Biology MCQs

Question #261

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The part of the nephron that creates a concentration gradient in the kidney medulla is:

A. Proximal convoluted tubule

B. Distal convoluted tubule

C. Loop of Henle

D. Bowman's capsule

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Correct Answer: Option C

Explanation:
The loop of Henle, especially the descending limb (permeable to water) and ascending limb (impermeable, active salt transport), creates an osmotic gradient in the medulla. This countercurrent multiplier system enables the kidney to produce concentrated or dilute urine depending on hydration status. ADH acts on the collecting duct to utilize this gradient.

This question belongs to: Science Biology

Question #262

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The myelin sheath around axons in the peripheral nervous system is formed by:

A. Schwann cells

B. Astrocytes

C. Microglia

D. Oligodendrocytes

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Correct Answer: Option A

Explanation:
In the peripheral nervous system (PNS), Schwann cells wrap around axons to form the myelin sheath, which insulates axons and increases the speed of nerve impulse conduction (saltatory conduction). In the central nervous system (CNS), oligodendrocytes perform this function. Astrocytes support neurons, microglia act as immune cells. Nodes of Ranvier are gaps in the myelin sheath.

This question belongs to: Science Biology

Question #263

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Which of the following is an excitatory neurotransmitter in the central nervous system?

A. Serotonin

B. Glycine

C. Acetylcholine

D. Gamma-aminobutyric acid (GABA)

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Correct Answer: Option C

Explanation:
Acetylcholine is an excitatory neurotransmitter at neuromuscular junctions and many CNS synapses. GABA and glycine are generally inhibitory in the CNS. Serotonin modulates mood but can have inhibitory or excitatory effects depending on receptors. Neurotransmitters are chemical messengers released from synaptic vesicles into the synaptic cleft to bind to postsynaptic receptors.

This question belongs to: Science Biology

Question #264

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The hormone that stimulates the synthesis of milk in the mammary glands is:

A. Estrogen

B. Oxytocin

C. Progesterone

D. Prolactin

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Correct Answer: Option D

Explanation:
Prolactin, secreted by the anterior pituitary, is the primary hormone responsible for milk synthesis (lactation) after childbirth. Oxytocin causes milk ejection (let-down reflex). Estrogen and progesterone stimulate mammary duct and alveolar development during pregnancy but inhibit milk secretion; after delivery, their levels drop, allowing prolactin to act.

This question belongs to: Science Biology

Question #265

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The corpus luteum secretes which hormone that maintains pregnancy in early stages?

A. Human chorionic gonadotropin (hCG)

B. Progesterone

C. Estrogen

D. Luteinizing hormone

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Correct Answer: Option B

Explanation:
The corpus luteum, formed from the ruptured follicle after ovulation, secretes progesterone (and some estrogen) which prepares the endometrium for implantation and maintains early pregnancy until the placenta takes over (around 8-10 weeks). hCG, produced by the placenta, maintains the corpus luteum. Progesterone is crucial for maintaining the uterine lining.

This question belongs to: Science Biology

Question #266

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Which of the following is a water-borne bacterial disease?

A. Influenza

B. Ringworm

C. Cholera

D. Dengue

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Correct Answer: Option C

Explanation:
Cholera is caused by Vibrio cholerae, transmitted through contaminated water and food, causing severe watery diarrhea and dehydration. Influenza is a viral respiratory disease, ringworm is a fungal infection, dengue is a mosquito-borne viral disease. Cholera toxin disrupts ion transport in intestinal cells, leading to massive fluid loss. ORS (oral rehydration solution) is a simple life-saving treatment.

This question belongs to: Science Biology

Question #267

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Which of the following diseases is caused by a protozoan transmitted by the tsetse fly?

A. Sleeping sickness

B. Malaria

C. Yellow fever

D. Filariasis

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Correct Answer: Option A

Explanation:
African sleeping sickness (trypanosomiasis) is caused by the protozoan Trypanosoma brucei, transmitted by the tsetse fly (Glossina). Malaria is by Plasmodium (Anopheles mosquito), filariasis by Wuchereria (Culex), yellow fever by a virus (Aedes). Sleeping sickness affects the central nervous system, causing lethargy, coma, and death if untreated.

This question belongs to: Science Biology

Question #268

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The antibiotic that inhibits bacterial cell wall synthesis is:

A. Chloramphenicol

B. Streptomycin

C. Penicillin

D. Tetracycline

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Correct Answer: Option C

Explanation:
Penicillin and other beta-lactam antibiotics (cephalosporins, carbapenems) inhibit the transpeptidase enzyme involved in cross-linking peptidoglycan, weakening the cell wall and causing bacterial lysis. Tetracycline and streptomycin inhibit protein synthesis by acting on ribosomes. Chloramphenicol also inhibits protein synthesis. Penicillin is most effective against actively dividing gram-positive bacteria.

This question belongs to: Science Biology

Question #269

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The disease 'osteomalacia' in adults is caused by the deficiency of:

A. Vitamin B₁₂

B. Vitamin C

C. Vitamin D

D. Vitamin A

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Correct Answer: Option C

Explanation:
Osteomalacia is the softening of bones in adults due to vitamin D deficiency, leading to inadequate mineralization of bone matrix. In children, the equivalent is rickets. Vitamin D aids calcium absorption. Deficiency may result from poor dietary intake, lack of sunlight, or malabsorption. Bones become weak and prone to fractures.

This question belongs to: Science Biology

Question #270

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The process of converting mRNA information into a polypeptide chain is:

A. Transcription

B. Replication

C. Transduction

D. Translation

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Correct Answer: Option D

Explanation:
Translation is the synthesis of a polypeptide chain at the ribosome, where the genetic code in mRNA is read by tRNA anticodons to assemble amino acids in the correct order. Transcription produces mRNA from DNA, replication duplicates DNA, transduction is gene transfer by viruses. Translation involves initiation, elongation, and termination factors.

This question belongs to: Science Biology

Question #271

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An individual with the genotype AaBb (if genes are unlinked) will produce how many types of gametes?

A. 1

B. 4

C. 2

D. 8

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Correct Answer: Option B

Explanation:
For an individual heterozygous at two unlinked loci (AaBb), the possible gamete combinations are AB, Ab, aB, ab, i.e., 4 types. This follows the law of independent assortment. Formula: 2^n, where n is the number of heterozygous gene pairs. Here n=2, so 2²=4. If genes are linked, recombination frequency determines the proportion.

This question belongs to: Science Biology

Question #272

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The 'Founder Effect' is a cause of:

A. Gene flow

B. Mutation

C. Natural selection

D. Genetic drift

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Correct Answer: Option D

Explanation:
The founder effect is a type of genetic drift that occurs when a small group of individuals becomes isolated from a larger population, establishing a new population with a different allele frequency. This can lead to reduced genetic variation and increased prevalence of certain traits. Examples: high frequency of polydactyly in Amish community.

This question belongs to: Science Biology

Question #273

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Which of the following is a direct evidence of evolution?

A. Fossils

B. Embryology

C. Molecular biology

D. Comparative anatomy

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Correct Answer: Option A

Explanation:
Fossils provide direct, tangible evidence of past life forms and evolutionary changes over geological time. Comparative anatomy (homologous/analogous organs), embryology (similar early stages), and molecular biology (DNA/protein sequence similarities) provide indirect evidence. Fossils show transitional forms like Archaeopteryx (between reptiles and birds).

This question belongs to: Science Biology

Question #274

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The biome characterized by permafrost and low biodiversity, dominated by mosses and lichens, is:

A. Temperate deciduous forest

B. Tundra

C. Savanna

D. Taiga

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Correct Answer: Option B

Explanation:
The tundra biome, found in Arctic and Antarctic regions, has permanently frozen subsoil (permafrost), extremely low temperatures, short growing seasons, and vegetation limited to mosses, lichens, grasses, and dwarf shrubs. Taiga is the boreal coniferous forest, savanna is tropical grassland, temperate deciduous forest has distinct seasons. Tundra is the coldest biome.

This question belongs to: Science Biology

Question #275

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The Biological Oxygen Demand (BOD) measures:

A. The amount of oxygen required by microorganisms to decompose organic matter in water

B. The oxygen produced by plants

C. The oxygen in blood

D. The oxygen content in the atmosphere

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Correct Answer: Option A

Explanation:
BOD is a measure of organic pollution in water bodies. High BOD indicates high levels of biodegradable organic matter, leading to oxygen depletion as microorganisms break it down. This can cause fish kills and eutrophication. Clean water has low BOD (

This question belongs to: Science Biology

Question #276

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Which of the following organisms is a decomposer in an ecosystem?

A. Eagle

B. Grass

C. Lion

D. Fungi

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Correct Answer: Option D

Explanation:
Fungi (and bacteria) are decomposers that break down dead organic matter and waste, releasing nutrients back into the ecosystem. They are saprotrophs. Grass is a producer, lion and eagle are consumers (carnivores). Decomposers are essential for nutrient cycling, especially carbon and nitrogen cycles. Without decomposers, ecosystems would be clogged with dead material.

This question belongs to: Science Biology

Question #277

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The primary reason for the depletion of the ozone layer is the emission of:

A. Carbon monoxide

B. Sulphur oxides

C. Chlorofluorocarbons (CFCs)

D. Methane

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Correct Answer: Option C

Explanation:
CFCs, used in refrigerants and aerosol propellants, release chlorine atoms when exposed to UV radiation in the stratosphere. A single chlorine atom can destroy thousands of ozone (O₃) molecules. This led to the formation of the 'ozone hole' over Antarctica. The Montreal Protocol (1987) successfully phased out CFCs, allowing the ozone layer to slowly recover.

This question belongs to: Science Biology

Question #278

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Which of the following is a correct statement about viruses?

A. They contain both DNA and RNA.

B. They have cell walls like bacteria.

C. They are living organisms that can reproduce outside a host.

D. They are obligate intracellular parasites.

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Correct Answer: Option D

Explanation:
Viruses are non-living outside a host cell and must hijack a host's cellular machinery to replicate, making them obligate intracellular parasites. They contain either DNA or RNA, not both (except some rare cases). They lack cell walls and metabolic machinery. Their status as living or non-living is debated, but they are biologically active only inside a host.

This question belongs to: Science Biology

Question #279

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The process by which two different organisms live together and both benefit is:

A. Commensalism

B. Parasitism

C. Mutualism

D. Amensalism

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Correct Answer: Option C

Explanation:
Mutualism is a symbiotic interaction where both species benefit (+/+). Examples: Rhizobium-legume association, lichens, mycorrhizae, and oxpecker-rhino. Parasitism benefits one at the expense of the other (+/-), commensalism benefits one without affecting the other (+/0), amensalism harms one without benefit to the other (-/0). Mutualism is crucial for ecosystem functioning.

This question belongs to: Science Biology

Question #280

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The 'central dogma' of molecular biology states that information flows:

A. DNA → RNA → Protein

B. RNA → DNA → Protein

C. Protein → RNA → DNA

D. DNA → Protein → RNA

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Correct Answer: Option A

Explanation:
The central dogma, proposed by Francis Crick, describes the flow of genetic information from DNA to RNA (transcription) and then to protein (translation). Information cannot flow from protein to nucleic acid. However, exceptions exist: reverse transcription (RNA → DNA) in retroviruses, and some regulatory RNA functions. The fundamental principle still holds.

This question belongs to: Science Biology

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