Science and Technology: UPSC Previous Year Questions (Prelims)

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

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

Statement 1 is correct: Tapeworms, belonging to the class Cestoda are fascinating parasites with a unique reproductive strategy. Each segment of their body, called a proglottid, contains both male and female reproductive organs, making them hermaphroditic. This adaptation allows tapeworms to self-fertilize or cross-fertilize, ensuring reproductive success even in limited host environments. Their reproductive efficiency is one reason they thrive as internal parasites.

Statement 2 is correct: Roundworms, or nematodes (phylum Nematoda), exhibit sexual dimorphism, meaning individuals are either male or female. Males are typically smaller and possess specialized structures like spicules for mating. This separation of sexes enhances genetic diversity, a critical factor for survival in diverse environments. Nematodes are one of the most abundant and diverse groups of animals on Earth, found in almost every ecosystem.

Statement 3 is correct: Filariasis is caused by parasitic nematodes like Wuchereria bancrofti and Brugia malayi, which are transmitted to humans via mosquito vectors. These tiny worms reside in the lymphatic system, causing lymphatic filariasis (commonly called elephantiasis). This disease exemplifies the intricate relationship between hosts, parasites, and vectors. Nematodes like these highlight the importance of vector control in managing parasitic diseases.

Statement 4 is incorrect: The Guinea worm (Dracunculus medinensis) is not an annelid (segmented worm) but a nematode. This parasitic roundworm causes dracunculiasis, a disease associated with painful blister formation as the adult worm emerges from the skin. Nematodes are unsegmented and possess a simple body structure adapted for parasitism. This parasite’s lifecycle is intricately linked to water sources, making safe drinking water a key preventive measure.

Explanation

DNA fingerprinting is biotechnique based on analysis of specific regions of DNA that are unique to each individual. This makes it a powerful forensic tool. Assertion (A) is true: DNA fingerprinting, also known as DNA profiling, has become a powerful tool in forensic science for identifying individuals. It is widely used to establish paternity and to identify suspects in criminal cases, including rape and assault. The unique genetic makeup of each individual allows forensic experts to match DNA samples from crime scenes with those of suspects or to determine biological relationships. Reason (R) is true: Trace evidence such as hair, saliva, and dried semen can provide sufficient DNA for analysis. Even small biological samples left at crime scenes can be collected and processed to extract DNA, which can then be compared to known samples to identify individuals. Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains why DNA fingerprinting is effective in establishing paternity and identifying criminals in rape and assault cases. The ability to analyze DNA from minimal trace evidence underpins the power of DNA fingerprinting in forensic investigations.