UPSC IAS / IPS - MCQ Practice Questions

Understanding:

This question tests knowledge of the specific target cells of the HIV virus within the human immune system.

Step 1: HIV's Mode of Entry

HIV carries a surface protein called gp120, which has a high affinity for the CD4 receptor present on the surface of T-helper cells. This specific binding is the key to HIV's tropism.

Step 2: Role of CD4+ T-helper Cells

CD4+ T-helper cells (also called T4 cells) are the master coordinators of the immune response. They signal B-cells to produce antibodies, activate cytotoxic T-cells, and stimulate NK cells. Their destruction cripples the entire immune response.

Step 3: Progression to AIDS

As HIV replicates inside CD4+ T-cells and kills them, the CD4+ count drops. When it falls below 200 cells per microlitre of blood (normal is 500–1500), the condition progresses to Acquired Immunodeficiency Syndrome (AIDS), leaving the body vulnerable to opportunistic infections.

Answer:

HIV primarily targets and destroys CD4+ T-helper lymphocytes, which are central to coordinating the adaptive immune response.

Quick Tip:

A healthy adult has approximately 500–1500 CD4+ cells per microlitre. The WHO defines AIDS as a CD4 count below 200/µL. This threshold figure often appears in UPSC Prelims science questions.

Understanding:

This question asks about the relative Global Warming Potential (GWP) of various greenhouse gases over a 100-year period, with ${\text{CO}}_2$ set as the baseline (GWP = 1).

Step 1: Define GWP

GWP measures how much energy the emission of 1 tonne of a gas will absorb over a given time period, relative to 1 tonne of ${\text{CO}}_2$. A higher GWP means greater warming potential per unit mass.

Step 2: Compare GWP values (100-year horizon)

Step 3: Identify the Highest

${\text{SF}}_6$ has a GWP of approximately 23,500, making it the most potent greenhouse gas among the options listed. It is used primarily in electrical switchgear and semiconductors.

Answer:

Sulphur Hexafluoride (${\text{SF}}_6$) has the highest GWP over a 100-year horizon among the given options.

Quick Tip:

For UPSC, remember the GWP order: ${\text{SF}}_6\gg {\text{N}}_2\text{O}\gg {\text{CH}}_4\gg {\text{CO}}_2$. ${\text{SF}}_6$ is also the most potent greenhouse gas listed in the Kyoto Protocol.

Understanding:

This question tests understanding of the chemical cause of stratospheric ozone depletion, specifically over Antarctica.

Step 1: Source of the Problem

Chlorofluorocarbons (CFCs), used in refrigerants, aerosol propellants, and foam-blowing agents, rise slowly into the stratosphere. Under normal conditions they are very stable.

Step 2: UV-Triggered Decomposition

In the stratosphere, intense ultraviolet (UV) radiation breaks CFC molecules, releasing highly reactive chlorine (Cl) atoms:

Step 3: Catalytic Destruction of Ozone

Each chlorine atom can catalytically destroy thousands of ozone molecules:

The chlorine atom is regenerated and continues the cycle. Over Antarctica, polar stratospheric clouds amplify this process, creating the 'ozone hole.'

Answer:

Chlorofluorocarbons (CFCs) are the primary cause of ozone depletion over Antarctica through catalytic destruction of ozone molecules.

Quick Tip:

The Montreal Protocol (1987) is the international treaty that phased out CFC production. It is often cited as the most successful environmental treaty. HCFCs (hydrochlorofluorocarbons) replaced CFCs but are also being phased out under the Kigali Amendment (2016).

Understanding:

This question asks for the correct physical explanation of the sensation of reduced weight experienced when a body is immersed in water.

Formula:

Archimedes' Principle states that the buoyant force on a submerged object equals the weight of the fluid it displaces:

where ${\rho }_{\text{fluid}}$ is the fluid density, $V_{\text{displaced}}$ is the volume of fluid displaced, and $g$ is acceleration due to gravity.

Step 1: Apparent Weight

The apparent weight of a body immersed in a fluid is:

Step 2: Why the Body Feels Lighter

Since $F_b>0$, the apparent weight is less than the actual weight. The upward buoyant force partially counters gravity. Gravity itself does not change; the mass does not change. Only the net downward force (apparent weight) decreases.

Step 3: Why Other Options Are Wrong

Answer:

The sensation of feeling lighter in water is due to the upward buoyant force exerted by water, which equals the weight of water displaced.

Quick Tip:

Archimedes' Principle also explains why ships float (density of the ship-system is less than water) and why hot air balloons rise (displaced air weighs more than the balloon system).

Understanding:

This question asks about the vitamin produced endogenously in skin on UV exposure and its role in mineral metabolism.

Step 1: Sunlight and Skin Synthesis

When ultraviolet B (UVB) radiation (wavelength 290–320 nm) strikes the skin, it converts 7-dehydrocholesterol (a cholesterol derivative) into pre-Vitamin D3 (cholecalciferol), which is then thermally isomerised to Vitamin D3.

Step 2: Activation Pathway

Vitamin D3 is metabolically inactive. It undergoes hydroxylation:

Step 3: Role in Calcium Absorption

Calcitriol (active Vitamin D) promotes the synthesis of calcium-binding proteins in intestinal epithelial cells, facilitating absorption of dietary calcium and phosphorus. Deficiency leads to rickets in children and osteomalacia in adults.

Step 4: Why Other Vitamins Are Incorrect

Answer:

Vitamin D is synthesised in the skin on sunlight exposure and is critical for intestinal calcium absorption.

Quick Tip:

Vitamin D is unique because it functions as a hormone, not just a vitamin. India has a paradoxically high rate of Vitamin D deficiency despite abundant sunshine, often linked to dietary habits and skin coverage.

Understanding:

This question tests basic knowledge of the digestive system, specifically the production and storage of bile.

Step 1: Production of Bile

Bile is produced continuously by hepatocytes (liver cells). The liver produces approximately 600–1000 mL of bile per day.

Step 2: Storage of Bile

Between meals, when digestion is not occurring, bile is diverted via the cystic duct into the gallbladder, where it is concentrated (water is absorbed, making it more potent) and stored.

Step 3: Release of Bile

When fatty food enters the duodenum (first part of the small intestine), the hormone cholecystokinin (CCK) is released, which stimulates the gallbladder to contract and release bile through the common bile duct into the duodenum.

Step 4: Function of Bile

Bile contains bile salts that emulsify fats — breaking large fat globules into smaller droplets — increasing the surface area available for lipase enzymes to act upon. Bile also neutralises stomach acid entering the duodenum.

Answer:

Bile is produced by the liver and stored in the gallbladder until needed for digestion.

Quick Tip:

The gallbladder can be surgically removed (cholecystectomy) with no life-threatening consequences, as bile still reaches the intestine — it just flows continuously rather than in concentrated bursts.

Understanding:

This question tests knowledge of the physical property of superconductivity and the condition under which it occurs.

Step 1: Definition of Superconductivity

Superconductivity is a quantum mechanical phenomenon where certain materials exhibit exactly zero electrical resistance when cooled below a characteristic critical temperature ($T_c$). It was first discovered by Heike Kamerlingh Onnes in 1911 in mercury at approximately 4.2 K.

Step 2: Key Characteristics

Step 3: Applications

Superconductors are used in MRI machines (magnetic resonance imaging), maglev trains, particle accelerators (like the LHC at CERN), and are a focus of quantum computing research.

Step 4: High-Temperature Superconductors

Conventional superconductors require cooling with liquid helium (~4 K). 'High-temperature' superconductors (like yttrium barium copper oxide, YBCO) work above 77 K (liquid nitrogen temperature), which is more practically achievable.

Answer:

Superconductivity is the phenomenon of exactly zero electrical resistance below a critical temperature $T_c$.

Quick Tip:

The search for room-temperature superconductors is one of the biggest goals in modern physics. A room-temperature superconductor would revolutionise power transmission, eliminating the approximately 10% energy lost in electrical grids worldwide.

Understanding:

This question tests knowledge of the molecular tools used in recombinant DNA technology, specifically the function of restriction enzymes.

Step 1: What Are Restriction Enzymes?

Restriction enzymes (restriction endonucleases) are naturally occurring bacterial enzymes that act as part of the bacterial immune system, protecting bacteria from foreign (viral) DNA by cutting it.

Step 2: How They Work

Restriction enzymes recognise specific short palindromic DNA sequences (typically 4–8 base pairs long), called recognition or restriction sites. They cleave both strands of the DNA at or near this site. For example, the enzyme EcoRI recognises the sequence 5'-GAATTC-3' and cuts between G and A, leaving 'sticky ends.'

Step 3: Why Other Options Are Wrong

Step 4: Importance in Recombinant DNA Technology

By using the same restriction enzyme to cut both the vector (e.g., plasmid) and the donor DNA, complementary sticky ends are produced, allowing the desired gene to be inserted into the vector. This forms the basis of genetic engineering.

Answer:

Restriction enzymes cut DNA at specific recognition sequences, making them essential molecular scissors in recombinant DNA technology.

Quick Tip:

Remember the analogy: Restriction enzymes = molecular scissors; DNA Ligase = molecular glue; Reverse Transcriptase = makes DNA from RNA. These three tools form the core of genetic engineering.

Understanding:

This question asks about the physiological role of the myelin sheath in nerve impulse conduction.

Step 1: What Is the Myelin Sheath?

The myelin sheath is a lipid-rich insulating layer formed by Schwann cells (in the peripheral nervous system) and oligodendrocytes (in the central nervous system). It wraps around the axon of neurons in a spiral manner.

Step 2: Mechanism — Saltatory Conduction

The myelin sheath insulates the axon electrically, preventing current leakage. Electrical impulses do not travel smoothly along the entire axon; instead, they 'jump' from one gap in the myelin sheath to the next. These gaps are called Nodes of Ranvier. This jumping transmission is called saltatory conduction (from Latin 'saltare' = to jump).

Step 3: Effect on Speed

Saltatory conduction dramatically increases the speed of nerve impulse transmission — from about 0.5 m/s in unmyelinated fibres to up to 120 m/s in heavily myelinated fibres. This allows rapid, coordinated responses.

Step 4: Clinical Relevance

In multiple sclerosis (MS), the immune system destroys the myelin sheath (demyelination), slowing or blocking nerve impulse transmission, leading to muscle weakness, vision problems, and coordination issues.

Answer:

The myelin sheath primarily increases the speed of nerve impulse transmission through saltatory conduction at Nodes of Ranvier.

Quick Tip:

The diameter of the axon also affects conduction speed — thicker axons conduct faster. But myelination has a far greater effect than axon diameter alone. Multiple sclerosis as an example of demyelination disease is a frequently tested fact in UPSC General Science.

Understanding:

This question tests understanding of Rayleigh scattering and its effect on the colour of the sky under different conditions.

Step 1: Rayleigh Scattering

When sunlight (which contains all wavelengths) passes through the atmosphere, it collides with gas molecules (mainly ${\text{N}}_2$ and ${\text{O}}_2$). These molecules scatter light. The intensity of scattering is inversely proportional to the fourth power of the wavelength:

Step 2: Why the Sky Is Blue

Blue light (wavelength ≈ 450 nm) has a shorter wavelength than red light (wavelength ≈ 700 nm). Since scattering is proportional to ${\lambda }^{-4}$, blue light is scattered approximately ${\left(\frac{700}{450}\right)}^4\approx 9$ times more than red light. Scattered blue light reaches our eyes from all directions — making the sky appear blue.

Step 3: Why Sunrise and Sunset Are Red/Orange

At sunrise and sunset, sunlight must travel through a much greater thickness of atmosphere to reach an observer (because the Sun is at the horizon). Over this longer path, nearly all the blue light is scattered away. Only the longer-wavelength red and orange light reaches the observer's eye directly, giving the horizon its characteristic colours.

Answer:

The blue sky results from preferential Rayleigh scattering of short-wavelength blue light; red sunsets occur because blue light is scattered out over the longer atmospheric path at low Sun angles.

Quick Tip:

Rayleigh scattering also explains why the Sun appears white/yellow overhead (less scattering path) but appears red near the horizon. This is a classic optics question that often appears in UPSC Prelims and NDA exams.

Understanding:

This question tests knowledge of the agricultural transformation known as the Green Revolution and the scientific basis behind it.

Step 1: Context of the Green Revolution

In the 1960s, India faced severe food scarcity and depended heavily on food aid (PL-480 programme from the USA). The Green Revolution (mid-1960s to 1970s) transformed Indian agriculture and achieved food self-sufficiency.

Step 2: Key Technology — High-Yielding Varieties (HYVs)

The core of the Green Revolution was the introduction of High-Yielding Varieties (HYVs) of wheat (notably 'Lerma Rojo' and 'Sonora' varieties from Mexico, developed by Norman Borlaug) and rice (IR-8 variety from the International Rice Research Institute, IRRI, Philippines). These were bred through conventional cross-breeding, not genetic modification (which came decades later).

Step 3: Other Accompanying Inputs

HYVs needed complementary inputs to realise their potential:

Step 4: Key Figures in India

M.S. Swaminathan is called the 'Father of the Green Revolution in India' for adapting and introducing these varieties to Indian conditions.

Answer:

The Green Revolution was based on High-Yielding Varieties (HYVs) of wheat and rice, developed through conventional plant breeding combined with intensive use of fertilisers and irrigation.

Quick Tip:

Do not confuse HYVs (conventional breeding, Green Revolution) with GMOs (genetic engineering, e.g., Bt cotton, a later development). This distinction is frequently tested in UPSC Prelims.

Understanding:

This question tests fundamental knowledge of cell biology — the defining structural differences between prokaryotic and eukaryotic cells.

Step 1: Prokaryotic Cells

Prokaryotes (from Greek: 'before nucleus') include Bacteria and Archaea. Their defining features:

Step 2: Eukaryotic Cells

Eukaryotes (from Greek: 'true nucleus') include all plants, animals, fungi, and protists. Their defining features:

Step 3: Why Other Options Are Wrong

Answer:

Prokaryotic cells lack a membrane-bound nucleus and organelles, whereas eukaryotic cells have both.

Quick Tip:

A useful mnemonic: 'Pro' = before, 'Eu' = true. Prokaryotes came before eukaryotes in evolutionary history. The endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

Understanding:

This question asks for the correct everyday example of the Doppler Effect in sound.

Formula:

The observed frequency $f^{\prime }$ when the source moves toward a stationary observer is:

where $f_0$ is the emitted frequency, $v$ is the speed of sound, $v_o$ is the observer's speed (0 if stationary), and $v_s$ is the source speed.

Step 1: Source Approaching

When the train approaches, successive sound wavefronts are compressed (shorter wavelength, higher frequency). The observer hears a higher pitch than the train actually emits:

Step 2: Source Receding

When the train moves away, successive wavefronts are stretched (longer wavelength, lower frequency). The observer hears a lower pitch:

Step 3: The Characteristic Change in Pitch

This shift from higher to lower pitch as the source passes the observer is the classic, unmistakable signature of the Doppler Effect.

Step 4: Why Other Options Are Wrong

Answer:

The Doppler Effect is demonstrated by the change in pitch of a train whistle — higher as it approaches, lower as it recedes.

Quick Tip:

The Doppler Effect applies to all waves, including light. Police radar guns and weather Doppler radar use this principle. Red-shift of light from distant galaxies (light Doppler effect) is evidence that the universe is expanding — a fact linking this concept to cosmology.

Understanding:

This question tests knowledge of water chemistry, specifically the cause and nature of temporary water hardness.

Step 1: What Is Hard Water?

Hard water contains dissolved salts of calcium (${\text{Ca}}^{2+}$) and magnesium (${\text{Mg}}^{2+}$) that prevent soap from lathering easily. These ions react with soap to form a scum (insoluble calcium stearate).

Step 2: Temporary Hardness

Temporary hardness is caused by dissolved bicarbonates of calcium and magnesium — ${\text{Ca(HCO}}_3{)}_2$ and ${\text{Mg(HCO}}_3{)}_2$. It is called 'temporary' because it can be removed simply by boiling:

Calcium carbonate (${\text{CaCO}}_3$) precipitates as a white solid (limescale/furring in kettles), removing the hardness.

Step 3: Permanent Hardness

Permanent hardness is caused by dissolved chlorides and sulfates of calcium (${\text{CaCl}}_2$, ${\text{CaSO}}_4$) and magnesium (${\text{MgCl}}_2$, ${\text{MgSO}}_4$). It cannot be removed by boiling and requires chemical treatment (e.g., ion exchange resins, addition of washing soda ${\text{Na}}_2{\text{CO}}_3$).

Answer:

Temporary hardness is caused by calcium and magnesium bicarbonate ions and can be removed by boiling.

Quick Tip:

The white deposit (limescale) inside kettles and geysers is ${\text{CaCO}}_3$ — a direct result of removing temporary hardness by heating. Ion exchange is the most effective method for removing permanent hardness in water purifiers.

Understanding:

This question tests understanding of the two fundamental nuclear reactions that release energy, and their distinction.

Step 1: Nuclear Fission

Fission is the splitting of a heavy nucleus (typically uranium-235 or plutonium-239) into two smaller (lighter) daughter nuclei, accompanied by the release of neutrons and a large amount of energy:

Fission is the principle behind nuclear power plants and atomic bombs.

Step 2: Nuclear Fusion

Fusion is the combining of two light nuclei (typically isotopes of hydrogen — deuterium and tritium) to form a heavier nucleus, releasing enormous energy:

Fusion is the energy source of the Sun and stars, and is the principle behind hydrogen bombs (thermonuclear weapons). Controlled fusion for power generation (e.g., ITER project) is still under development.

Step 3: Energy Comparison

Fusion releases more energy per unit mass than fission. The mass defect in fusion reactions is larger proportionally.

Answer:

Fission splits heavy nuclei; fusion combines light nuclei — both release energy via mass-energy equivalence ($E=m{c}^2$).

Quick Tip:

Remember: 'Fission = split' (like fissure = crack) and 'Fusion = join' (like fuse = merge). The Sun runs on fusion; nuclear power plants on Earth run on fission. ITER (International Thermonuclear Experimental Reactor) in France aims to achieve commercial fusion power.

Understanding:

This question tests knowledge of CRISPR-Cas9, one of the most significant scientific advances of the 21st century.

Step 1: Origin of CRISPR

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is a natural defence mechanism found in bacteria. Bacteria incorporate fragments of viral DNA into their own genome as 'memory.' When the same virus attacks again, the bacteria use Cas9 protein guided by RNA to cut and destroy the viral DNA.

Step 2: The CRISPR-Cas9 System as a Gene-Editing Tool

Scientists Jennifer Doudna and Emmanuelle Charpentier (Nobel Prize in Chemistry, 2020) adapted this bacterial immune system into a precise gene-editing tool. The components are:

Step 3: Applications

Step 4: Ethical Considerations

The technology is ethically controversial, especially after the case of He Jiankui (China, 2018) who claimed to have created gene-edited babies — widely condemned by the scientific community.

Answer:

CRISPR-Cas9 is a precise gene-editing technology that uses guide RNA and Cas9 protein to cut and modify specific DNA sequences in living organisms.

Quick Tip:

Jennifer Doudna and Emmanuelle Charpentier won the Nobel Prize in Chemistry in 2020 for developing CRISPR-Cas9. This makes it a highly probable UPSC current affairs + science question combination.

Understanding:

This question tests knowledge of the SONAR (Sound Navigation and Ranging) technology and the physical principle behind it.

Formula:

The distance to an object is calculated using:

where $v$ is the speed of sound in water (approximately 1500 m/s) and $t$ is the total time for the pulse to travel to the object and back.

Step 1: Why Not Radio Waves?

Radio waves (electromagnetic radiation) are strongly absorbed by water and cannot travel significant distances underwater. Sound waves, however, travel very efficiently through water.

Step 2: How Sonar Works

A sonar transducer emits pulses of ultrasonic sound (frequency above 20,000 Hz) into the water. These pulses travel through the water, hit an object (submarine, fish school, ocean floor), and reflect back as echoes. The time elapsed between emission and detection of the echo is measured.

Step 3: Calculation Example

If the speed of sound in water is $v=1500\text{ m/s}$ and the echo returns in $t=2\text{ s}$:

So the object is 1500 m away.

Step 4: Active vs Passive Sonar

Answer:

Sonar uses ultrasonic sound waves and measures echo return time to calculate the distance to underwater objects.

Quick Tip:

The same principle applies to bats (echolocation), medical ultrasound imaging, and fish-finders. SONAR stands for Sound Navigation And Ranging; RADAR stands for Radio Detection And Ranging — the key difference is sound vs. radio waves.

Understanding:

This question tests knowledge of the hormone insulin, its source, and its primary physiological function.

Step 1: Source of Insulin

Insulin is produced and secreted by the beta (β) cells of the islets of Langerhans, which are clusters of endocrine cells embedded in the pancreas. (Note: Alpha cells produce glucagon, which has the opposite effect.)

Step 2: Trigger for Insulin Release

When blood glucose levels rise (e.g., after a meal), the beta cells are stimulated to release insulin into the bloodstream.

Step 3: Mechanism of Action

Insulin acts on target cells (liver, muscle, and adipose tissue) to:

Net result: Blood glucose levels fall back to the normal range (70–100 mg/dL fasting).

Step 4: Disease Connection

Answer:

Insulin is produced by pancreatic beta cells and lowers blood glucose by facilitating cellular glucose uptake and promoting glycogen synthesis.

Quick Tip:

Remember the pancreatic cell types: Alpha (α) = Glucagon (raises blood sugar), Beta (β) = Insulin (lowers blood sugar), Delta (δ) = Somatostatin (inhibits both). The 'A-B-D' rule helps: A for glucAgon, B for Blood-sugar lowering insulin.

Understanding:

This question tests knowledge of Faraday's Law of Electromagnetic Induction — the foundation of electric generators and transformers.

Formula:

Faraday's Law states that the induced EMF in a closed loop is proportional to the rate of change of magnetic flux through the loop:

where $\mathcal{E}$ is the induced EMF, ${\Phi }_B=B\cdot A\cdot \cos \theta$ is the magnetic flux, $B$ is the magnetic field strength, $A$ is the area of the loop, and $\theta$ is the angle between the field and the normal to the loop.

Step 1: Key Condition

The key word is 'changing' magnetic flux. A static (unchanging) magnetic field produces no EMF and no current. Only when the flux changes — by moving the magnet, moving the loop, or changing the field strength — is an EMF induced.

Step 2: Lenz's Law (The Negative Sign)

The negative sign in Faraday's Law represents Lenz's Law: the induced EMF acts in a direction to oppose the change in flux that caused it. This is a consequence of the conservation of energy.

Step 3: Applications

Electromagnetic induction is the operating principle of:

Step 4: Why Other Options Are Wrong

Answer:

Electromagnetic induction occurs when a changing magnetic flux through a loop induces an EMF in that loop, as described by Faraday's Law.

Quick Tip:

Faraday's discovery in 1831 is arguably the single most important discovery for modern civilisation — nearly all electricity we use is generated using electromagnetic induction. Michael Faraday never used mathematics himself; James Clerk Maxwell later gave it the mathematical form we use today.

Understanding:

We need to identify which of the given rivers flows through a rift valley (also called a graben).

Step 1: Nature of each river

The Narmada and Tapi rivers are classic examples of rivers that flow through rift valleys — structural depressions formed by faulting. The Narmada flows westward through a rift valley between the Vindhya Range to the north and the Satpura Range to the south.

Step 2: Eliminate other options

The Ganga, Godavari, and Mahanadi are consequent/subsequent rivers that follow the natural slope of the land and do not occupy rift valleys. They are drainage rivers that have developed over sedimentary or Deccan terrain.

Answer:

The Narmada flows through a rift valley formed by the down-faulting of the block between the Vindhya and Satpura ranges.

Quick Tip:

Remember the pair: Narmada and Tapi (Tapti) are the two major peninsular rivers flowing westward through rift valleys. All other major peninsular rivers flow eastward.