Read the given paragraph carefully and answer the following questions:
Since the DNA is enclosed within the membranes, we have to break the cell open to release DNA along with other macromolecules such as RNA, proteins, polysaccharides and also lipids. This can be achieved by treating the bacterial cells/plant or animal tissue with enzymes such as lysozyme (bacteria), cellulase (plant cells), chitinase (fungus). There are some basic steps in genetically modifying an organism – (i) identification of DNA with desirable genes; (ii) introduction of the identified DNA into the host; (iii) maintenance of introduced DNA in the host and transfer of the DNA to its progeny. Agrobacterium tumifaciens, a pathogen of several dicot plants is able to deliver a piece of DNA known as 'T-DNA' to transform normal plant cells into a tumor and direct these tumor cells to produce the chemicals required by the pathogen. Other molecules can be removed by appropriate treatments and purified DNA ultimately precipitates out after the addition of chilled ethanol. Selection of recombinants due to inactivation of antibiotics is a cumbersome procedure because it requires simultaneous plating on two plates having different antibiotics. Therefore, alternative selectable markers have been developed which differentiate recombinants from non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate. In this, a recombinant DNA is inserted within the coding sequence of an enzyme, β-galactosidase. This results into inactivation of the gene for synthesis of this enzyme, which is referred to as insertional inactivation.
The purified DNA ultimately precipitates after the addition of:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Chilled ethanol
To isolate DNA, the cell is broken open and the other molecules are removed one by one: RNA with ribonuclease, proteins with protease. DNA does not dissolve in cold alcohol, so when chilled ethanol is added, the purified DNA comes out of the solution as fine threads.
Option (1) Methanol — Incorrect. Methanol is not the alcohol used to precipitate DNA in this procedure.
Option (2) Chilled ethanol — Correct. In DNA isolation, the cell is broken, RNA and proteins are removed by enzymes, and finally chilled ethanol is added. The purified DNA precipitates out and can be seen as a collection of fine threads.
Option (3) Protease — Incorrect. Protease is an enzyme that removes proteins; it acts before the DNA is precipitated.
Option (4) Ethidium bromide — Incorrect. It is a stain used to see DNA bands in a gel under UV light.
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.