Q.(a)
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Recombinant DNA Technology
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. …
Part (b)Concept understanding — Bt Toxin Inactivity
Let’s start with something you already know. Imagine you eat a piece of stale, spoiled food. Your stomach immediately feels uneasy — maybe you get cramps or nausea. Your body is reacting to something harmful that entered it. Now, what if that harmful thing was a tiny crystal that only dissolves and becomes active in your stomach’s specific acidic environment? That’s the basic idea behind Bt toxin inactivity.
The everyday intuition
Think of a medicine capsule that has a special coating. That coating is designed to survive the acid in your stomach and only dissolve in your intestines, where the medicine is needed. If the coating dissolved too early, the medicine would be destroyed before it could work. If it never dissolved, the medicine would pass right through you, useless.
Bt toxin works on a similar principle — but in reverse. The toxin is produced by a bacterium called Bacillus thuringiensis (Bt). This bacterium makes a protein crystal that is harmless to most organisms, including humans, because it is produced in an inactive form. It only becomes dangerous when it reaches the very specific environment of an insect’s midgut.
The precise meaning
Bt toxin inactivity means that the toxin protein is initially non-toxic. It is a protoxin — a precursor that needs to be activated inside the body of a target insect. The activation happens only when the protoxin is eaten by a susceptible insect larva. Inside the insect’s midgut, which has a high pH (alkaline conditions) and specific digestive enzymes, the inactive crystal dissolves and gets chopped into a smaller, active toxin molecule.
That active toxin then binds to the insect’s gut wall, creating pores that cause the gut to leak. The insect stops feeding and eventually dies. But for any other animal — including humans, cattle, birds, or fish — the toxin remains inactive because their digestive systems lack the right alkaline pH and the specific enzymes needed to activate it.
This is why Bt toxin is considered safe for humans and non-target animals. The toxin is not poisonous by itself; it is the insect’s own digestive chemistry that turns it into a poison. If you ate the Bt crystal, it would pass through your body unchanged, like an undigested seed.
Why it matters
This concept is the foundation of Bt crops — genetically modified plants like Bt cotton, Bt corn, and Bt brinjal. Scientists take the gene that codes for this inactive toxin from the bacterium and insert it into the plant’s DNA. The plant then produces the inactive toxin in its tissues. When a pest insect chews on the plant, it ingests the inactive toxin, which becomes active only inside that insect.
The key advantage is target specificity. The toxin does not harm beneficial insects like bees, ladybugs, or earthworms, because their gut conditions are different. It also does not harm humans or livestock. This reduces the need for chemical insecticides, which are broad-spectrum poisons that kill many organisms, including helpful ones.
What the NCERT textbook says
The NCERT Class 12 Biology textbook (Chapter 12: Biotechnology and its Applications) explains this clearly: …
Part (a)
- Competent cells and calcium treatment. DNA is a large, hydrophilic, negatively charged molecule that cannot cross the bacterial cell wall/membrane on its own, so the cell must be made competent (able to take up DNA). Using calcium ions: cells (e.g. E. coli) are incubated in ice-cold CaCl2, whose Ca2+ ions neutralise the negative charges on the membrane and on the DNA; a brief heat shock (~42 C for about 90 s) then creates transient pores through which the DNA enters, after which the cells are returned to ice so the DNA is trapped inside.
- Gel electrophoresis.
- Importance: it separates DNA fragments by size, needed to check restriction digests, confirm cloning/PCR products, purify fragments and do DNA fingerprinting.
- Principle: DNA is negatively charged, so in an electric field it moves towards the anode (+) through the sieve-like agarose gel; smaller fragments move faster/farther, giving size-based separation. …
Part (a): cells must be made competent (CaCl2 + heat shock open transient pores) to take up alien DNA; gel electrophoresis separates DNA by size — negatively charged DNA migrates to the anode through agarose — and ethidium bromide intercalates into DNA to fluoresce under UV.
Part (b): Bt cotton is made resistant by inserting the B. thuringiensis cry gene so it makes Cry protein; the ingested protoxin is activated in the bollworm's alkaline gut, binds midgut cells and forms pores that lyse them, killing the insect.
Part (a)
(i) Why competence is needed and how calcium provides it. The bacterial wall and membrane form a barrier that a large, negatively charged, hydrophilic DNA molecule cannot cross unaided; most bacteria actively resist foreign DNA. To transform cells with a recombinant plasmid we make them competent:
- Cells (typically E. coli) are held in ice-cold calcium chloride. The divalent Ca2+ binds the negatively charged membrane phospholipids and the DNA backbone, neutralising the repulsion that keeps DNA away from the cell.
- A brief heat shock at ~42 C (about 90 s) stresses the membrane and opens transient pores, letting the DNA–calcium complex slip in.
- The cells are returned to ice so the pores close and the DNA stays trapped inside.
(ii) Gel electrophoresis.
- (1) Importance: it is the standard way to separate DNA fragments by size, used to confirm restriction digestion and cloning, analyse PCR products, purify specific bands and carry out DNA fingerprinting.
- (2) Principle: the phosphate backbone gives DNA a uniform negative charge, so in an electric field all fragments move towards the positive electrode (anode). The agarose gel acts as a molecular sieve: small fragments thread through the pores quickly and travel far, large ones are held back — so separation is purely by size, producing a ladder of bands. …
Showing the 12 most recent of 53 on this concept.
- CBSE 2026Set 57/1/11 markMCQQ.Which of the following statements about plasmids is incorrect ? (A) Plasmids have the ability to replicate within the bacterial cell. (B) Their replication is controlled by chromosomal DNA. (C) They are autonomously replicating circular extra-chromosomal DNA. (D) They often carry antibiotic resistant genes.
›Reveal solutionSolution
The incorrect statement about plasmids is that their replication is controlled by chromosomal DNA; plasmids replicate autonomously.
Plasmids are fascinating and crucial components in the world of microbiology and biotechnology. They are essentially small, circular, extra-chromosomal DNA molecules found predominantly in bacteria, but also in some eukaryotes like yeast. Think of them as accessory genetic units that carry non-essential but often beneficial genes for the host cell.
Let's break down the given statements to understand which one is incorrect:
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Statement (A): Plasmids have the ability to replicate within the bacterial cell.
This statement is correct. A defining feature of plasmids is their ability to self-replicate. They possess their own origin of replication (ori) sequence, which allows them to initiate DNA synthesis independently of the main bacterial chromosome. This autonomous replication is what makes them incredibly useful as vectors in genetic engineering, as they can multiply along with the host cell, carrying the inserted foreign DNA.
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Statement (B): Their replication is controlled by chromosomal DNA.
This statement is incorrect. As mentioned above, plasmids replicate autonomously. This means their replication is independent of the bacterial cell's main chromosomal DNA replication. While the host cell's machinery (enzymes, nucleotides) is used for plasmid replication, the initiation and control of plasmid replication are governed by sequences within the plasmid itself, not by the bacterial chromosome. This independence is a key distinction between plasmids and the main chromosome.
ImportantThe autonomous replication of plasmids, independent of the host cell's chromosomal DNA, is a fundamental characteristic that distinguishes them and makes them invaluable tools in molecular biology.
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Statement (C): They are autonomously replicating circular extra-chromosomal DNA.
This statement is correct. This is a precise definition of a plasmid.
- Autonomously replicating: They have their own origin of replication and can replicate independently.
- Circular: Most plasmids are found in a closed circular form. …
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- CBSE 2026Set 57/2/11 markMCQQ.Assertion (A) : DNA Ligase is used to join DNA fragments. Reason (R) : It catalyses the formation of glycosidic bonds between nucleotides. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not correct explanation for Assertion (A). (C) Assertion (A) is true, Reason (R) is false. (D) Assertion (A) is false, Reason (R) is true.
›Reveal solutionSolution
DNA Ligase joins DNA fragments by catalysing phosphodiester bonds, not glycosidic bonds. So Assertion is true, Reason is false — option (C).
The question tests a very specific piece of molecular biology: what bond does DNA Ligase actually form? Many students memorise "DNA Ligase joins fragments" but forget the exact chemistry. Let's break it down cleanly.
Why the Assertion is correct
DNA Ligase is the enzyme used in DNA replication, repair, and genetic engineering to seal nicks in the DNA backbone. It joins two DNA fragments — for example, an Okazaki fragment to the growing lagging strand, or an insert into a plasmid vector. This is a standard fact in biotechnology and molecular biology.
Why the Reason is wrong
The Reason claims DNA Ligase catalyses the formation of glycosidic bonds between nucleotides. That is a critical error.
- A glycosidic bond links a sugar (deoxyribose) to a nitrogenous base (A, T, G, C) — that bond is formed during nucleotide synthesis, not by DNA Ligase.
- DNA Ligase catalyses the formation of a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of the next nucleotide. This is the bond that holds the sugar-phosphate backbone together.
Watch outA common mistake is confusing "joining nucleotides" with "forming the bond between sugar and base." The bond between nucleotides in a DNA strand is always phosphodiester, never glycosidic.
Step-by-step reasoning
- Evaluate Assertion (A): "DNA Ligase is used to join DNA fragments." …
- CBSE 2026Set V11 markMCQQ.The corn borers are controlled by the protein encoded by the gene(a) Cry I Ab(b) Cry II Ab(c) Cry IV Ab(d) Cry I Ac
›Reveal solutionSolution
The cryIAb protein controls the corn borer.
Different cry genes encode Bt toxins active against different insects. The cryIAb protein controls the corn borer, whereas cryIAc and cryIIAb control the cotton bollwor …
- CBSE 2026Set A1 markMCQQ.Why does Bt toxin protein not kill the Bacillus?(a) Bt toxin protein exists as inactive protoxins(b) Bt toxin protein is converted into harmless form by Bacillus(c) One enzyme of Bacillus inactivates Bt toxin protein(d) All of these
›Reveal solutionSolution
Bt toxin is stored as an inactive protoxin, so it does not kill Bacillus; the correct option is (a).
Bacillus thuringiensis produces the Bt toxin as inactive crystalline protoxin (a pro-toxin). Because it is inactive in this form, it does no harm to the bacterium itself. When an insect larva ingests the crystals, the alkaline pH of the insect's gut solubilises them, and gut proteases convert the inactive protoxin into the active toxin. The active toxin binds to the midgut epit …
- CBSE 2026Set BOTANY1 markMCQQ.Bacillus thuringiensis has _____ property.(a) fungicidal(b) bactericidal(c) insecticidal(d) herbicidal
›Reveal solutionSolution
Bacillus thuringiensis is a soil bacterium whose Cry (crystal) protein toxins kill specific insect pests, so its property is insecticidal.
Bacillus thuringiensis (Bt) forms protein crystals during sporulation that contain a toxin called delta endotoxin, encoded by cry genes. These crystals are inactive as produced, but once an insect larva ingests them, the alkaline pH of its gut solubilises the crystal, releasing the active toxin. The activated toxin binds to specific receptors on the midgut epithelial cells, creating pores that cause the cells to swell and lyse, killing the larva. Because each Cry protein binds a narrow range of insect receptors, a given Bt toxin (e.g. Cry I Ac, Cry II Ab) is specific to a particular group of insects (bollworms, corn borer, etc.) and harmless to non-target organisms, including humans. This natural insecticidal property is exploited biotec …
- CBSE 2026Set ANNUAL1 markQ.The protein that controls cotton bollworm is encoded by which gene?
›Reveal solutionSolution
The Bt cotton (bollworm-resistant) toxin protein is encoded by cry genes borrowed from the soil bacterium Bacillus thuringiensis.
Bacillus thuringiensis produces crystal (Cry) proteins that are toxic to specific insect larvae, including cotton bollworm. Genes encoding these toxins (e.g., cryIAc and cryIIAb, effective against cotton bollworms) were isolated from the bacterium and introduced into the cotton genome, producing Bt cotton. The Cry protein remains inactive in the bacterium/plant but is activated in the insect' …
- CBSE 2026Set ANNUAL1 markMCQQ.The Bt gene is isolated from which organism?(a) E.coli(b) Lactobacillus(c) Streptococcus cerevisae(d) Bacillus thuringiensis
›Reveal solutionSolution
The 'Bt' in Bt cotton/Bt crops refers to the bacterium Bacillus thuringiensis, whose cry genes encode insecticidal toxin proteins.
Bacillus thuringiensis is a Gram-positive soil bacterium that, during a particular phase of its growth, produces crystal (cry) proteins toxic to specific insect larvae (such as the bollworm affecting cotton). Different strains produce different cry proteins toxic to particular insect groups — e.g., cryIAc and cryIIAb control cotton bollworms, while cryIAb controls corn borer.
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- CBSE 2026Set ANNUAL1 markMCQQ.Given below are the steps carried out to construct a recombinant DNA.(i) Isolation of genetic material(ii) Insertion of recombinant DNA in the host cell /organism(iii) Obtaining the foreign gene product(iv) Amplification of gene of interest(v) Downstream processing. Which one of the following gives the correct sequences of these steps?(a)(i)(iii)(iv)(ii)(v)(b)(i)(iv)(ii)(iii)(v)(c)(ii)(i)(iii)(iv)(v)(d)(ii)(iv)(v)(iii) (i)
›Reveal solutionSolution
Constructing and using recombinant DNA follows a fixed logical sequence: first isolate and amplify the gene of interest, then insert the recombinant DNA into a host, let the host express the gene product, and finally process that product for use — i.e., (i) → (iv) → (ii) → (iii) → (v).
Recombinant DNA technology (genetic engineering) involves several sequential processes:
- (i) Isolation of genetic material (DNA) — the DNA must first be extracted in a pure form from the source cell, free of other macromolecules.
- (iv) Amplification of the gene of interest — using restriction enzymes to cut out the desired gene and, typically, PCR to amplify it, then ligating it into a suitable vector to form the recombinant DNA.
- (ii) Insertion of the recombinant DNA into the host cell/organism — the recombinant DNA (vector + gene of interest) is introduced into a competent host cell (transformation) and the host is then cultured/multiplied so the gene is expressed. …
- CBSE 2026Set ANNUAL1 markQ.CRY gene protects which type of crop by bollworms?
›Reveal solutionSolution
The cry gene (from Bacillus thuringiensis) codes for the Bt toxin protein that protects cotton crop from bollworm attack.
The soil bacterium Bacillus thuringiensis (Bt) produces crystal proteins, coded by cry genes (e.g. cryIAc, cryIIAb), that are toxic to specific insect larvae (in their inactive form, they are non-toxic pro-toxins). When a susceptible insect ingests the toxin, it is activated by the alkaline pH of the insect gut, binds to the gut epithelium, creates pores, causing cell swelling/lysis and death of the larva. Genetically engineering th …
- CBSE 2026Set ANNUAL1 markMCQQ.Assertion (A): Plants having genes from Bacillus thuringiensis are resistant to insects. Reason (R): The transgenic plants have receptors which convert protoxin into toxin.(a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.(b) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.(c) Assertion is true but Reason is false.(d) Assertion is false but Reason is true.
›Reveal solutionSolution
Bt cotton plants ARE insect-resistant (Assertion true), but the protoxin-to-toxin conversion happens inside the INSECT'S GUT (due to its alkaline pH), not via any 'receptor' in the plant — so the given Reason is false.
Assertion: Bt cotton is genetically engineered to carry a cry gene from the soil bacterium Bacillus thuringiensis, which codes for a Bt toxin (Cry) protein. This makes the plant resistant to bollworms and other lepidopteran/coleopteran/dipteran pests — true.
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- CBSE 2026Set ANNUAL1 markMCQQ.What causes the inactive form of Bt toxin (Protoxin) to get converted into it's active form in the body of an insect?(a) Temperature of the gut(b) Enzymes present in saliva(c) Alkaline pH of the gut(d) There is no specific reason
›Reveal solutionSolution
The inactive Bt protoxin is solubilised and activated by the alkaline pH of the insect's gut, so the toxin only becomes active after the insect eats it.
Bacillus thuringiensis makes the insecticidal Cry protein as an inactive protoxin (a crystal, Cry protein). When a susceptible insect (e.g. a bollworm larva) ingests it:
- The alkaline pH of the insect midgut dissolves the crystal, releasing the protoxin.
- Gut proteases then cleave it into the active toxin.
- The active toxin binds the midgut epithelium and creates pores, so the cells swell, lyse and the insect dies. …
- CBSE 2025Set F1 markMCQQ.What does Cry-toxin do?(a) Swelling of cells(b) Binding of epithelium of midgut(c) Lysis of intestinal cells(d) More than one of these
›Reveal solutionSolution
The Cry (Bt) toxin binds the midgut epithelium and causes cell swelling and lysis.
The Cry proteins of Bacillus thuringiensis are produced as inactive protoxin crystals. When an insect ingests them, the alkaline pH of its gut solubilises and activates the toxin. The activated toxin binds to the surface of the midgut epithelial cells and creates pores in the membrane, which makes the cells swell and lys …
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