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.
The role of calcium in transformation with recombinant DNA is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Increases the efficiency of DNA uptake by bacterial cells
To take up DNA, a bacterial cell must first be made competent. Then the cells are incubated with the recombinant DNA on ice, given a heat shock and put back on ice.
(1) Increases the efficiency of DNA uptake by bacterial cells — Correct. This is the role of calcium in transformation.
(2) Decreases the efficiency of DNA uptake by fungal cells — Incorrect. Calcium raises, not lowers, the uptake. It is used for bacteria, not for fungal cells.
(3) Helps in the precipitation of DNA — Incorrect. Purified DNA precipitates out when chilled ethanol is added, not calcium.
(4) Helps in the release of DNA from animal cells — Incorrect. DNA is released from cells by treating them with enzymes. Calcium does not do this.
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:
Competent Host (For Transformation with Recombinant DNA) – “… Since DNA is a hydrophilic molecule, it cannot pass through cell membranes.”
Competent Host (For Transformation with Recombinant DNA) – “… In order to force bacteria to take up the plasmid, the bacterial cells must first be made ‘competent’ to take up DNA. This is done by treating them with a specific concentration of a divalent cation, such as calcium, which increases the efficiency with which DNA enters the bacterium through pores in its cell wall. Recombinant DNA can then be forced into such cells by incubating the cells with recombinant DNA on ice, followed by placing them briefly at 42°C (heat shock), and then putting them back on ice. This enables the bacteria to take up the recombinant DNA.”