incorrect. Centrifugation separates materials by spinning them according to their density. It is not the method used to separate DNA fragments by size.
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 07-06-2026 (Slot 2) and on 29-05-2026 (Slot 2), and in CUET (UG) 2025 on 29-05-2025 (Slot 1).
NCERT Extract:
Restriction Enzymes – …
