Biology MCQs

Question #461

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The process that results in the formation of gametes with half the chromosome number is:

A. Mitosis

B. Meiosis

C. Budding

D. Amitosis

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Correct Answer: Option B

Explanation:
Meiosis involves two divisions reducing the chromosome number from diploid (2n) to haploid (n), producing gametes (sperm and egg). Mitosis maintains the chromosome number and produces identical somatic cells. Amitosis is direct nuclear division, budding is asexual reproduction. Meiosis ensures genetic diversity.

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Question #462

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The phenomenon that explains the movement of water from roots to leaves against gravity is mainly:

A. Capillary action

B. Root pressure

C. Active transport

D. Transpiration-cohesion-tension mechanism

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Correct Answer: Option D

Explanation:
The cohesion-tension theory (transpiration pull) is the primary mechanism for water ascent in tall trees. Transpiration creates negative pressure (tension) in leaves, water molecules cohere, and the column is pulled up. Root pressure and capillary action play minor roles. The cohesion of water is due to hydrogen bonds.

This question belongs to: Science Biology

Question #463

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The element that is a key component of amino acids and nucleic acids is:

A. Phosphorus

B. Nitrogen

C. Calcium

D. Potassium

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Correct Answer: Option B

Explanation:
Nitrogen is an essential component of amino acids (proteins) and nitrogenous bases of nucleic acids (DNA, RNA). Phosphorus is in nucleic acids and ATP, potassium is an ion, calcium is structural. The nitrogen cycle makes atmospheric N₂ available to organisms through fixation.

This question belongs to: Science Biology

Question #464

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The enzyme that breaks down hydrogen peroxide into water and oxygen in cells is:

A. Lipase

B. Catalase

C. Peroxidase

D. Amylase

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Correct Answer: Option B

Explanation:
Catalase is found in peroxisomes and protects cells from oxidative damage by converting H₂O₂ (a toxic byproduct) into water and oxygen. Peroxidase also acts on peroxides but catalase is highly efficient. Amylase acts on starch, lipase on lipids. Liver and RBCs have high catalase activity.

This question belongs to: Science Biology

Question #465

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The genetic exchange between bacteria via direct physical contact through a pilus is called:

A. Transformation

B. Mutation

C. Conjugation

D. Transduction

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Correct Answer: Option C

Explanation:
Conjugation involves the transfer of plasmid or chromosomal DNA from a donor (F+) to a recipient (F-) through a sex pilus. Transformation is uptake of free DNA, transduction is gene transfer by bacteriophage, mutation is a change in DNA sequence. Conjugation is often considered bacterial mating.

This question belongs to: Science Biology

Question #466

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The vitamin that is essential for the normal development of the fetal neural tube is:

A. Vitamin B₉ (folic acid)

B. Vitamin A

C. Vitamin D

D. Vitamin C

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Correct Answer: Option A

Explanation:
Folic acid (vitamin B₉) is crucial for DNA synthesis and cell division; its deficiency during early pregnancy increases the risk of neural tube defects like spina bifida. Many countries fortify flour with folic acid. Vitamin A is for vision, C for collagen, D for calcium. Women of childbearing age are advised to take supplements.

This question belongs to: Science Biology

Question #467

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The organism that causes citrus canker is:

A. Bacterium

B. Mycoplasma

C. Fungus

D. Virus

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Correct Answer: Option A

Explanation:
Citrus canker is a bacterial disease caused by Xanthomonas citri (syn. X. axonopodis pv. citri). It produces raised necrotic lesions on leaves, fruit, and stems, causing defoliation and reduced yield. It spreads via rain splash and contaminated tools. No viral or fungal causal agent.

This question belongs to: Science Biology

Question #468

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The scientist who proposed the 'one gene-one enzyme' hypothesis was:

A. Watson and Crick

B. Beadle and Tatum

C. Hershey and Chase

D. Meselson and Stahl

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Correct Answer: Option B

Explanation:
George Beadle and Edward Tatum, working with Neurospora crassa, proposed that each gene controls the synthesis of a specific enzyme (later refined to one gene-one polypeptide). Watson and Crick discovered DNA structure, Hershey-Chase confirmed DNA as genetic material, Meselson-Stahl demonstrated semi-conservative replication.

This question belongs to: Science Biology

Question #469

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The process by which plants produce glucose using light energy is called:

A. Respiration

B. Photosynthesis

C. Chemosynthesis

D. Fermentation

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Correct Answer: Option B

Explanation:
Photosynthesis is the anabolic process in which plants, algae, and cyanobacteria convert CO₂ and H₂O into glucose and O₂ using light energy, catalyzed by chlorophyll. Respiration breaks down glucose to release energy, fermentation is anaerobic breakdown, chemosynthesis uses chemical energy (in some bacteria).

This question belongs to: Science Biology

Question #470

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Which of the following diseases is vector-borne?

A. Typhoid

B. Plague

C. Common cold

D. Tetanus

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Correct Answer: Option B

Explanation:
Plague is caused by Yersinia pestis and transmitted to humans by the bite of infected fleas (from rodents), thus vector-borne. Common cold is airborne, typhoid is water/food-borne, tetanus enters through wounds from soil. Other vector-borne diseases include malaria (mosquito), dengue (mosquito), Lyme disease (tick).

This question belongs to: Science Biology

Question #471

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The enzyme that converts sucrose into glucose and fructose is:

A. Invertase

B. Maltase

C. Amylase

D. Lactase

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Correct Answer: Option A

Explanation:
Invertase (sucrase) hydrolyzes sucrose into its constituent monosaccharides, glucose and fructose. Maltase acts on maltose, lactase on lactose, amylase on starch. Invertase is found in the intestinal brush border and is also produced by yeast. The resulting mixture is called invert sugar, sweeter than sucrose.

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Question #472

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The protein that forms the thick filaments in muscle sarcomeres is:

A. Tropomyosin

B. Troponin

C. Actin

D. Myosin

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Correct Answer: Option D

Explanation:
Myosin is the motor protein forming thick filaments; its heads bind to actin thin filaments and generate force via the cross-bridge cycle. Actin forms thin filaments, troponin and tropomyosin regulate contraction. The sliding filament theory describes muscle shortening without filament length change.

This question belongs to: Science Biology

Question #473

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The condition in which the lens of the eye becomes cloudy, impairing vision, is:

A. Hypermetropia

B. Myopia

C. Cataract

D. Glaucoma

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Correct Answer: Option C

Explanation:
Cataract is clouding of the crystalline lens leading to decreased vision, commonly age-related. Glaucoma is optic nerve damage often due to increased intraocular pressure. Myopia is nearsightedness, hypermetropia is farsightedness. Cataract surgery replaces the lens with an artificial one.

This question belongs to: Science Biology

Question #474

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The term 'operon' in genetics refers to:

A. A set of linked genes regulated by a single promoter

B. A viral DNA integrated into host genome

C. A single gene with multiple functions

D. A DNA repair mechanism

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Correct Answer: Option A

Explanation:
An operon is a functional unit of DNA in prokaryotes containing a cluster of genes under the control of a single promoter, transcribed as a single mRNA. Example: lac operon. It includes operator, promoter, and structural genes. Operons allow coordinate regulation. Eukaryotes generally lack operons.

This question belongs to: Science Biology

Question #475

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The marine algae that form the basis of many aquatic food chains and produce much of Earth's oxygen are:

A. Green algae

B. Red algae

C. Phytoplankton

D. Brown algae

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Correct Answer: Option C

Explanation:
Phytoplankton, including cyanobacteria, diatoms, and dinoflagellates, are microscopic photosynthetic organisms that drift in oceans and freshwaters. They are primary producers responsible for about half of global oxygen production. Brown, red, and green macroalgae also contribute but phytoplankton are the primary oxygen producers.

This question belongs to: Science Biology

Question #476

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The gland that secretes a hormone that regulates calcium levels by lowering blood calcium is:

A. Parathyroid

B. Pancreas

C. Adrenal cortex

D. Thyroid (parafollicular cells)

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Correct Answer: Option D

Explanation:
Calcitonin, produced by parafollicular (C) cells of the thyroid gland, lowers blood calcium by inhibiting osteoclast activity and promoting calcium deposition in bones. Parathyroid hormone (PTH) raises calcium. Adrenal cortex secretes corticosteroids, pancreas secretes insulin/glucagon. Calcitonin opposes PTH.

This question belongs to: Science Biology

Question #477

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The stage of human embryonic development in which the embryo implants in the uterine wall is the:

A. Zygote

B. Morula

C. Blastocyst

D. Gastrula

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Correct Answer: Option C

Explanation:
The blastocyst, a hollow ball of cells with an inner cell mass, hatches from the zona pellucida and implants into the endometrium around day 6-7 after fertilization. The zygote is the fertilized egg, morula is a solid ball, gastrula forms the three germ layers post-implantation.

This question belongs to: Science Biology

Question #478

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The scientist who developed the first vaccine against polio was:

A. Louis Pasteur

B. Edward Jenner

C. Jonas Salk

D. Robert Koch

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Correct Answer: Option C

Explanation:
Jonas Salk developed the first successful inactivated polio vaccine (IPV) in 1955. Albert Sabin later developed the oral attenuated vaccine (OPV). Jenner developed smallpox vaccine, Pasteur rabies, Koch identified tuberculosis bacillus. Polio has been nearly eradicated due to vaccination.

This question belongs to: Science Biology

Question #479

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The process by which carbon dioxide is transported in the blood mainly as:

A. Carbon monoxide

B. Bicarbonate ions

C. Dissolved in plasma

D. Carbaminohemoglobin

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Correct Answer: Option B

Explanation:
About 70% of CO₂ is transported as bicarbonate (HCO₃⁻) ions in plasma. CO₂ reacts with water in RBCs, catalyzed by carbonic anhydrase, to form carbonic acid, which dissociates into H⁺ and HCO₃⁻. The rest is bound to hemoglobin or dissolved. Bicarbonate is the major transport form.

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Question #480

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The phylum that includes organisms with jointed appendages and an exoskeleton of chitin is:

A. Echinodermata

B. Arthropoda

C. Mollusca

D. Annelida

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Correct Answer: Option B

Explanation:
Arthropoda is the largest phylum, characterized by jointed appendages, segmented body, and chitinous exoskeleton. Examples: insects, crustaceans, arachnids. Mollusca have a muscular foot and shell, Annelida have segmented worms, Echinodermata have spiny skin and radial symmetry. Arthropods have an open circulatory system.

This question belongs to: Science Biology

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