Q.C-peptide of human insulin is:

The key here is to recall that different Cry proteins from Bacillus thuringiensis are specific to different insect groups. This specificity arises because each Cry protein binds to distinct receptors in the gut of particular insect larvae. You don’t need to memorise every Cry protein, but the ones that appear frequently in Indian exams — Cry I Ab, Cry I Ac, Cry II Ab — are worth knowing.

Cry I Ab is effective against the European corn borer, a major pest of maize. Cry II Ab, on the other hand, is effective against cotton bollworms (like Helicoverpa armigera). This is a standard fact from the chapter on biotechnology and its applications in agriculture.

Let’s walk through the options:

1. Option (A): Cotton bollworms and corn borer — This matches: Cry II Ab → cotton bollworms, Cry I Ab → corn borer. This is correct.

2. Option (B): Corn borer and cotton bollworms — This reverses the pairing. Cry I Ab does not control cotton bollworms, and Cry II Ab does not control corn borer. So this is wrong.

3. Option (C): Tobacco bud worms and nematodes — Cry proteins are not used against nematodes; nematode control uses different approaches. Tobacco budworms are controlled by Cry I Ac, not by Cry II Ab or Cry I Ab. So this is incorrect.

4. Option (D): Army worms and tobacco bud worms — Army worms are not the primary targets of these two Cry proteins. Again, incorrect.

Understanding Cry Proteins and Their Specificity

Cry proteins (crystal proteins) are insecticidal toxins produced by the bacterium Bacillus thuringiensis (Bt). These proteins are highly specific to certain insect orders, making them valuable tools in agricultural biotechnology. The naming convention follows a systematic pattern where different Cry protein types target different pest groups.

The Key Cry Proteins in Question

Let me work through the specificity of each protein mentioned:

1. CryIIAb proteins belong to the CryII family, which primarily targets Lepidopteran insects (moths and butterflies). Within this family, CryIIAb is specifically effective against cotton bollworms (Helicoverpa species), which are major pests of cotton crops causing significant economic damage.

2. CryIAc proteins belong to the CryI family, another group targeting Lepidopterans. CryIAc shows particular efficacy against the corn borer (Ostrinia nubilalis and related species), a devastating pest of maize/corn crops that burrows into stalks and ears.

3. Why this specificity matters: Each Cry protein binds to specific receptors in the midgut of target insects. The protein-receptor interaction is highly selective, which is why CryIIAb and CryIAc have different target spectra despite both affecting Lepidopterans.

4. Eliminating other options:
   • Nematodes are controlled by different Cry proteins (like Cry5, Cry6, Cry14) or entirely different mechanisms
   • Tobacco budworms, while Lepidopterans, are not the primary targets of these specific Cry proteins

Insulin is a crucial peptide hormone that regulates glucose metabolism in the body. Understanding its structure is fundamental to comprehending its function and is a common topic in biochemistry and biology exams. Insulin is a relatively small protein, but its precise arrangement of amino acids into two distinct chains is vital for its biological activity.

Here's a breakdown of insulin's structure:

1. Insulin's Polypeptide Chains: Mature insulin is composed of two polypeptide chains, referred to as chain A and chain B. These chains are not synthesized separately but are initially part of a single precursor protein called proinsulin. Proinsulin undergoes enzymatic cleavage to remove a connecting peptide (C-peptide), leaving behind the active two-chain insulin molecule.

2. Chain A: This polypeptide chain is shorter. It consists of 21 amino acids. Within chain A itself, there is an intra-chain disulfide bond formed between cysteine residues at positions 6 and 11 ($A_6 - A_{11}$).

3. Chain B: This polypeptide chain is longer than chain A. It consists of 30 amino acids.

4. Disulfide Bonds Linking Chains A and B: The two chains, A and B, are held together by two inter-chain disulfide bonds. These bonds are formed between specific cysteine residues:

   • One bond connects cysteine at position 7 of chain A to cysteine at position 7 of chain B ($A_7 - B_7$).
   • The other bond connects cysteine at position 20 of chain A to cysteine at position 19 of chain B ($A_{20} - B_{19}$).
   Important

   The presence and correct formation of these disulfide bonds are critical for the three-dimensional structure and biological activity of insulin.

5. Matching with Options: Based on the structure, chain A has 21 amino acids and chain B has 30 amino acids. This corresponds to option (C).

Concept and Intuition

Recombinant DNA technology is a cornerstone of modern molecular biology, allowing scientists to combine genetic material from different sources to create new DNA sequences. The fundamental idea is to introduce a specific gene or DNA segment into a "vector" molecule, which can then carry and replicate this foreign DNA within a host cell. This process relies on several key molecular tools:

1. Plasmids: Small, circular DNA molecules found in bacteria, separate from the main bacterial chromosome. They can replicate independently and often carry genes beneficial to the bacterium, such as antibiotic resistance. Plasmids are commonly used as vectors because they can be easily manipulated and introduced into host cells.
2. Restriction Enzymes: These are molecular "scissors" that recognize and cut DNA at specific nucleotide sequences. They are essential for cutting both the vector DNA and the foreign DNA, creating compatible ends for joining.
3. DNA Ligase: This enzyme acts as molecular "glue," joining the cut DNA fragments together by forming phosphodiester bonds.
4. Host Cell: An organism (typically a bacterium or yeast) into which the recombinant DNA is introduced, allowing it to replicate and express the foreign gene.

The first successful construction of recombinant DNA was a landmark event that demonstrated the feasibility of genetic engineering. It involved carefully selecting a suitable plasmid vector, a source of foreign DNA, and a host organism to propagate the newly formed recombinant molecule.

Step-by-Step Explanation

1. The Pioneers: The groundbreaking work of constructing the first recombinant DNA molecule was carried out by Stanley Cohen and Herbert Boyer in 1972. Their experiment marked the beginning of modern biotechnology.

2. Choosing the Vector: For their experiment, Cohen and Boyer needed a suitable vector to carry the foreign DNA. They chose a plasmid, which is a naturally occurring extrachromosomal DNA molecule found in bacteria. Plasmids are ideal vectors because they can replicate independently within a host cell and can be engineered to carry desired genes.

3. Source of the Plasmid: The specific plasmid used by Cohen and Boyer in this pioneering experiment was isolated from the bacterium Salmonella typhimurium. This plasmid naturally carried genes conferring resistance to certain antibiotics.

4. The Foreign DNA: They then took a gene responsible for resistance to the antibiotic kanamycin from another plasmid. Both the Salmonella typhimurium plasmid and the kanamycin resistance gene were cut with the same restriction enzyme. This created complementary "sticky ends" on both DNA fragments.

5. Ligation and Recombinant DNA: The cut Salmonella typhimurium plasmid and the kanamycin resistance gene were then mixed, and DNA ligase was used to join them together. This resulted in the formation of the first recombinant DNA molecule, which was essentially the Salmonella typhimurium plasmid carrying the inserted kanamycin resistance gene.

6. The Host Cell: Finally, this newly constructed recombinant plasmid was introduced into Escherichia coli (E. coli) cells. E. coli served as the host organism where the recombinant plasmid could replicate and express the kanamycin resistance gene. This demonstrated that the recombinant DNA was stable, functional, and could be propagated in a different bacterial species.

   Watch out

   It is important to distinguish between the source of the plasmid used as the vector and the host cell into which the recombinant plasmid was introduced. The plasmid itself originated from Salmonella typhimurium, while Escherichia coli was used as the host cell for replication and expression of the recombinant DNA. The question specifically asks about the bacterium from which the plasmid was constructed.

This experiment not only created the first recombinant DNA but also proved the concept of gene cloning and laid the foundation for all subsequent genetic engineering applications.

The first recombinant DNA was constructed using a plasmid from Salmonella typhimurium.

Concept & Intuition

Each List-I item has a characteristic role in genetic engineering. Three of the four are unambiguous; the printed option that carries those three correct pairings identifies the key.

Step-by-step

• A) Retroviruses $\to$ II) Used to transfer genes to animals. Retroviruses integrate into host genomes and are engineered as vectors for animal (including human) cells.
• C) Cosmids $\to$ I) A type of cloning vector. Cosmids carry the phage-$\lambda$ $cos$ site and clone large ($\sim$45 kb) inserts.
• D) Colony hybridisation $\to$ IV) Small specific gene probe + radioactive molecule. A labelled probe screens colonies transferred to a membrane.
• B) Disarmed pathogen vector $\to$ V. By the official key, the remaining descriptor pairs with B once II, I and IV are assigned.

Testing the printed options, only (D) gives A-II, C-I, D-IV together, so it is the key.

The key to this question lies in understanding what actually makes you sick with diphtheria. The bacterium Corynebacterium diphtheriae colonises the throat, but the damage — the thick grey pseudomembrane, the heart and nerve complications — comes from a powerful exotoxin it releases. So when we design a vaccine, we don't need to stop the bacterium from growing; we need to neutralise that toxin.

That changes the whole strategy. A whole-agent vaccine (live attenuated or killed) would target the bacterium itself. But the immune system's best defence against a toxin is an antibody that binds and inactivates the toxin molecule. The most efficient way to generate those antibodies is to inject a harmless version of the toxin itself — a toxoid.

1. Why not attenuated or inactivated whole-agent vaccines?

   Options (A) and (B) would present the whole bacterium to the immune system. That would produce antibodies against many bacterial surface proteins, but the critical anti-toxin response would be weaker and less focused. Worse, an attenuated live vaccine could theoretically revert to virulence, and a killed whole-cell vaccine often causes more local reactions. Neither is the standard for diphtheria.

2. Why not a recombinant vector vaccine?

   Option (D) uses a harmless virus or bacterium to carry a gene for a diphtheria antigen into the body. This is a modern approach used for some diseases (e.g., Ebola, COVID-19), but it is not the established vaccine for diphtheria. The classic, proven method is far simpler.

3. How a toxoid vaccine works

   The diphtheria toxin is treated with formaldehyde, which destroys its toxic activity but leaves its three-dimensional shape largely intact. This modified, non-toxic molecule is the toxoid. When injected, the immune system sees it as the real toxin and produces anti-toxin antibodies. Later, if the real toxin appears during an infection, those antibodies neutralise it before it can cause damage.

The core concept here is recombinant DNA technology and how it is used to produce human insulin. The classic pitfall is confusing the source of the production (bacteria in a lab) with the natural location (human pancreas). Let’s examine each statement carefully.

1. Statement (A): "Bacterial Insulin is now made in human pancreas"

   This is biologically impossible. The human pancreas naturally produces human insulin, but "bacterial insulin" refers to insulin synthesized by genetically modified bacteria (e.g., E. coli). The bacteria are grown in fermentation tanks, not inside a human organ. This statement is false — it confuses the production site.

2. Statement (B): "A and B chains of human insulin are produced separately by bacterial genome"

   This is true. In the early Eli Lilly method, the genes for the A and B chains of human insulin were inserted separately into bacterial plasmids. Each chain was produced independently in different bacterial cultures, then purified and chemically joined to form active insulin.

3. Statement (C): "DNA sequences corresponding to A and B chains of human insulin were introduced into bacterial plasmid"

   This is also true. This is the standard recombinant DNA technique: the human insulin gene sequences are spliced into a plasmid vector (often using restriction enzymes and DNA ligase), which is then transformed into bacteria.

4. Statement (D): "Specific mRNA of the nematodes have been silenced by dsRNA"

   This refers to RNA interference (RNAi). Double-stranded RNA (dsRNA) triggers degradation of complementary mRNA, silencing specific genes. This was famously discovered in the nematode C. elegans (Fire & Mello, 1998). The statement is correct.

The key here is to recall the biological role of each protein, not just its name. Many students confuse "receptor" with a general function, but here it's specifically listed as "taste receptor" — a specialized sensory receptor. Similarly, GLUT-4 is a transporter, not a hormone, and collagen is a structural protein, not an enzyme.

Let’s work through each one:

1. Trypsin — This is a digestive enzyme produced by the pancreas. It breaks down proteins in the small intestine. Its function is clearly enzymatic. So it matches with B) Enzymes.

2. Receptor — In this list, the function given is "Taste receptor" (option E). A receptor is a protein that detects signals; taste receptors are a specific type. So it matches E) Taste receptor.

3. Insulin — This is a hormone secreted by the beta cells of the pancreas. It regulates blood glucose levels. So it matches D) Hormone.

4. GLUT-4 — This is a glucose transporter protein, found mainly in muscle and fat cells. It moves glucose into cells in response to insulin. So it matches A) Enables glucose transport into cells.

5. Collagen — This is a structural protein found in connective tissues. It forms the extracellular matrix and provides strength. So it matches C) Intercellular ground substance.

Now, let’s see which option lists them in the correct order (i, ii, iii, iv, v → B, E, D, A, C):

• Option (A): A, B, D, C, E → wrong
• Option (B): B, A, D, C, E → wrong (receptor matched to A, GLUT-4 to B)
• Option (C): B, E, D, A, C → correct
• Option (D): A, D, C, B, E → wrong

The concept first: what molecular farming is

Biopharmaceuticals — antibodies, vaccines, insulin, interferons, industrial enzymes — are traditionally made in microbial fermenters or animal cell culture. Both are expensive: sterile stainless-steel bioreactors, costly media, tight containment, and (for animal cells) a risk of human pathogens.

Molecular farming (molecular pharming) replaces the fermenter with a field of plants. A gene for the desired protein is introduced into a plant; the plant then synthesises that protein in its leaves, seeds, tubers or even in its milk-like sap, and the product is extracted downstream.

Step-by-step

1. Assertion. "Molecular farming is large-scale production of biochemicals from plants." That is the definition. A is TRUE.

2. Reason. "Transgenic plants are bioreactors for commercial production of antibodies." Also TRUE — and it names the flagship product. Antibodies made this way are literally called plantibodies. Tobacco, maize, rice and safflower are the workhorses; the same route yields the hepatitis-B antigen and human serum albumin.

3. Does R explain A? Yes — R supplies the mechanism and rationale behind A:

   • How can plants yield biochemicals at scale? Because a transgenic plant is a bioreactor — every cell of every plant in the field expresses the transgene.
   • Why plants? Because scaling up means simply sowing more hectares; there is no fermenter to build, the substrate is sunlight, water and $CO_2$, and plants harbour no human pathogens.

   Remove the bioreactor property of transgenic plants and molecular farming would not exist. So R is not a stray fact — it is the operative principle of A.

ANSWER: A

Human insulin is a small protein hormone made of 51 amino acids in total. But it doesn't exist as a single long chain — it's composed of two separate polypeptide chains held together by disulfide bonds. This is a classic fact from biology, often tested in exams on the structure of insulin.

The two chains are named A and B. Chain A is shorter, with 21 amino acids. Chain B is longer, with 30 amino acids. So when the question asks for the number of amino acids in the polypeptide chain B, you're being asked to recall that specific number.

1. Total amino acids in human insulin = 51.
2. These are split into two chains: A and B.
3. Chain A has 21 amino acids.
4. Chain B has 30 amino acids.
5. Therefore, the number of amino acids in chain B is 30.

The key here is to recall the structure of human insulin. Insulin is a peptide hormone that regulates blood glucose. It is made of two separate polypeptide chains — called the A chain and the B chain — that are linked together by disulfide bridges. The total number of amino acids in the entire insulin molecule is 51, but that sum is split unevenly between the two chains.

1. Chain A is the shorter one. It contains 21 amino acids.
2. Chain B is the longer one. It contains 30 amino acids.

So when the question asks for the number of amino acids in the B chain, you are looking for the number specific to that chain, not the total for the whole molecule.