UPSC Prelims 2021 Science and Technology questions: topic-wise weightage
UPSC Prelims 2021 asked 10 science and technology questions in GS Paper I, spread over 3 topics. See the science and technology weightage for 2021 below, then attempt each question with the official answer and an explanation.
Science and Technology 2021: questions by topic
- Biology, health and biotechnology6
- Space technology1
- Physics and chemistry in daily life; energy and nuclear technology3
Counted from the 10 questions on this page, tagged by topic in the Battle IAS PYQ bank.
Seeing 10 Science and Technology questions spread over 3 topics can feel overwhelming. That's exactly why the Battle IAS PYQ Book maps every question topic by topic, so you know what to study first. Download free sample
Biology, health and biotechnology 6
With reference to recent developments regarding ‘Recombinant Vector Vaccines’, consider the following statements :
- 1. Genetic engineering is applied in the development of these vaccines.
- 2. Bacteria and viruses are used as vectors.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). A recombinant vector vaccine puts a gene for a pathogen's antigen into a harmless carrier, which then makes the antigen inside the body and trains the immune system. Statement 1 is right: making the vaccine relies on genetic engineering. Statement 2 is right: the carrier can be a virus, such as the adenovirus in Covishield and Sputnik V, or a weakened bacterium, such as Salmonella or BCG.
Notice other COVID vaccine types: Covaxin used a whole inactivated virus; Pfizer and Moderna used mRNA; ZyCoV-D was the world's first DNA vaccine for people; Corbevax was a protein subunit vaccine. India also approved an intranasal vaccine (iNCOVACC).
Answer as per the official UPSC answer key.
In the context of hereditary diseases, consider the following statements :
- 1. Passing on mitochondrial diseases from parent to child can be prevented by mitochondrial replacement therapy either before or after in vitro fertilization of egg.
- 2. A child inherits mitochondrial diseases entirely from mother and not from father.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). Statement 2 is right: mitochondria come only from the egg, because the sperm's mitochondria are destroyed after fertilisation. So mitochondrial DNA diseases pass down entirely from the mother. Statement 1 is right: mitochondrial replacement therapy swaps the mother's faulty mitochondria for healthy donor mitochondria. This can be done before fertilisation (spindle transfer) or after it (pronuclear transfer), with in vitro fertilisation. The child then carries DNA from three people, hence "three-parent babies".
Notice that the UK legalised the method in 2015, and in 2025 reported eight healthy babies born this way. Mitochondria are the cell's powerhouses, making energy as ATP.
Answer as per the official UPSC answer key.
Bollgard I and Bollgard II technologies are mentioned in the context of
Answer (b) & explanation
Bollgard refers to genetically modified crops (b). Bollgard is Monsanto's (now Bayer's) Bt cotton. Bollgard I carries the cry1Ac gene from the soil bacterium Bacillus thuringiensis, and Bollgard II adds cry2Ab. These genes make toxins that kill the bollworm, a major cotton pest. It is not about clonal propagation (a), growth substances (c) or biofertilisers (d).
Notice that Bt cotton, approved in 2002, is India's only commercially approved GM crop, though GM mustard (DMH-11) got environmental release approval in 2022. Pink bollworm has developed resistance, weakening Bt cotton. The GEAC under the Environment Ministry approves GM crops. India released two genome-edited rice varieties in 2025.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Bacteria
- 2. Fungi
- 3. Virus
Which of the above can be cultured in artificial/synthetic medium?
Answer (a) & explanation
Bacteria and fungi can be grown on artificial media (a). Item 1 is right: bacteria grow on nutrient agar or in broth, which supply sugars, salts and proteins. Item 2 is right: fungi also grow on artificial media, such as Sabouraud agar. Item 3 is wrong: viruses cannot reproduce on their own. They are not cells and have no machinery to make proteins, so they need living cells, such as cell cultures, embryonated eggs or animals. They cannot be grown on a cell-free synthetic medium.
Notice that most flu vaccines are still made by growing the virus in hens' eggs. The difference matters in medicine: antibiotics work against bacteria, not viruses. Bacteriophages are viruses that infect bacteria.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. Adenoviruses have single-stranded DNA genomes whereas retroviruses have double-stranded DNA genomes.
- 2. Common cold is sometime caused by an adenovirus whereas AIDS is caused by a retrovirus.
Which of the statements given above is/are correct?
Answer (b) & explanation
Only statement 2 is correct (b). Statement 1 is wrong on both counts. Adenoviruses have double-stranded DNA genomes. Retroviruses, such as HIV, carry single-stranded RNA, which an enzyme called reverse transcriptase copies into DNA that joins the host's genome. Statement 2 is right: adenoviruses cause some common colds, sore throats and conjunctivitis, while HIV, a retrovirus, causes AIDS.
Notice that most colds are caused by rhinoviruses, with adenoviruses and coronaviruses behind some. Adenoviruses are also used as vectors in vaccines such as Covishield. HIV treatment uses antiretroviral therapy (ART), and India's National AIDS Control Programme began in 1992.
Answer as per the official UPSC answer key.
The term ‘ACE2’ is talked about in the context of
Answer (d) & explanation
ACE2 relates to the spread of viral diseases (d). ACE2 (angiotensin-converting enzyme 2) is a protein on the surface of cells in the lungs, gut, heart and blood vessels. The spike protein of SARS-CoV-2 binds to ACE2 to enter cells, like a key in a lock. That is why it became central to the study of how COVID-19 spreads and harms the body. It is not a gene in GM plants (a), part of NavIC (b) or radio collars (c).
Notice that the earlier SARS virus (2002–03) also used ACE2. Normally, ACE2 helps control blood pressure. Many vaccines target the spike protein to stop it binding to ACE2.
Answer as per the official UPSC answer key.
Space technology 1
Which one of the following is a reason why astronomical distances are measured in light-years?
Answer (d) & explanation
Light-years work because the speed of light is constant (d). A light-year is the distance light covers in one year in a vacuum, about 9.46 × 10¹² km. It can serve as a unit only because the speed of light in a vacuum is always the same, about 3 × 10⁵ km per second, for every observer. Option (a) is wrong: distances between stars and galaxies do change; the universe is even expanding. Option (b) is wrong: gravity of stellar bodies changes as they evolve. Option (c) is wrong: light does not always travel straight; gravity bends it (gravitational lensing).
Notice the other units: the astronomical unit (Earth–Sun distance, about 150 million km) and the parsec (about 3.26 light-years). The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away.
Answer as per the official UPSC answer key.
Physics and chemistry in daily life; energy and nuclear technology 3
In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following?
- 1. Area of the hole in the lid
- 2. Temperature of the flame
- 3. Weight of the lid
Select the correct answer using the code given below.
Answer (c) & explanation
The hole area and lid weight set the temperature (c). Water boils at a higher temperature when the pressure is higher. In a pressure cooker, the pressure inside is set by the weight of the whistle on the vent divided by the area of the hole. Item 3 is right: a heavier weight holds more pressure. Item 1 is right: a smaller hole means the same weight holds more pressure. Item 2 is wrong: a hotter flame only makes the cooker reach that pressure sooner and release more steam; it does not raise the cooking temperature.
Notice the reverse case: at high altitude, low air pressure makes water boil below 100 °C, so food cooks slowly. That is why pressure cookers are vital in the mountains. Typical cookers reach about 120 °C.
Answer as per the official UPSC answer key.
Water can dissolve more substances than any other liquid because
Answer (a) & explanation
Water dissolves so much because it is dipolar (a). A water molecule is polar, or dipolar: the oxygen end carries a slight negative charge, and the two hydrogen ends carry slight positive charges. These partial charges surround ions and other polar molecules and pull them apart, so water dissolves more substances than any other liquid. That is why it is called the "universal solvent". Options (b) and (c) are true of water but do not explain dissolving. Option (d) is true, but being an oxide of hydrogen is not the reason.
Notice water's other special traits: high specific heat, which moderates climate; hydrogen bonding; and ice being less dense than liquid water, so it floats.
Answer as per the official UPSC answer key.
With reference to street-lighting, how do sodium lamps differ from LED lamps?
- 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
- 2. As street-lights, sodium lamps have longer life span than LED lamps.
- 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.
Select the correct answer using the code given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). Statement 1 is right: a sodium lamp glows in all directions (360°), so much light is wasted unless reflectors are used, while LEDs are directional. Statement 3 is right: sodium lamps give a nearly monochromatic yellow light, so colours are hard to tell apart, while LEDs render colours far better. Statement 2 is wrong: LEDs last longer than sodium lamps, often 50,000 hours or more, and use much less power.
Notice India's programmes: UJALA (2015) for LED bulbs in homes, and the Street Lighting National Programme (SLNP), run by EESL, which replaced over 1.3 crore street lights with LEDs. The 2014 Nobel Prize in Physics went for the blue LED.
Answer as per the official UPSC answer key.
All years by topic
Every question from 2011 to 2026, grouped by syllabus topic, with year-wise trends.