Recombinant DNA technology
Read the following passage carefully and answer the given questions
Biotechnology has been used by humans since times immemorial in the form of unmodified microbe mediated processes. However, rDNA technology uses genetically modified organisms and employs number of tools. The key tools are restriction enzymes, polymerase enzymes, ligases, vectors and the host organism. Restriction enzymes are also called as nucleases as they cut the DNA. Exonucleases cuts the DNA from the ends whereas endonucleases cuts the DNA at specific positions within the DNA. Endonucleases recognize a sequence of six base pairs called ‘recognition sequence’. Restriction enzymes cut both foreign DNA and vector DNA at specific point. Plasmids and bacteriophages are the commonly used vectors for transferring rDNA into the host organism. Vectors used now a days are engineered in such a way that help in easy linking with foreign DNA as well as selection of recombinants from non-recombinants. Maintenance of introduced DNA in the host and transfer of DNA to its progeny is one of the basic step of biotechnology.
Recombinant DNA is inserted within the coding sequence of beta galactosidase which helps in selection of recombinants as:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Recombinants do not produce any colour in presence of chromogenic substrate
The enzyme β-galactosidase turns a chromogenic substrate into a blue colour. If the foreign DNA is inserted inside the gene of this enzyme, the gene is inactivated. This is called insertional inactivation.
(1) Recombinants do not produce any colour in presence of antibiotic in the medium — Incorrect. This colour test needs a chromogenic substrate. An antibiotic is not part of it.
(2) Non-recombinants do not produce colour in presence of chromogenic substrate — Incorrect. A non-recombinant has no insert, so its β-galactosidase works and its colonies turn blue.
(3) Recombinants do not produce any colour in presence of chromogenic substrate — Correct. The insert has inactivated the enzyme, so the colonies stay colourless and are picked as recombinants.
(4) Recombinants produce blue colour in presence of chromogenic substrate — Incorrect. This is reversed. Blue colonies are the non-recombinants.
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 – …
