Previous Year UPSC Questions

Explanation

The complementary nature of the two DNA strands ensures accurate replication and transmission of genetic information. Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C) on the opposite strand. This allows for precise copying of the genetic information during cell division.

Explanation

Metastasis is the process through which cancer cells spread from the primary tumor to distant parts of the body, forming secondary tumors in organs such as the lungs, liver, bones, or brain. This process is responsible for nearly 90% of cancer-related deaths, showcasing the aggressive and life-threatening nature of malignant tumors. The process begins with Epithelial-to-Mesenchymal Transition (EMT), where cancer cells gain the ability to migrate and invade surrounding tissues. These cells then enter the bloodstream or lymphatic system (intravasation) and travel to other parts of the body. While in circulation, they evade immune system attacks and other hostile conditions. Upon reaching a new site, the cells exit blood vessels (extravasation) and adapt to the local environment to form secondary tumors. Interestingly, some metastatic cells can remain dormant for years before becoming active again, contributing to cancer recurrence.

Explanation

Enzymes are biological catalysts, crucial for various biological processes, including digestion, metabolism, and DNA replication. Hormones are chemical messengers produced by endocrine glands which regulate various physiological processes, such as growth, development, metabolism, reproduction, and mood. A is matched with 2: Ptyalin, also known as salivary amylase, is an enzyme present in saliva that breaks down starch into maltose and smaller polysaccharides during digestion. B is matched with 3: Pepsin is a proteolytic enzyme secreted in the stomach. It helps digest proteins into smaller peptides by breaking peptide bonds in acidic conditions. C is matched with 1: Renin is an enzyme secreted by the kidneys in response to low blood pressure. It converts angiotensinogen (produced by the liver) into angiotensin I, which is further converted to angiotensin II, a potent vasoconstrictor. D is matched with 5: Oxytocin is a hormone produced by the hypothalamus and secreted by the pituitary gland. It induces the contraction of smooth muscles, particularly in the uterus during childbirth and in milk ejection during breastfeeding.

Explanation

Antigens are recognized by the immune system as foreign substances, which stimulates the production of specific antibodies by B-cells (a type of white blood cell). Antibodies bind to antigens, marking them for destruction by other immune cells or neutralizing their harmful effects. Antigens can be proteins, polysaccharides, or lipids. For example, the spike protein of the SARS-CoV-2 virus acts as an antigen, triggering an immune response and the production of antibodies in infected individuals or after vaccination. The immune response involves two types of lymphocytes:

1. B-cells produce antibodies that target antigens.

2. T-cells destroy infected cells displaying antigens.

Explanation

Option (d) is correct: Chloroplasts and mitochondria are unique organelles as they contain their own circular DNA, similar to bacterial chromosomes, along with ribosomes and protein synthesis machinery. Mitochondrial DNA (mtDNA), present in all eukaryotic cells, is maternally inherited and essential for ATP production. Chloroplast DNA (cpDNA) is found in photosynthetic eukaryotes. It encodes proteins for photosynthesis. Their extranuclear DNA supports the endosymbiotic theory which suggests that these organelles evolved from free-living bacteria engulfed by ancestral eukaryotes. Despite their partial independence, mitochondria and chloroplasts still rely on nuclear-encoded proteins for various functions, distinguishing them from other organelles that depend entirely on nuclear DNA. Options (a), (b) and (c) are incorrect:

Centrioles are involved in cell division and the formation of spindle fibers but do not contain DNA. They play a structural role and have no genetic material. The Golgi apparatus modifies, packages, and transports proteins but does not contain DNA or protein synthesis machinery, depending entirely on the cell’s nucleus. Lysosomes are enzyme-filled organelles responsible for digesting cellular waste but lack DNA. They function solely under the control of nuclear-encoded proteins.

Explanation

The combination of ribosomes and the endoplasmic reticulum plays a pivotal role in protein synthesis. Ribosomes are the molecular machines responsible for translating messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins. They can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum. The ER is a network of membranous tubules within the cell. The rough ER (RER) is studded with ribosomes on its cytoplasmic surface, making it the site of protein synthesis for proteins destined for secretion or for use in the cell membrane. After synthesis, these proteins are folded and undergo post-translational modifications within the RER.

Explanation

Cell death is a fundamental biological process crucial for maintaining cellular homeostasis, development, and defense mechanisms in multicellular organisms. There are two primary types of cell death: papoptosis and necrosis.

Option (a) is correct: The cellular and molecular control of programmed cell death is called apoptosis, a regulated process triggered by physiological or pathological signals. It ensures the removal of damaged or harmful cells without harming the organism. Key processes include cell shrinkage, DNA fragmentation, and the formation of apoptotic bodies, which are engulfed by nearby cells or macrophages.

Explanation

If an observer sees the stars rising perpendicular to the horizon, they are positioned at the equator. At the Earth’s equator, stars appear to rise straight up from the eastern horizon, move directly overhead, and set in the west. This occurs because the Earth’s rotational axis is perpendicular to the observer’s horizon at this location. The celestial equator, which is an imaginary extension of Earth’s equator into space, perfectly aligns with the observer’s horizon. As a result, stars follow a vertical trajectory as they rise and set. In contrast, at other latitudes, such as the Tropic of Cancer, the North Pole, or the South Pole, stars rise and set at different angles due to the observer’s varying position on the globe and the tilt of the Earth.

Explanation

Human bones are rigid organs that form the skeleton. They provide structure, support, protection for internal organs, and enable movement. Bones are also classified based on their development and function as either axial (skull, spine, ribs, and sternum) or appendicular (limbs and girdles).The human skeleton consists of approximately 206 bones. A is matched with 4: The sternum, commonly known as the breastbone is a flat bone situated at the center of the thoracic cage, connects to the ribs via costal cartilage. It plays a vital role in protecting the heart and lungs while serving as a structural anchor for the ribcage. B is matched with 1: The clavicle, or collarbone is an S-shaped bone which stabilizes the shoulder girdle and assists in transmitting mechanical forces from the upper limb to the axial skeleton. C is matched with 2: The patella, known as the kneecap is a sesamoid bone embedded within the tendon of the quadriceps muscle, protecting the knee joint. D is matched with 3: The scapula, or shoulder blade is a flat, triangular bone located on the posterior aspect of the ribcage, playing a key role in shoulder movement and provides attachment sites for numerous muscles involved in upper limb motion.

Explanation

Option (b) is correct: Australia is home to a wide variety of marsupials, including kangaroos, koalas, wombats, and tasmanian devils. About 70% of all marsupial species are found in Australia. Options (a), (c) and (d) are incorrect:

South America does have a few marsupials, such as the opossum, but the diversity and number of marsupial species are far greater in Australia. Africa, and South East Asia do not have significant populations of marsupials. Marsupials are rare or absent in these regions.