Gel Electrophoresis
Read the following passage carefully and answer the questions given below:
As a result of cutting DNA by restriction endonucleases, DNA becomes fragmented. These fragments can be separated by gel electrophoresis. Since DNA fragments are negatively charged, they are made to move towards the anode under an electric field and are separated according to their size. The matrix agarose, which is commonly used nowadays, is a natural polymer extracted from seaweeds. DNA fragments move through the sieving effect of the agarose gel and get separated according to their size. Therefore, the smaller the fragment size, the farther they move. After staining the separated DNA fragments with a compound called ethidium bromide, they can be seen as bright orange coloured bands when exposed to UV radiation. The separated DNA fragments are cut out from the agarose gel and extracted from the gel pieces. This process is called elution. The DNA purified in this way is joined to cloning vectors for the construction of recombinant DNA.
In gel electrophoresis, DNA fragments are separated according to their:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Size
The agarose gel acts like a sieve. All DNA fragments carry negative charge and move towards the anode. Small fragments slip through the pores of the gel easily and move farther. Large fragments move slowly and stay behind. So the fragments are separated according to their size. The smaller the fragment, the farther it moves.
(1) Temperature — Incorrect. All the fragments are at the same temperature in the gel, so temperature cannot separate them.
(2) Size — Correct. The sieving effect of the gel separates fragments by size.
(3) Volume — Incorrect. The separation is described by fragment size (length of DNA), not by volume.
(4) Charge — Incorrect. Charge makes all the fragments move towards the anode, but it does not sort them. It is the size that decides how far each one goes.
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.