General Science MCQs – Page 55 (Q541-550)

A single complete round of meiotic cell division typically results in four daughter cells, each containing only half the original chromosome number found in the parent cell, achieved through two consecutive rounds of cell division, meiosis I and meiosis II, following just a single initial round of DNA replication. This produces genetically diverse haploid gametes, sperm or egg cells, each slightly genetically different from one another due to genetic recombination occurring during meiosis I, contributing significantly to the genetic diversity observed among offspring within sexually reproducing species like humans.

Carbohydrates generally serve as the body's primary and quickest source of energy, since they are broken down relatively rapidly during digestion into simple sugars like glucose, which cells can immediately use through cellular respiration to produce usable ATP energy. Dietary carbohydrates are found abundantly in foods like grains, fruits, vegetables, and sugar, and while proteins and fats can also be metabolized for energy when needed, the body generally prioritizes using readily available carbohydrates first, reserving fat as a longer-term, more energy-dense storage reserve for times when quick carbohydrate energy sources are unavailable or depleted.

Proteins in the diet are broken down into amino acids, their basic building block units, during digestion, through the action of digestive enzymes like pepsin in the stomach and trypsin in the small intestine, which break the peptide bonds linking amino acids together within larger protein molecules. These absorbed amino acids are then used by the body to synthesize new proteins needed for growth, tissue repair, enzyme production, and countless other essential biological functions. There are 20 standard amino acids used in building human proteins, of which nine are considered essential, meaning they must be obtained through diet.

Dietary proteins are primarily important in the body mainly for growth and repair of body tissues, providing the essential amino acid building blocks needed to construct and maintain muscles, skin, organs, enzymes, hormones, and antibodies throughout an organism's life. Unlike carbohydrates, which the body prioritizes for quick, readily available energy, protein serves a more structural and functional role, and while excess protein can technically be metabolized for energy when needed, this is generally a less efficient and less preferred energy pathway compared to the body's primary reliance on dietary carbohydrates and fats for routine energy needs.

A deficiency of vitamin C, also known as ascorbic acid, is classically known to cause the disease scurvy, historically a significant and often fatal problem among sailors on long ocean voyages who lacked access to fresh fruits and vegetables for extended periods. Vitamin C is essential for synthesizing collagen, a key structural protein needed for healthy skin, blood vessels, gums, and connective tissue, and its deficiency causes symptoms including bleeding gums, joint pain, and poor wound healing. British naval surgeon James Lind demonstrated in 1747 that citrus fruits could effectively prevent and treat scurvy, an early landmark in the history of clinical nutritional science.

A deficiency of vitamin D is classically known to cause the bone disease rickets in children, characterized by softened, weakened bones that can become visibly bowed or deformed due to inadequate mineralization during critical growth periods, since vitamin D is essential for the body's proper absorption and utilization of calcium and phosphorus needed for healthy bone development. Vitamin D is somewhat unique among vitamins in that the human body can synthesize it naturally in the skin upon adequate exposure to sunlight, which is why it is sometimes informally called the sunshine vitamin, alongside dietary sources like fortified milk and fatty fish.

A deficiency of vitamin A is classically associated with causing night blindness, an early symptom resulting from vitamin A's essential role in producing rhodopsin, the light-sensitive pigment found in the retina's rod cells that enables vision in dim light conditions. Beyond its critical role in vision, vitamin A is also important for maintaining healthy skin, supporting proper immune system function, and cellular growth, and severe or prolonged deficiency can progress from initial night blindness to more serious, potentially permanent eye damage, making vitamin A deficiency a significant global public health concern, particularly in developing regions with limited dietary diversity.

A deficiency of vitamin B1, also known as thiamine, is classically known to cause the disease beriberi, characterized by symptoms affecting the nervous and cardiovascular systems, including muscle weakness, nerve damage, and in severe cases, heart failure, since thiamine plays an essential role in converting carbohydrates into usable cellular energy. Historically prevalent in populations relying heavily on polished white rice as a primary dietary staple, since the milling process removes the thiamine-rich outer bran layer, beriberi's link to diet was a landmark early discovery in the developing field of nutritional science and vitamin deficiency research.

Vitamin K is primarily important in the body for enabling normal blood clotting, serving as an essential cofactor needed for the liver to produce several critical clotting factor proteins that allow blood to properly form stable clots and stop bleeding following an injury. Deficiency in vitamin K can therefore lead to excessive, prolonged bleeding and easy bruising, and newborn infants are commonly given a vitamin K supplement shortly after birth as a preventive measure, since their digestive systems have not yet developed the bacterial gut flora that normally contributes significantly to the body's ongoing natural vitamin K production and supply.

Citrus fruits such as oranges and lemons are generally considered a particularly rich dietary source of vitamin C, an essential water-soluble nutrient that the human body, unlike many other animals, cannot synthesize on its own and must therefore obtain regularly through diet. Beyond citrus fruits, vitamin C is also found abundantly in foods like strawberries, bell peppers, and broccoli, and beyond its well-known historical association with preventing scurvy, vitamin C also functions as a powerful antioxidant and plays important roles in immune system support and collagen synthesis throughout the body.