Recombinant DNA
Read the following passage carefully and answer the given questions.
Recombinant DNA can be forced into competent cells by incubating the cells with recombinant DNA on ice, followed by heat shock and placing them back on ice. This enables the bacteria to take up the recombinant DNA. In a method known as micro-injection, recombinant DNA is directly injected into the nucleus of an animal cell. In another method, suitable for plants, cells are bombarded with high velocity micro-particles of gold or tungsten coated with DNA in a method known as gene gun and the last method uses ‘disarmed pathogen’ vectors, which when allowed to infect the cell, transfer the recombinant DNA into the host. 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. The RNA can be removed by treatment with ribonuclease whereas proteins can be removed by treatment with protease. Other molecules can be removed by appropriate treatments and purified DNA ultimately precipitates out after the addition of chilled ethanol.
DNA can not pass through cell membranes due to:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Hydrophilic nature
A cell membrane has a lipid bilayer, whose inner part repels water. A hydrophilic (water-loving) molecule cannot pass through it. DNA is a hydrophilic molecule, so it cannot cross the cell membrane by itself. This is why bacteria must be made competent to take up DNA.
(1) Hydrophobic nature — Incorrect. The feature is reversed. A hydrophobic molecule would mix with the lipid part of the membrane more easily. DNA is hydrophilic.
(2) Hydrophilic nature — Correct. This is why DNA cannot pass through the membrane.
(3) Basic in nature — Incorrect. DNA is an acidic substance, not a basic one.
(4) Neutral in nature — Incorrect. DNA carries a negative charge, so it is not neutral.
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.
NCERT Extract:
Cloning Vectors – …
