Read the Passage carefully and answer the questions.
There are two kinds of nucleases - exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of the DNA whereas, endonucleases make cuts at specific positions within the DNA. 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. Small volume cultures cannot yield appreciable quantities of products. To produce in large quantities, the development of bioreactors, where large volumes (100-1000 litres) of culture can be processed, was required. Thus, bioreactors can be thought of as vessels in which raw materials are biologically converted into specific products, individual enzymes, etc., using microbial plant, animal or human cells. PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene (or DNA) of interest are synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase.
Which one of the following statements is correct about DNA separation and isolation?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → DNA fragments are negatively charged and move towards anode under the electric field.
Option (1) DNA fragments are positively charged and move towards cathode under the electric field. — Incorrect. The charge and the electrode are both reversed. The backbone of DNA carries phosphate groups, and these make DNA negatively charged. A negative molecule is pulled to the positive electrode, the anode, not to the cathode.
Option (2) DNA fragments are negatively charged and move towards anode under the centrifugal field. — Incorrect. The charge and the electrode are right, but the force is wrong. A centrifugal field comes from spinning a tube fast in a centrifuge. Gel electrophoresis moves DNA with an electric field, not by spinning.
Option (3) DNA fragments are negatively charged and move towards anode under the electric field. — Correct. This is the principle of gel electrophoresis. The negatively charged fragments are forced through a matrix such as agarose gel towards the anode under an electric field. The gel acts like a sieve, so the smaller fragments move farther, and the fragments get separated by size.
Option (4) DNA fragments are negatively charged and move towards cathode under the centrifugal field. — Incorrect. Two parts are wrong here. A negative fragment cannot move to the cathode, which is also negative, and the separation uses an electric field, not a centrifugal one.
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.