UPSC Prelims 2021
Science & Technology Previous Year Questions (PYQs)

Detailed Explanation:

Answer: Option 3 — 1 and 3 Only

In a pressure cooker, the cooking temperature depends on the internal pressure, which determines the boiling point of water. The area of the hole in the lid and the weight of the lid regulate how much pressure builds up inside the cooker, thereby controlling the cooking temperature. The flame temperature only affects the rate of heating, not the maximum temperature achieved.

✅ Statement 1 – Correct: The size of the hole in the lid regulates steam release; a larger hole allows more steam to escape, reducing internal pressure and thus lowering the cooking temperature.

❌ Statement 2 – Incorrect: The temperature of the flame affects only the rate of heating, not the final cooking temperature, which is determined by the internal pressure of the cooker.

✅ Statement 3 – Correct: A heavier weight on the lid requires higher internal pressure to lift it, allowing steam to escape only when higher pressure is reached, which increases the boiling point and thus the cooking temperature.

📝 Short Notes: Pressure Cooker Mechanism

• Principle: Pressure cookers work on the principle that increasing pressure raises the boiling point of water above 100°C.
• Normal Boiling Point: At atmospheric pressure (1 atm), water boils at 100°C.
• In Pressure Cooker: Pressure can reach 15 psi (about 2 atm), raising the boiling point to approximately 121°C.
• Pressure Regulation: The weight (regulator) on the vent controls the maximum pressure by allowing steam to escape when pressure exceeds a threshold.
• Vent Hole Size: The area of the vent hole determines how easily steam can escape, affecting pressure buildup.
• Cooking Time: Higher temperature due to increased pressure reduces cooking time significantly.
• Flame Role: The flame intensity affects how quickly the cooker reaches operating pressure, not the final cooking temperature.

Detailed Explanation:

Answer: Option 2 — 2 Only

Statement 1 is incorrect because adenoviruses have double-stranded DNA genomes, not single-stranded DNA. Retroviruses have single-stranded RNA genomes (not double-stranded DNA), which they reverse-transcribe into DNA using reverse transcriptase. Statement 2 is correct as adenoviruses are a common cause of the common cold, and AIDS is indeed caused by HIV, a retrovirus.

❌ Statement 1 – Incorrect: Adenoviruses have double-stranded DNA genomes; retroviruses have single-stranded RNA genomes, not double-stranded DNA.
✅ Statement 2 – Correct: The common cold can be caused by adenoviruses, and AIDS is caused by HIV, which is a retrovirus.

📝 Short Notes: Viral Genome Types

• DNA Viruses: Generally replicate in the nucleus using host cell machinery (exceptions: poxviruses replicate in cytoplasm)
• RNA Viruses: Most replicate in the cytoplasm; carry their own RNA polymerase
• Retroviruses: Unique in possessing reverse transcriptase enzyme that synthesizes DNA from RNA template
• Common Cold Causes: Rhinoviruses (most common), coronaviruses, adenoviruses, and others
• HIV Classification: Lentivirus genus of the Retroviridae family; targets CD4+ T cells

Detailed Explanation:

Answer: Option 4 — Speed of light is always same.

Astronomical distances are measured in light-years because the speed of light in a vacuum is a universal constant (approximately 299,792 km/s). This constant nature makes it a reliable and standardized unit for measuring the vast distances in space. A light-year represents the distance light travels in one year, providing a practical scale for cosmic distances.

❌ Statement A – Incorrect: Distances between stellar bodies do change due to orbital motion, stellar evolution, and galactic movements; this is not why light-years are used as a unit.

❌ Statement B – Incorrect: While gravity is fundamental, the gravitational effects of stellar bodies vary with mass and distance changes; this has no bearing on the choice of light-year as a distance unit.

❌ Statement C – Incorrect: Light does not always travel in a straight line; it bends around massive objects due to gravitational lensing, as predicted by Einstein's General Theory of Relativity.

✅ Statement D – Correct: The speed of light in a vacuum is constant at approximately 3 × 10⁸ m/s, making it an ideal standard for measuring astronomical distances.

📝 Short Notes: Astronomical Distance Measurements

• Light-Year: The distance light travels in one year in a vacuum (approximately 9.46 trillion kilometers or 5.88 trillion miles).
• Astronomical Unit (AU): The average distance from Earth to the Sun (approximately 150 million km); used for measuring distances within the solar system.
• Parsec: Another unit for astronomical distances, equal to about 3.26 light-years; commonly used by professional astronomers.
• Speed of Light (c): A fundamental physical constant equal to 299,792,458 meters per second in vacuum; remains constant regardless of the observer's motion or the source's motion.
• Gravitational Lensing: The bending of light around massive objects due to gravity, predicted by Einstein's General Theory of Relativity and observed around galaxies and black holes.

Detailed Explanation:

Answer: Option 4 — spread of viral diseases

ACE2 (Angiotensin-Converting Enzyme 2) is a protein receptor found on human cell surfaces that became widely discussed during the COVID-19 pandemic. The SARS-CoV-2 virus uses ACE2 as its primary entry point into human cells, binding to this receptor through its spike protein to infect cells in the lungs, heart, kidneys, and other organs.

📝 Short Notes: ACE2 Receptor

• Full Form: Angiotensin-Converting Enzyme 2
• Normal Function: ACE2 is an enzyme that plays a crucial role in regulating blood pressure and cardiovascular function by converting angiotensin II to angiotensin (1-7)
• Location: Found on cell surfaces in multiple organs including lungs (alveolar cells), heart, kidneys, intestines, and blood vessels
• Role in COVID-19: Acts as the cellular receptor that SARS-CoV-2 spike protein binds to for viral entry into cells
• Research Significance: Target for therapeutic interventions including soluble ACE2, monoclonal antibodies, and small molecule inhibitors to block viral entry
• Previous Connection: The original SARS-CoV virus (2003) also used ACE2 as its receptor, making it a known vulnerability in coronavirus infections

Detailed Explanation:

Answer: Option 3 — Both 1 and 2

Mitochondrial replacement therapy (MRT) is a reproductive technique designed to prevent the transmission of mitochondrial diseases from mother to child by replacing defective mitochondria with healthy donor mitochondria, either before or after in vitro fertilization. Mitochondrial DNA is inherited exclusively through the maternal line, as mitochondria in sperm are typically destroyed after fertilization, making it impossible for a child to inherit mitochondrial diseases from the father.

✅ Statement 1 – Correct: MRT can be performed either before fertilization (using maternal spindle transfer) or after fertilization (using pronuclear transfer) to prevent mitochondrial disease transmission.
✅ Statement 2 – Correct: Mitochondrial DNA is inherited entirely from the mother, as only the egg contributes mitochondria to the embryo.

📝 Short Notes: Mitochondrial Inheritance and Replacement Therapy

Detailed Explanation:

Answer: Option 3 — 1 and 3

This question compares traditional sodium lamps with modern LED lamps used in street lighting. Statement 1 correctly identifies that sodium lamps emit light omnidirectionally (360 degrees), while LEDs are directional, reducing light wastage. Statement 3 correctly notes that sodium lamps produce nearly monochromatic yellow light, whereas LEDs offer better color rendering and spectrum diversity.

✅ Statement 1 – Correct: Sodium lamps emit light in all directions (360 degrees), while LED lamps are directional and focus light where needed, improving efficiency.
❌ Statement 2 – Incorrect: LED lamps have a significantly longer lifespan (50,000-100,000 hours) compared to sodium lamps (15,000-24,000 hours).
✅ Statement 3 – Correct: Sodium lamps produce monochromatic yellow-orange light (narrow spectrum), while LED lamps offer a broad spectrum with better color rendering index (CRI) for street lighting.

📝 Short Notes: Street Lighting Technologies

Detailed Explanation:

Answer: Option 3 — Lemongrass

Lemongrass (also known as Citronella grass) is widely used in preparing natural mosquito repellents due to its strong citrus-like odor. The essential oil extracted from lemongrass contains citronella oil, which is highly effective in repelling mosquitoes. Studies have shown that lemongrass oil can repel or kill up to 95% of certain mosquito species, with effects lasting approximately 2-3 hours.

📝 Short Notes: Natural Mosquito Repellents

• Lemongrass (Cymbopogon): Contains citronella oil, the most commonly used natural mosquito repellent; effective against multiple mosquito species
• Mechanism: The strong citrus odor masks human scent and carbon dioxide, making it difficult for mosquitoes to locate their targets
• Citronella Oil: Extracted from lemongrass leaves; used in candles, sprays, lotions, and incense sticks
• Duration of Effect: Typically lasts 2-3 hours; requires reapplication for extended protection
• Other Natural Repellents: Neem oil, eucalyptus oil, lavender, basil, and marigold also possess mosquito-repelling properties
• Safety: Natural repellents are generally safer than chemical alternatives like DEET, with fewer side effects

Detailed Explanation:

Answer: Option 3 — Both 1 and 2

Recombinant vector vaccines utilize genetic engineering to insert pathogen genes into harmless carrier organisms (vectors), which then deliver these genes into human cells to produce antigens and trigger immunity. Both viruses (like Adenoviruses in Covishield) and bacteria (like attenuated Salmonella or BCG) can serve as vectors in these vaccines.

✅ Statement 1 – Correct: Genetic engineering is essential for developing recombinant vector vaccines, involving the insertion of pathogen antigen genes into a vector's genetic material.
✅ Statement 2 – Correct: Both bacteria (e.g., Salmonella, BCG) and viruses (e.g., Adenovirus, VSV) are used as vectors to deliver antigens.

📝 Short Notes: Recombinant Vector Vaccines

• Technology: Uses genetic engineering to insert genes encoding pathogen antigens into harmless viral or bacterial vectors
• Mechanism: Vectors deliver genetic material into host cells, which then produce antigens triggering immune response
• Viral Vectors: Adenoviruses (Covishield/Sputnik V), Vesicular Stomatitis Virus (Ebola vaccine - Ervebo), Modified Vaccinia Ankara (MVA)
• Bacterial Vectors: Attenuated Salmonella typhimurium, Mycobacterium bovis (BCG), Listeria monocytogenes strains
• Advantages: Strong cellular and humoral immunity, no live pathogen used, scalable production
• Examples: COVID-19 vaccines (AstraZeneca/Covishield, J&J, Sputnik V), Ebola vaccine (Ervebo), experimental TB and malaria vaccines

Detailed Explanation:

Answer: Option 2 — developing genetically modified crop plants.

Bollgard I and Bollgard II are biotechnology products developed for creating genetically modified (GM) cotton plants with insect resistance. These technologies use genes from Bacillus thuringiensis (Bt) bacteria to produce insecticidal proteins that protect cotton plants from bollworm and other lepidopteran pests, thereby reducing the need for chemical pesticides.

📝 Short Notes: Bt Cotton Technologies

Detailed Explanation:

Answer: Option 1 — it is dipolar in nature

Water is known as the "universal solvent" because of its dipolar (polar) nature. Water molecules have a bent structure with partial positive charges on hydrogen atoms and a partial negative charge on the oxygen atom, creating a dipole. This polarity enables water molecules to surround and interact with ions and other polar molecules through electrostatic attraction, effectively separating and dissolving them. The dipolar nature is the fundamental reason why water can dissolve more substances than any other liquid.

📝 Short Notes: Properties of Water as a Solvent

• Dipolar/Polar Nature: Water has a bent molecular structure (H-O-H angle ~104.5°) with oxygen being more electronegative, creating partial charges (δ+ on H, δ- on O).
• Universal Solvent: Water dissolves ionic compounds (like salts) by surrounding ions with water molecules (hydration) and polar covalent compounds through hydrogen bonding.
• Hydrogen Bonding: The polar nature allows water molecules to form hydrogen bonds with each other and with solute particles, facilitating dissolution.
• High Dielectric Constant: Water has a high dielectric constant (~80), which reduces the electrostatic attraction between dissolved ions, keeping them separated in solution.
• Not Related to Solvent Properties: Good heat conductor, high specific heat, and being an oxide of hydrogen are properties of water but not the primary reasons for its exceptional solvent capabilities.

Detailed Explanation:

Answer: Option 1 — 1 and 2 Only

Bacteria and fungi can be cultured in artificial/synthetic media as they possess complete cellular machinery for independent growth and reproduction. Viruses, being obligate intracellular parasites, lack cellular machinery and can only replicate inside living host cells, making them incapable of growth in synthetic media.

✅ Statement 1 (Bacteria) – Correct: Bacteria can be cultured in various artificial media like nutrient agar, nutrient broth, and other specialized media as they are self-sufficient organisms with complete metabolic machinery.

✅ Statement 2 (Fungi) – Correct: Fungi can be cultured in artificial media such as Potato Dextrose Agar (PDA), Sabouraud Dextrose Agar, and Czapek Dox Agar, as they can independently utilize nutrients for growth.

❌ Statement 3 (Virus) – Incorrect: Viruses cannot be cultured in artificial/synthetic media as they are acellular and require living host cells (embryonated eggs, cell cultures, or living organisms) for replication.

📝 Short Notes: Microorganism Cultivation Methods

• Artificial Media: Chemically defined or complex media prepared in laboratory without living cells
• Bacterial Culture: Can grow on solid (agar plates) or liquid (broth) media at specific temperature and pH
• Fungal Culture: Require slightly acidic pH and longer incubation periods compared to bacteria
• Viral Culture: Requires tissue culture, embryonated chicken eggs, or experimental animals
• Key Difference: Viruses are acellular and depend entirely on host cell ribosomes, enzymes, and nucleotides for replication