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The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
Having learned the tools in the previous chapter, here you see what they are actually used for — insect-resistant cotton, insulin made by bacteria, gene therapy for a child born without a working immune system, and DNA-based diagnosis of infections long before symptoms appear. The chapter also asks harder questions about who owns a gene, which is where biopiracy and patent disputes come in. Keep the mechanism of Bt toxin and RNA interference clear in your mind, as these are asked most often.
The Bt toxin protein does not kill the Bacillus thuringiensis cell that produces it because:
Answer
It exists in the bacterium as an inactive protoxin crystal is correct — the protein is stored as inactive crystals and becomes toxic only when it is solubilised and activated by the alkaline pH of an insect's gut. Since the bacterium's own interior is not alkaline in this way, the protoxin remains harmless to it, which is exactly why the toxin can be safely produced and stored by the organism that makes it.
Human insulin produced by recombinant DNA technology is preferred over insulin extracted from cattle and pigs because:
Answer
Animal insulin can cause an allergic or immune reaction in some patients is correct — insulin from slaughtered cattle and pigs differs slightly in sequence from human insulin, and some diabetics develop an immune response to it. Recombinant humulin is identical to the human hormone, so this problem is avoided, and it also removes dependence on an animal supply. Both forms of insulin have A and B chains joined by disulphide bridges.
In the first reported case of gene therapy, the patient suffered from a deficiency of which enzyme?
Answer
Adenosine deaminase is correct — the four-year-old girl treated in 1990 had ADA deficiency, in which the missing enzyme causes a severe failure of the immune system. Lymphocytes taken from her blood were given a functional ADA cDNA and returned to her body. Beta-galactosidase is used as a selection marker, while reverse transcriptase and restriction endonucleases are laboratory tools.
The unauthorised use of a country's bio-resources and associated traditional knowledge by a foreign company without fair compensation is called:
Answer
Biopiracy is correct — it refers to the exploitation of bio-resources and the traditional knowledge attached to them without proper authorisation or benefit-sharing with the country or community concerned, as in the disputed patents over Basmati rice and the wound-healing use of turmeric. Bioremediation is cleaning up pollution using organisms, biofortification is breeding crops for higher nutrient content, and biomagnification is the concentration of toxicants up a food chain.
Assertion (A): A tobacco plant made to produce a specific double-stranded RNA becomes resistant to the nematode Meloidogyne incognita. Reason (R): The double-stranded RNA silences the parasite's essential mRNA, so the nematode cannot survive in the transgenic host.
Answer
Both A and R are true and R is the correct explanation of A — genes introduced through an Agrobacterium vector make the host produce both sense and antisense RNA, which pair to form a double-stranded molecule. Taken up by the feeding nematode, it initiates RNA interference against the parasite's own complementary mRNA, so that essential transcript is degraded and the parasite cannot survive, while the transgenic plant itself is unharmed.
State two advantages a farmer gains from growing a Bt crop rather than spraying a chemical insecticide.
Answer
First, the toxin is produced within the plant tissue itself and is activated only in the alkaline gut of the target insect, so it kills the specific pest that feeds on the crop while leaving pollinators, natural predators, and other non-target organisms unharmed. A broad-spectrum chemical spray kills useful insects too. Second, the farmer needs far fewer applications of chemical pesticide, which lowers input cost, reduces the pesticide residues that persist on the harvested produce and in the soil and groundwater, and reduces the health risk to the person applying the spray.
Explain how molecular diagnosis using PCR and ELISA allows a disease to be detected earlier than conventional methods.
Answer
Conventional methods such as serum and urine analysis usually detect a disease only after the pathogen has multiplied enough to produce symptoms or measurable changes, by which time treatment may already be less effective. PCR works directly on nucleic acid. Even a very small amount of a pathogen's DNA or RNA in a patient's sample can be amplified exponentially by repeated cycles until there is enough to detect, so an infection such as HIV can be identified long before symptoms appear. The same method is used to detect gene mutations in suspected cancer patients. ELISA is based on the antigen-antibody interaction. It detects either the antigens of the pathogen itself, such as proteins or glycoproteins, or the antibodies that the patient's body has synthesised against the pathogen. Because antibodies appear early in the course of infection, the disease is detected well before the patient becomes seriously ill.
Give two reasons why transgenic animals are produced.
Answer
They are produced to serve as models for human disease and for the study of gene regulation. A transgenic animal carrying a gene that makes it develop a condition resembling a human disease allows researchers to investigate how the disease arises and to test possible treatments, and comparing a transgenic animal with a normal one shows how the introduced gene is regulated and how it affects the body. They are also produced to obtain useful biological products and to test the safety of vaccines and drugs. For example, the transgenic cow Rosie produced human protein-enriched milk containing alpha-lactalbumin, which is more nutritionally balanced for human infants than natural cow milk, and transgenic animals allow vaccines and chemicals to be tested without exposing human volunteers.
Describe how Bt cotton is produced and explain, step by step, how the Bt toxin kills an insect that feeds on the plant.
Answer
Production of Bt cotton: the soil bacterium Bacillus thuringiensis forms protein crystals during a particular phase of its growth, and these crystals contain a toxic insecticidal protein. The genes coding for this protein are called cry genes, and different cry genes act on different insects — cryIAc and cryIIAb code for proteins that control the cotton bollworm, while cryIAb controls the corn borer. The chosen cry gene is isolated from Bacillus thuringiensis and incorporated into the cotton plant using recombinant DNA technology, so that the plant itself expresses the toxin in its tissues. The resulting plants are called Bt cotton. Why the toxin is harmless to the bacterium and to the plant: the cry protein is produced and stored as an inactive protoxin in the form of a crystal. In this crystalline form it is not toxic, so neither the bacterium that makes it nor the transgenic plant that expresses it is harmed. How it kills the insect: when an insect larva such as the cotton bollworm feeds on the plant, it ingests the inactive protoxin. The gut of the insect is strongly alkaline, and this alkaline pH solubilises the crystal, converting the protoxin into the active toxin form. The activated toxin then binds to specific receptor sites on the surface of the epithelial cells lining the midgut of the insect. Binding creates pores in these cell membranes, so water and ions move in uncontrollably. The epithelial cells swell, burst, and lyse. With its midgut lining destroyed, the larva can no longer feed or absorb nutrients, and it dies. Significance: because activation requires the alkaline gut of the target insect, the toxin is specific to those insects and does not harm humans, cattle, or beneficial insects. The farmer therefore needs far fewer chemical pesticide sprays, which lowers cost, reduces residues on the crop and in the environment, and reduces the health hazard of spraying.
Explain the production of recombinant human insulin and outline the principle of gene therapy, using a named example.
Answer
Recombinant human insulin: insulin is a small protein made of two polypeptide chains — the A chain of 21 amino acids and the B chain of 30 — held together by disulphide bridges. In the human body it is first synthesised as a single long precursor called proinsulin, which contains an additional stretch known as the C peptide. This C peptide is removed during the maturation of the hormone and is absent from the functional insulin, which is why simply expressing the natural precursor is not straightforward. Before recombinant technology, diabetics were treated with insulin extracted from the pancreas of slaughtered cattle and pigs. This animal insulin differs slightly in sequence from the human hormone, and some patients developed an allergic or immune reaction to it, quite apart from the problem of supply. In 1983 the American company Eli Lilly solved this by preparing two DNA sequences corresponding to the A chain and the B chain of human insulin and introducing them into plasmids of Escherichia coli. The bacteria produced the two chains separately, and these were then extracted and joined together in vitro by creating the disulphide bonds, giving a product identical to human insulin. This product is called humulin. Gene therapy — principle: gene therapy is a collection of methods that allow correction of a genetic defect by delivering a normal, functional gene into the cells or tissues of a person who has inherited a defective one. The introduced gene takes over the function of the missing or faulty one, so the underlying defect is compensated rather than merely the symptoms being treated. Where the correction can be introduced into cells of the early embryo, the effect could be permanent and heritable. Example: the first clinical case was performed in 1990 on a four-year-old girl with adenosine deaminase deficiency. In this disorder the gene coding for the enzyme adenosine deaminase is missing or defective, and its absence causes a severe failure of the immune system. Bone marrow transplantation and enzyme replacement therapy are possible but not completely curative. In gene therapy, lymphocytes were taken from the patient's blood and grown in culture, a functional ADA cDNA was introduced into them using a retroviral vector, and the cells were returned to her body. Because these lymphocytes are not immortal, the infusion of genetically engineered cells has to be repeated periodically, so this treatment is not a permanent cure. A permanent cure would require the gene to be introduced into cells at an early stage of development, such as bone marrow stem cells.
A district agriculture office receives complaints from farmers growing a conventional cotton variety who report heavy bollworm damage and say they are spraying insecticide up to eight times a season. An officer recommends a transgenic variety instead. At the same public meeting, a farmers' collective raises a separate concern: a foreign company has applied for a patent on a traditional aromatic rice variety long grown in the region, without any agreement with the growers. (a) Name the gene the recommended transgenic cotton variety would carry and the organism it comes from. (b) Explain why this variety would reduce the number of insecticide sprays needed. (c) Name the practice the farmers' collective is objecting to and explain what it means. (d) Name the Indian body that regulates the safety of releasing genetically modified organisms to the public.
Answer
(a) The variety would carry a cry gene — cryIAc or cryIIAb, since these code for proteins that control the cotton bollworm. The gene is obtained from the soil bacterium Bacillus thuringiensis, which produces the corresponding insecticidal protein crystals naturally. (b) The plant itself expresses the toxin in its tissues, so any bollworm larva that feeds on it ingests the protoxin. The alkaline gut of the larva activates the toxin, which binds to the midgut epithelium, creates pores, and destroys the lining, killing the insect. Since the crop is thus protected continuously from within, the farmer no longer needs repeated external sprays, which cuts input cost, reduces pesticide residues on the produce and in the soil, and lowers the health risk to the person spraying. (c) The collective is objecting to biopiracy — the use of a country's bio-resources and the traditional knowledge associated with them by a company or organisation without proper authorisation from, or fair compensation to, the country or community concerned. Similar disputes arose over patents on Basmati rice and on the wound-healing use of turmeric, and led India to amend its Patents Bill to protect indigenous knowledge. (d) The Genetic Engineering Approval Committee, GEAC, set up by the Government of India. It examines the validity of GM research and the safety of introducing genetically modified organisms for public services.
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