UPSC Prelims 2013 Science and Technology questions: topic-wise weightage

All three are correct (d). Because the genetic code is universal, a gene from one organism can work in another. Recombinant DNA technology therefore allows transfer across plant species (1), from animals to plants, as in trials with a fish antifreeze gene in tomato (2), and from microbes to higher organisms, as with the Bt gene in cotton and the human insulin gene in bacteria in reverse (3). So options (a), (b) and (c) leave out real possibilities.

Notice the tools: restriction enzymes cut DNA, ligase joins it, and plasmids or Agrobacterium carry it. Newer CRISPR-Cas9 editing (Nobel 2020) changes a plant's own genes; India exempted such SDN-1 and SDN-2 edits from GM rules in 2022. Trap: the universality of the code makes all three transfers possible.

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Moulds is correct (c). Aflatoxins are produced by the moulds (fungi) Aspergillus flavus and A. parasiticus, which grow on damp or poorly stored groundnuts, maize, cottonseed, spices and other grains. They are potent liver toxins and among the strongest natural carcinogens, and normal cooking does not destroy them. (a) is wrong: bacteria cause other food poisoning, such as botulinum toxin. (b) and (d) are wrong: protozoa and viruses do not make aflatoxins.

Notice the safety rules: FSSAI sets limits on aflatoxin in food, and aflatoxin M1 appears in milk when cattle eat contaminated feed. Good drying to low moisture and proper storage prevent it. Aflatoxin rejections have hit Indian groundnut and spice exports to the EU. Trap: toxins in food are often blamed on bacteria.

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Nostoc only is correct (b). Nostoc is a blue-green alga (cyanobacterium) that fixes atmospheric nitrogen in special cells called heterocysts, so it is used as a biofertiliser in paddy fields, along with Anabaena (2). Item 1 is wrong: Agaricus is an edible mushroom, a fungus that does not fix nitrogen. Item 3 is wrong: Spirogyra is a green alga that does not fix nitrogen. So (a), (c) and (d) fail.

Notice the other biofertilisers: Rhizobium in legume root nodules, free-living Azotobacter and Azospirillum, the Azolla–Anabaena partnership in rice fields, and mycorrhizal fungi that help absorb phosphorus. Biofertilisers are promoted under the National Mission on Natural Farming. Trap: all three are 'lower' organisms, but only one fixes nitrogen.

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Hepatitis B and HIV are correct (b). Tattoo needles shared or reused without sterilising can carry blood-borne viruses: Hepatitis B (2), Hepatitis C, and HIV (3). Item 1 is wrong: chikungunya spreads through the bite of infected Aedes mosquitoes (aegypti and albopictus), not through blood contact in tattooing. So (a), (c) and (d) fail.

Notice the prevention: single-use needles, sterilised equipment and fresh ink pots. India's blood banks screen donors for HBV, HCV and HIV and often defer people with recent tattoos for 6–12 months. Hepatitis B has a vaccine in India's Universal Immunisation Programme; HIV and Hepatitis C do not. Trap: chikungunya is a viral disease, but its route is the mosquito.

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All three per UPSC's key (d), but iron's place is doubtful. Sodium ions rush into nerve and muscle cells to create the action potential that triggers contraction (3). Calcium released inside the muscle cell binds troponin and lets actin and myosin slide (1). Iron is not directly part of the contraction mechanism; it sits in myoglobin and haemoglobin, supplying the oxygen muscles need to keep working, so many argue for (c). UPSC counted iron as required. (a) and (b) fail on any reading.

Notice the others: potassium restores the resting state, and magnesium helps muscles relax; low calcium causes tetany and cramps. Anaemia from iron deficiency, common in India, causes muscle fatigue. Trap: 'required for contraction' can be read narrowly or broadly.

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The rate of respiration is decreased is correct (c). Harvested fruit is still alive and keeps respiring, burning its stored sugars, losing water and ripening further. Low temperature slows respiration and enzyme action, and also slows the growth of microbes that cause rot, so the fruit lasts longer. (a) is wrong: cold chambers are dark, but blocking sunlight is not the main reason. (b) is wrong: higher CO₂ helps only in special controlled-atmosphere storage, not in a simple cold chamber. (d) is wrong: humidity is kept moderate to prevent shrivelling, but it is not the main reason.

Notice the related methods: ethylene scrubbing slows ripening, and irradiation extends shelf life of onions and potatoes. India loses a large share of fruits and vegetables after harvest; the Integrated Cold Chain scheme under PM Kisan SAMPADA tackles this. Trap: cold slows life processes; that is the core idea.

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Most of the root hairs are lost is correct (c). Root hairs, thin outgrowths near the root tips, absorb most of a plant's water and minerals. They are delicate and are torn off when a seedling is uprooted, so the transplanted plant cannot take up enough water and wilts, sometimes dying. (a) is wrong: the new soil usually has adequate minerals. (b) is wrong: root hairs do not 'grip too hard'. (d) is wrong: leaf damage can happen but is not the main reason.

Notice the practice: gardeners transplant with a ball of soil around the roots, water well, shade the seedlings, and transplant in the evening to cut transpiration. Paddy is transplanted from nurseries, while Direct Seeded Rice avoids transplanting altogether and saves water. Trap: the problem is at the roots, not the soil.

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Statement 1 only is correct (a). Viruses have no energy-making enzymes or metabolism of their own; they use the host cell's machinery to make copies (1). Statement 2 is wrong: viruses grow only in living cells, such as cell cultures or embryonated eggs, not in a synthetic, cell-free medium. Statement 3 is wrong: viruses spread by many routes: air (influenza, COVID-19), water and food (hepatitis A), contact, blood (HIV) and also vectors (dengue). So (b), (c) and (d) fail.

Notice the debate: viruses are at the edge of life, active only inside cells. Bacteriophages infect bacteria and are studied as alternatives to antibiotics. India's NIV, Pune is the main virology institute. Trap: 'only' makes statement 3 too narrow.

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All three are correct (d). Desert plants (xerophytes) cut transpiration in every way. Hard, waxy leaves with a thick cuticle reduce evaporation (1). Tiny leaves or no leaves reduce the surface losing water (2). Thorns or spines instead of leaves, as in cactus, reduce water loss further, while the green stem does photosynthesis (3). So options (a), (b) and (c) leave out real adaptations.

Notice the other adaptations: sunken stomata, stomata that open at night (CAM photosynthesis in cacti and pineapple), deep tap roots, and fleshy succulent stems that store water. India's Thar has khejri, which has deep roots, and ker with tiny leaves. Trap: all three are textbook xerophyte features.

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All three are correct (d). Mycorrhiza is a partnership between a fungus and plant roots. The fungal threads extend the absorbing area of roots far into the soil, helping the plant take up water and resist drought (1). They help plants tolerate extremes of pH, salinity and heavy metals in poor soils (2). They also protect roots against soil pathogens by forming a barrier and competing with them (3). This is why mycorrhizal inoculants are used to rehabilitate degraded sites such as mine spoils. So (a), (b) and (c) leave out real benefits.

Notice the main benefit: mycorrhiza greatly improves phosphorus uptake. About 80% of land plants form such partnerships. TERI in India developed mycorrhizal biofertilisers for wasteland reclamation. Trap: all three effects are documented.

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Statements 1 and 3 are correct (c). Bagasse, the fibrous residue left after crushing cane, is burnt in co-generation plants to produce steam and electricity (1). Molasses, the thick syrup left after sugar crystallises, is fermented into ethanol for blending with petrol and for alcohol (3). Statement 2 is wrong: synthetic fertilisers such as urea are made from natural gas or naphtha, not molasses, though press mud from sugar mills is used as organic manure. So (a), (b) and (d) fail.

Notice the ethanol drive: India reached E20 (20% ethanol in petrol) in 2025, five years ahead of the original target, with sugarcane juice, B-heavy molasses and grains as feedstock. Bagasse is also used to make paper. Trap: organic manure is not synthetic fertiliser.

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The effective area of contact is reduced is correct (c). Ball bearings replace sliding with rolling. The balls touch the races only at tiny points, so the effective area of contact is reduced, and rolling friction is far less than sliding friction. Less friction means less heat, wear and energy loss. (a) and (b) are wrong: more contact area would increase friction. (d) fails because (c) is correct.

Notice the related ideas: lubricants such as oil and grease add a thin film to cut friction further, and roller bearings handle heavier loads. Friction is not always bad: tyres need grip, and treads increase it on roads. The invention of the wheel was the first use of rolling over sliding. Trap: 'actual' versus 'effective' area is the subtle distinction.

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Items 1, 2 and 4 are correct (c). The sun looks larger near the horizon at dusk, a perception effect called the moon illusion, which also applies to the sun (1). Its reddish colour at dawn comes from scattering of shorter blue wavelengths as light travels through more air (2). The twinkling of stars comes from refraction by moving layers of air of different density (4). Items 3 and 5 are wrong: the moon at dawn and the pole star are really there, so they are not illusions. So (a), (b) and (d) fail.

Notice the related effects: atmospheric refraction lets us see the sun about two minutes before it actually rises and after it sets. Planets do not twinkle much because they appear as discs, not points. Trap: 'colour' is a real scattering effect, but UPSC groups it with appearances.

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All three are correct (d). Sunlight is refracted, or bent, as it enters a raindrop (2), and split into its colours because each wavelength bends differently: dispersion (1). The light is then reflected internally at the back of the drop (3) and refracted again as it leaves. Together, these make a rainbow at about 42° from the point opposite the sun. So options (a), (b) and (c) leave out real processes.

Notice the related facts: a rainbow is seen only with the sun behind you; a secondary rainbow, from two internal reflections, has reversed colours and lies outside the main bow. Red is on the outside of the primary rainbow. Newton used a prism to show dispersion. Trap: internal reflection is often forgotten.

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Gravity is the strongest is the statement not correct (a). Gravity is the weakest of the four forces; it dominates only at large scales because masses always attract and add up, while electric charges cancel. The strong nuclear force is the strongest. (b) is correct: electromagnetism acts on charged particles. (c) is correct: the weak nuclear force causes beta decay, a form of radioactivity. (d) is correct: the strong force holds protons and neutrons in the nucleus despite the protons' repulsion.

Notice the ranking: strong (1) > electromagnetic (about 1/137) > weak > gravity (about 10⁻³⁸). Electromagnetism and the weak force are unified as the electroweak force (Nobel 1979). Gravity is explained by general relativity, not yet joined with quantum theory. Trap: gravity feels powerful in daily life but is the weakest.

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Statement 1 only is correct (a). The Higgs boson, discovered at CERN's Large Hadron Collider in July 2012, confirmed the Higgs field, which explains why elementary particles have mass (1). Statement 2 is wrong: it has no link to teleporting matter. Statement 3 is wrong: it does not help make nuclear fuels. So (b), (c) and (d) fail.

Notice the Indian link: the particle is a boson, a class named after Satyendra Nath Bose. Peter Higgs and François Englert won the 2013 Nobel Prize in Physics. India became an associate member of CERN in 2017. The popular name 'God particle' comes from a book title, not from physics. Trap: dramatic applications are attached to basic science.

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