Q.Explain the roles of the following with the help of an example each in recombinant DNA technology :
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Imagine you have a cookbook with recipes from all over the world. Normally, you can only cook what's in one book at a time. But what if you could cut out the best recipe from one book and paste it into another, so your new book has a dish that no single cuisine ever had before? That is the basic idea behind Recombinant DNA Technology.
At its simplest, this technology is a way to take a piece of DNA (the genetic instruction manual) from one organism and join it with the DNA of a completely different organism. The result is a new, "recombinant" DNA molecule — a hybrid that never existed in nature. Think of it as genetic tailoring: you cut a gene from a human, a bacterium, or a plant, and stitch it into the DNA of another organism, often a bacterium or yeast. That host organism then reads the new instructions and starts producing the protein the inserted gene codes for.
Why does this matter? Because it lets us manufacture things that living organisms naturally make, but in a controlled, large-scale way. For example, the human insulin gene can be inserted into E. coli bacteria. These bacteria then become tiny factories, churning out human insulin that can be purified and given to diabetic patients. Before this technology, insulin had to be extracted from the pancreases of cows and pigs — a slow, expensive, and sometimes allergenic process.
The NCERT textbook (Class 12 Biology, Chapter 11) defines it precisely: Recombinant DNA Technology is the technique of joining DNA from two different species and inserting it into a host organism to produce a new genetic combination. The textbook highlights three key tools that make this possible:
- Restriction Enzymes – These are the "molecular scissors" that cut DNA at specific, predictable points. They allow scientists to cut out a desired gene cleanly.
- Vectors – These are the "delivery vehicles," usually plasmids (small circular DNA in bacteria) or viruses, that carry the foreign DNA into the host cell.
- Host Organisms – The living factory (like bacteria, yeast, or plant cells) that will replicate the recombinant DNA and produce the desired protein.
The core principle is genetic recombination — creating a DNA molecule that contains sequences from two or more different sources. This is not the same as natural reproduction or mutation; it is a deliberate, laboratory-made hybrid.
The process itself follows a clear sequence:
- Isolation of the desired gene (say, the human insulin gene) from the donor organism's DNA.
- Cutting both the gene and the vector DNA with the same restriction enzyme, creating matching "sticky ends."
- Ligation — using an enzyme called DNA ligase to permanently join the gene and the vector, forming the recombinant DNA.
- Transformation — inserting this recombinant DNA into a host cell (like a bacterium).
- Selection — identifying and growing only those host cells that successfully took up the recombinant DNA.
- Expression — getting the host cells to produce the desired protein in large quantities.
A common confusion is thinking this technology creates "new life." It does not. It creates a new genetic combination inside an existing living cell. The host organism remains the same species, but it now carries an extra instruction — like a factory that gets a new blueprint for a product it never made before. …
Restriction enzymes and plasmids are the two basic tools of recombinant DNA technology, described in the NCERT Class 12 Biology chapter on biotechnology principles and processes. …
(a) Restriction enzymes cut DNA at specific sites (e.g. EcoRI cuts GAATTC), giving fragments/sticky ends to recombine. (b) Plasmids are small circular bacterial DNA molecules used as vectors to carry foreign genes into a host (e.g. pBR322).
Concept. Recombinant DNA technology requires cutting and joining DNA and a vehicle to carry the gene into the host. Restriction enzymes and plasmid vectors are two of its essential tools.
(a) Restriction Enzymes. These are enzymes (a class of nucleases) that recognise a specific short palindromic sequence in DNA and cut the two strands at defined points. Because they cut only at specific recognition sites, they act as 'molecular scissors' to produce defined DNA fragments and, when they cut in a staggered way, leave single-stranded 'sticky ends' that can base-pair with complementary ends of another DNA. Example: EcoRI (from Escherichia coli) recognises and cuts the sequence 5'-GAATTC-3', producing sticky ends; this allows the gene of interest and the vector, cut by the same enzyme, to be joined by DNA ligase.
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- KCET 2024Set B-41 markMCQQ.Which of the following is based upon the principle of antigen-antibody interaction ? (A) PCR (B) ELISA (C) rDNA technology (D) Gel Electrophoresis
›Reveal solutionSolution
Only ELISA is an immunological assay; the other three are nucleic-acid techniques.
Step 1 — Read the discriminating phrase: "antigen–antibody interaction."
An antibody binds its specific antigen with a lock-and-key specificity determined by the antigen's epitope. Any technique "based on" this must therefore be an immunological/serological technique, not a DNA-handling one. That single criterion sorts the options immediately.
Step 2 — How ELISA exploits it.
ELISA = Enzyme-Linked Immuno-Sorbent Assay (the name itself contains immuno). It is used for the diagnosis of AIDS, among others. The principle:
- The antigen (e.g. a viral protein) is adsorbed onto a solid surface (a microtitre well).
- A specific antibody is added, which binds the antigen — the antigen–antibody interaction.
- A secondary antibody linked to an enzyme binds the complex.
- The enzyme's substrate is added and converted into a coloured product; the intensity of the colour is proportional to the amount of antigen present.
It detects infection by finding either the antigen (proteins, glycoproteins) of the pathogen, or the antibodies synthesised against it. This is the answer. ✓
Step 3 — Eliminate the nucleic-acid techniques.
- (A) PCR — amplifies a gene of interest through denaturation, primer annealing and Taq-polymerase extension. It detects nucleic acid (used to detect very low amounts of a virus, e.g. HIV in suspected AIDS patients), by base complementarity, not by antibodies. ✗ …
- KCET 2022Set A-11 markMCQQ.Enzyme which is useful to remove the oily stains in laundry? (A) Amylase (B) Renin (C) Lipase (D) Protease
›Reveal solutionSolution
Match the enzyme to its substrate: an oily stain is a lipid, and lipase is the lipid-hydrolysing enzyme.
Step 1 — The concept: enzymes are substrate-specific.
Enzymes act on one class of substrate (the lock-and-key / induced-fit specificity). So the way to answer any "which enzyme removes X" question is simply to identify what X is chemically, then name the enzyme that digests that class.
Step 2 — Identify the stain.
An oily stain is fat / triglyceride — a lipid.
Step 3 — Match the enzyme.
Triglyceride lipase Glycerol+Fatty acids
The products (glycerol and fatty acid salts) are water-soluble/soap-like, so the grease lifts off the fabric and rinses away. Lipases are therefore added to detergent formulations exactly for this purpose.
Step 4 — Why the other options are wrong (each digests a different substrate). …
- KCET 2021Set C-31 markMCQQ.Silencing of a gene could be achieved through the use of (A) Short interfering RNA (RNAi) (B) Antisense RNA (C) By both A & B (D) None of the above
›Reveal solutionSolution
Both siRNA (RNAi) and antisense RNA silence a gene by base-pairing with its mRNA so that it can never be translated — so both routes are valid.
Step 1 — What 'gene silencing' means
A gene is expressed when its mRNA is translated into protein. Silencing blocks this at the post-transcriptional level: the mRNA is made but prevented from being read.
Step 2 — Mechanism of the two methods
- Antisense RNA: an RNA molecule complementary to the target ('sense') mRNA is introduced or transcribed in the cell. It base-pairs with the mRNA to give a double-stranded RNA. Ribosomes cannot translate dsRNA, so the protein is never made.
- RNA interference (RNAi) / short interfering RNA (siRNA): dsRNA is cleaved by Dicer into short (≈21–23 nt) siRNA duplexes. One strand is loaded into the RISC complex, which then finds the complementary mRNA and cleaves/degrades it.
Note that both rest on the same underlying principle — complementary base pairing with the target mRNA.
Step 3 — The classic application …
- KCET 2019Set A-11 markMCQQ.A student while extracting DNA from Aspergillus fungus requires __________ enzyme to break open the cell wall. (A) Lysozyme (B) Chitinase (C) Cellulase (D) Pectinase
›Reveal solutionSolution
The cell wall of Aspergillus (a fungus) is made of chitin, so the enzyme needed to break it open is chitinase. The correct option is (B).
The key to this question lies in knowing the chemical composition of the cell wall of the organism you're working with. Different organisms have different structural polysaccharides in their cell walls, and the enzyme you choose must specifically target that polymer.
Aspergillus is a filamentous fungus. Fungal cell walls are primarily composed of chitin — a tough, nitrogen-containing polysaccharide made of N-acetylglucosamine units. This is very different from the cellulose found in plant cell walls or the peptidoglycan in bacterial cell walls.
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Identify the target material. The cell wall of Aspergillus contains chitin as its main structural component. To break it open during DNA extraction, you need an enzyme that can hydrolyse chitin.
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Match the enzyme to the substrate.
- Lysozyme breaks down peptidoglycan (bacterial cell walls) — not useful here.
- Chitinase specifically hydrolyses chitin — exactly what is needed.
- Cellulase breaks down cellulose (plant cell walls) — ineffective on chitin.
- Pectinase breaks down pectin (found in plant middle lamella) — irrelevant for fungi. …
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- KCET 2018Set A-11 markMCQQ.From which of the given plants is the drug whose skeletal structure is given below extracted? \chemfig∗6((=−(−OH)−=(−(CH2)5CH3)−=−(−OH)−=)) (A) Papaver somniferum (B) Atropa belladonna (C) Cannabis sativa (D) Erythroxylum coca
›Reveal solutionSolution
The drawn skeleton is an alkyl-substituted resorcinol (two phenolic −OH groups on a benzene ring plus a long −(CH2)5CH3 chain) with no nitrogen — that is the cannabinoid core, so the source plant is Cannabis sativa.
Step 1 — Read the skeleton
The
chemfigstring describes a benzene ring bearing:- two hydroxyl (−OH) groups, and
- one long alkyl chain −(CH2)5CH3.
So it is an alkyl-resorcinol / phenolic terpenoid-type skeleton. Crucially, there is no nitrogen atom anywhere in the structure.
Step 2 — Use the N-test to eliminate the alkaloids
The three other plants are the classic sources of alkaloids, and every alkaloid by definition contains a basic nitrogen atom in a heterocyclic ring:
Plant Drug Contains N? Papaver somniferum (opium poppy) morphine / heroin Yes (isoquinoline N) Atropa belladonna atropine Yes (tropane N) Erythroxylum coca cocaine Yes (tropane N) Cannabis sativa cannabinoids (THC, cannabinol, charas, ganja) No - KCET 2018Set A-11 markMCQQ.Silencing of a specific mRNA translation could be achieved through (A) Antisense RNA (B) RNA interference technique (C) Both (A) and (B) (D) Microinjection
›Reveal solutionSolution
Antisense RNA and RNA interference are BOTH post-transcriptional gene-silencing methods that act on a specific mRNA, so the combined option is correct.
Step 1 — Antisense RNA technology.
A gene is engineered so the cell makes an RNA strand complementary to the target mRNA. This antisense RNA base-pairs with the mRNA to form a double-stranded hybrid. A ribosome cannot translate double-stranded RNA, so translation of that particular mRNA is blocked (and the duplex is degraded). Classic example: the Flavr Savr tomato, where antisense RNA silences the polygalacturonase mRNA to delay softening.
Step 2 — RNA interference (RNAi). …
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