Biology MCQs

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Biology MCQs

Practice complete Biology MCQs covering Introduction to Biology, Cell Biology, Cell Structure, Cell Division, Biomolecules, Plant Physiology, Human Physiology, Genetics, Evolution, Biotechnology, Ecology, Classification of Living Organisms, Plant Kingdom, Animal Kingdom, Microbiology, Human Diseases, Nutrition, Reproduction, Environmental Biology, and all other important topics. Includes chapter-wise and exam-oriented multiple-choice questions with detailed answers and explanations for JKSSB, SSC, Banking, Railway, UPSC, CUET, NEET, State PSCs, and other competitive exams.

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Question #81
Which of the following diseases is caused by a protozoan?
A. Tuberculosis
B. Kala-azar
C. Measles
D. Tetanus
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Correct Answer: Option B

Explanation:
Kala-azar (visceral leishmaniasis) is caused by the protozoan Leishmania donovani, transmitted by sandfly bite. Tetanus is bacterial (Clostridium tetani), tuberculosis bacterial (Mycobacterium tuberculosis), measles viral. Protozoan diseases include malaria, amoebiasis, sleeping sickness, giardiasis. Kala-azar causes fever, weight loss, splenomegaly, and is endemic in parts of India.
This question belongs to: Science Biology
Question #82
The process by which bacteria convert atmospheric nitrogen into ammonia is called:
A. Denitrification
B. Nitrification
C. Ammonification
D. Nitrogen fixation
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Correct Answer: Option D

Explanation:
Nitrogen fixation is the conversion of inert atmospheric N₂ into ammonia (NH₃) by free-living (e.g., Azotobacter) or symbiotic (e.g., Rhizobium) bacteria, or by industrial processes. Nitrification is the oxidation of ammonia to nitrites then nitrates. Denitrification converts nitrates to N₂ gas. Ammonification is the release of ammonia from organic nitrogen by decomposers. Fixed nitrogen is essential for plant growth.
This question belongs to: Science Biology
Question #83
Which of the following is an example of a viral disease?
A. Dengue
B. Cholera
C. Diphtheria
D. Leprosy
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Correct Answer: Option A

Explanation:
Dengue fever is caused by the dengue virus (flavivirus) transmitted by Aedes aegypti mosquito. Cholera is bacterial (Vibrio cholerae), diphtheria bacterial (Corynebacterium diphtheriae), leprosy bacterial (Mycobacterium leprae). Viral diseases include influenza, common cold, AIDS, COVID-19, hepatitis, polio, and rabies. Dengue symptoms include high fever, severe headache, joint pain, and rash.
This question belongs to: Science Biology
Question #84
Which type of immunity is achieved by vaccination?
A. Artificial active immunity
B. Natural passive immunity
C. Artificial passive immunity
D. Natural active immunity
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Correct Answer: Option A

Explanation:
Vaccination introduces antigens (weakened or killed pathogens) to stimulate the body's own immune system to produce antibodies and memory cells without causing disease. This is artificial active immunity. Natural active immunity is acquired by recovering from infection. Artificial passive immunity is direct administration of antibodies (e.g., antiserum). Natural passive immunity is transfer of maternal antibodies to fetus.
This question belongs to: Science Biology
Question #85
The deficiency of vitamin B₃ (niacin) causes:
A. Scurvy
B. Beriberi
C. Rickets
D. Pellagra
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Correct Answer: Option D

Explanation:
Pellagra is characterized by dermatitis, diarrhea, dementia, and if untreated, death (the four Ds). It results from deficiency of niacin (vitamin B₃) or its precursor tryptophan. Scurvy is vitamin C deficiency, rickets vitamin D, beriberi vitamin B₁ (thiamine). Niacin is a component of coenzymes NAD and NADP, essential in redox reactions. Pellagra historically common in corn-based diets.
This question belongs to: Science Biology
Question #86
The pancreas functions as both an exocrine and endocrine gland. Which hormone is secreted by the islets of Langerhans?
A. Insulin
B. Pepsin
C. Bile
D. Gastrin
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Correct Answer: Option A

Explanation:
The endocrine part of the pancreas—the islets of Langerhans—contains β-cells that secrete insulin (lowers blood glucose), α-cells secrete glucagon (raises blood glucose), and δ-cells secrete somatostatin. Exocrine pancreas secretes digestive enzymes (trypsin, lipase, amylase). Pepsin is from stomach, bile from liver, gastrin from stomach. Insulin deficiency causes diabetes mellitus.
This question belongs to: Science Biology
Question #87
The condition caused by insufficient secretion of thyroxine in adults is:
A. Acromegaly
B. Myxedema
C. Cretinism
D. Goiter
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Correct Answer: Option B

Explanation:
Hypothyroidism in adults causes myxedema, characterized by puffy face, edema, lethargy, weight gain, and mental dullness. Goiter is an enlarged thyroid due to iodine deficiency but thyroxine may be normal or low. Cretinism is congenital hypothyroidism causing stunted growth and mental retardation. Acromegaly is due to excess growth hormone in adults.
This question belongs to: Science Biology
Question #88
In the male reproductive system, the spermatozoa are stored and mature in the:
A. Epididymis
B. Prostate gland
C. Seminal vesicles
D. Vas deferens
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Correct Answer: Option A

Explanation:
The epididymis is a coiled tube on the posterior surface of the testis where spermatozoa undergo maturation and are stored until ejaculation. Sperm acquire motility here. Seminal vesicles and prostate gland secrete seminal fluid. Vas deferens transports sperm from epididymis to ejaculatory duct. Testes produce sperm and testosterone. Sperm maturation includes gaining ability to fertilize.
This question belongs to: Science Biology
Question #89
The outermost extraembryonic membrane that contributes to the formation of the placenta is the:
A. Allantois
B. Yolk sac
C. Amnion
D. Chorion
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Correct Answer: Option D

Explanation:
The chorion is the outermost membrane surrounding the embryo. It forms chorionic villi that interdigitate with maternal uterine tissue to form the placenta, facilitating nutrient and gas exchange. The amnion surrounds the embryo, forming a fluid-filled cavity. The yolk sac is involved in early nutrition, allantois forms part of the umbilical cord. The chorion is derived from trophoblast and extraembryonic mesoderm.
This question belongs to: Science Biology
Question #90
Which of the following is an antibiotic produced by a bacterium?
A. Streptomycin
B. Cephalosporin
C. Griseofulvin
D. Penicillin
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Correct Answer: Option A

Explanation:
Streptomycin is an antibiotic produced by the bacterium Streptomyces griseus. Penicillin and cephalosporins are from fungi (Penicillium and Cephalosporium). Griseofulvin is a fungal antibiotic. Streptomycin is a broad-spectrum aminoglycoside, effective against gram-negative bacteria and Mycobacterium tuberculosis. It inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit.
This question belongs to: Science Biology
Question #91
Which of the following nutrients provides the highest amount of energy per gram?
A. Vitamin
B. Fat
C. Carbohydrate
D. Protein
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Correct Answer: Option B

Explanation:
Fats provide about 9 kcal per gram, while carbohydrates and proteins provide about 4 kcal/g each. Vitamins are not energy-yielding. Therefore, fats are the most energy-dense macronutrient, serving as long-term energy storage. However, carbohydrates are the primary fuel. Excess fat storage leads to obesity. Fats also aid in absorption of fat-soluble vitamins.
This question belongs to: Science Biology
Question #92
Which of the following is a trace element essential for human nutrition?
A. Iron
B. Phosphorus
C. Calcium
D. Sodium
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Correct Answer: Option A

Explanation:
Iron is a trace mineral (micronutrient) needed in small amounts for hemoglobin and myoglobin function. Calcium and phosphorus are macro-minerals required in larger amounts for bones. Sodium is a major extracellular electrolyte. Trace elements include iron, zinc, copper, iodine, selenium, manganese, etc. Iron deficiency causes anemia. Iron is absorbed in duodenum and stored as ferritin.
This question belongs to: Science Biology
Question #93
Beriberi is caused by the deficiency of vitamin:
A. Vitamin B₁₂ (Cobalamin)
B. Vitamin B₂ (Riboflavin)
C. Vitamin B₆ (Pyridoxine)
D. Vitamin B₁ (Thiamine)
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Correct Answer: Option D

Explanation:
Beriberi is due to thiamine (B₁) deficiency. It presents as two forms: wet beriberi (cardiovascular) and dry beriberi (neurological). Thiamine is a coenzyme (TPP) in decarboxylation reactions. Common in populations consuming polished rice. Riboflavin deficiency causes cheilosis, pyridoxine deficiency causes peripheral neuropathy and anemia, B₁₂ deficiency causes pernicious anemia.
This question belongs to: Science Biology
Question #94
The codon AUG codes for:
A. Alanine
B. Glycine
C. Arginine
D. Methionine
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Correct Answer: Option D

Explanation:
AUG is the initiation codon and codes for methionine in eukaryotes (formylmethionine in prokaryotes). It signals the start of translation. Arginine codons include CGU, CGC, etc. Alanine is coded by GCU, etc. Glycine by GGU, etc. The genetic code is degenerate, meaning multiple codons can code for the same amino acid, but AUG is unique as the initiator.
This question belongs to: Science Biology
Question #95
Which enzyme is responsible for unwinding the DNA double helix during replication?
A. Helicase
B. Primase
C. DNA polymerase
D. Ligase
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Correct Answer: Option A

Explanation:
Helicase unwinds the DNA helix at the replication fork by breaking hydrogen bonds between base pairs. DNA polymerase adds nucleotides, ligase seals Okazaki fragments, primase synthesizes RNA primer. This coordinated action ensures accurate semi-conservative replication. Without helicase, the strands would not separate. Energy for unwinding comes from ATP hydrolysis.
This question belongs to: Science Biology
Question #96
The 'Urey-Miller experiment' simulated the conditions of early Earth to demonstrate the synthesis of:
A. Proteins
B. Amino acids
C. DNA
D. Lipids
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Correct Answer: Option B

Explanation:
Stanley Miller and Harold Urey (1953) simulated primitive Earth conditions by mixing methane, ammonia, hydrogen, and water vapor and providing electric discharge (simulating lightning). They obtained amino acids (glycine, alanine, etc.), supporting the chemical origin of life hypothesis. It didn't produce proteins, DNA, or lipids directly. This experiment was crucial for abiogenesis theories.
This question belongs to: Science Biology
Question #97
Which of the following is an example of a vestigial organ in humans?
A. Appendix
B. Lung
C. Heart
D. Kidney
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Correct Answer: Option A

Explanation:
The vermiform appendix is a vestigial organ, reduced in size and function, serving no essential digestive role in humans. Other vestigial structures include wisdom teeth, ear muscles, and coccyx. They are remnants of organs that were functional in ancestors, providing evidence for evolution. Heart, lung, and kidney are vital functional organs.
This question belongs to: Science Biology
Question #98
Which of the following is a non-renewable energy resource?
A. Tidal energy
B. Coal
C. Solar energy
D. Wind energy
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Correct Answer: Option B

Explanation:
Coal is a fossil fuel formed over millions of years and is finite, thus non-renewable. Solar, wind, and tidal energy are renewable and replenishable. Non-renewable resources include coal, petroleum, natural gas, and nuclear minerals. Overexploitation leads to depletion and environmental pollution. Conservation of such resources is essential for sustainable development.
This question belongs to: Science Biology
Question #99
The biological process that converts ammonia to nitrites and nitrates is called:
A. Ammonification
B. Nitrification
C. Nitrogen fixation
D. Denitrification
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Correct Answer: Option B

Explanation:
Nitrification is a two-step process: oxidation of ammonia to nitrites by bacteria like Nitrosomonas, and then nitrites to nitrates by Nitrobacter. Ammonification is the release of ammonia from organic matter. Denitrification reduces nitrates to N₂ gas. Nitrogen fixation converts atmospheric N₂ to ammonia. Nitrification makes nitrogen available to plants as nitrates, which are easily absorbed.
This question belongs to: Science Biology
Question #100
Which of the following is an algal biofertilizer used in rice fields?
A. Nostoc
B. Trichoderma
C. Azospirillum
D. Rhizobium
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Correct Answer: Option A

Explanation:
Nostoc is a cyanobacterium (blue-green alga) that fixes atmospheric nitrogen and is used as a biofertilizer in rice paddies. Anabaena azollae with Azolla is also common. Rhizobium is bacterial biofertilizer for legumes, Azospirillum is a free-living nitrogen-fixing bacterium. Trichoderma is a fungal biocontrol agent. BGA biofertilizers increase organic nitrogen in soil.
This question belongs to: Science Biology

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