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.
Agrobacterium tumefaciens, a pathogen of several dicot plants, is able to deliver a piece of DNA known as:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → T-DNA
Agrobacterium tumefaciens is a natural genetic engineer of plants. It carries the Ti plasmid, and passes a part of it, the T-DNA (transferred DNA), into plant cells, where it turns normal cells into tumour cells. Scientists use a disarmed Ti plasmid to carry useful genes into plants.
Option (1) R-DNA — Incorrect. rDNA usually means recombinant DNA, made by joining DNA from two sources.
Option (2) C-DNA — Incorrect. cDNA is complementary DNA, made from mRNA by the enzyme reverse transcriptase.
Option (3) T-DNA — Correct. Agrobacterium tumefaciens delivers a piece of DNA called T-DNA into the plant cell, which transforms normal cells into tumour cells. Its Ti plasmid has been modified into a cloning vector that no longer causes disease.
Option (4) ds-DNA — Incorrect. It only means double-stranded DNA, a description of almost all DNA.
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.