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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Page 68 of 103
Question #1341
The 'Bundle of His' and 'Purkinje fibers' are specialized structures found in the:
A. Heart
B. Lungs
C. Brain
D. Kidneys
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Correct Answer: Option A

Explanation:
The Bundle of His and Purkinje fibers are part of the electrical conduction system of the heart. The Bundle of His carries the electrical impulse from the atrioventricular (AV) node down the interventricular septum. It then divides into right and left bundle branches, which terminate in the Purkinje fibers. These fibers rapidly distribute the impulse throughout the ventricular myocardium, ensuring coordinated ventricular contraction.
This question belongs to: Science Biology
Question #1342
In a standard Electrocardiogram (ECG), the 'QRS complex' represents:
A. Atrial depolarization
B. Ventricular depolarization
C. Atrial repolarization
D. Ventricular repolarization
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Correct Answer: Option B

Explanation:
The QRS complex in an ECG represents the depolarization of the ventricles, which initiates ventricular contraction (systole). The P wave represents atrial depolarization. The T wave represents ventricular repolarization, which leads to ventricular relaxation (diastole). Atrial repolarization is usually masked by the larger QRS complex. Exam tip: P = atrial depolarization; QRS = ventricular depolarization; T = ventricular repolarization.
This question belongs to: Science Biology
Question #1343
Erythroblastosis fetalis is a severe condition caused by the incompatibility of blood groups between the mother and fetus, specifically involving the:
A. Rh factor (Rhesus factor)
B. ABO blood group system
C. Duffy antigen
D. Kell antigen
Discuss
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Correct Answer: Option A

Explanation:
Erythroblastosis fetalis (hemolytic disease of the newborn) is primarily caused by Rh incompatibility. It occurs when an Rh-negative mother carries an Rh-positive fetus. During the first pregnancy, fetal Rh+ red blood cells may enter the mother's circulation, prompting her to produce anti-Rh antibodies. In subsequent pregnancies with an Rh+ fetus, these antibodies cross the placenta and destroy the fetal red blood cells, causing severe anemia and jaundice.
This question belongs to: Science Biology
Question #1344
The juxtaglomerular apparatus (JGA) in the kidney is formed by the modification of cells in the:
A. Proximal convoluted tubule and descending limb of Henle
B. Bowman's capsule and glomerulus
C. Distal convoluted tubule and afferent arteriole
D. Collecting duct and efferent arteriole
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Correct Answer: Option C

Explanation:
The juxtaglomerular apparatus (JGA) is a specialized structure where the distal convoluted tubule (DCT) comes into contact with the afferent arteriole. It consists of juxtaglomerular cells (modified smooth muscle cells in the afferent arteriole that secrete renin), macula densa cells (modified DCT cells that sense NaCl concentration), and extraglomerular mesangial cells. It plays a crucial role in regulating blood pressure and glomerular filtration rate via the renin-angiotensin-aldosterone system.
This question belongs to: Science Biology
Question #1345
The hormone aldosterone, which acts on the distal convoluted tubule and collecting duct to increase sodium reabsorption and potassium excretion, is secreted by the:
A. Adrenal medulla
B. Zona glomerulosa of the adrenal cortex
C. Zona fasciculata of the adrenal cortex
D. Anterior pituitary
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Correct Answer: Option B

Explanation:
Aldosterone is a mineralocorticoid hormone secreted by the zona glomerulosa, the outermost layer of the adrenal cortex. Its primary function is to regulate electrolyte balance by promoting the active reabsorption of sodium (Na+) and the excretion of potassium (K+) and hydrogen ions (H+) in the distal tubules and collecting ducts of the kidneys. The zona fasciculata secretes glucocorticoids (cortisol).
This question belongs to: Science Biology
Question #1346
The resting membrane potential of a typical neuron is approximately -70 mV. This negative charge inside the cell is primarily maintained by the:
A. High concentration of calcium ions inside the cell
B. Sodium-potassium pump and the differential permeability of the membrane to K+ and Na+
C. Active transport of chloride ions into the cell
D. Continuous influx of sodium ions through leak channels
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Correct Answer: Option B

Explanation:
The resting membrane potential (-70 mV) is primarily established and maintained by the Na+/K+ ATPase pump, which actively transports 3 Na+ out and 2 K+ into the cell, creating an electrochemical gradient. Additionally, the membrane is much more permeable to K+ than to Na+ due to K+ leak channels. K+ diffuses out of the cell down its concentration gradient, leaving the inside negatively charged.
This question belongs to: Science Biology
Question #1347
In the human brain, the 'limbic system' is primarily associated with:
A. Coordination of voluntary motor movements
B. Regulation of emotions, memory, and motivation
C. Regulation of heart rate and breathing
D. Processing of visual and auditory information
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Correct Answer: Option B

Explanation:
The limbic system is a complex set of structures in the medial temporal lobe, including the hippocampus, amygdala, and hypothalamus. It is often called the 'emotional brain' and is primarily responsible for processing emotions (fear, anger, pleasure), forming and retrieving memories, and regulating motivation and homeostatic drives (hunger, thirst). The medulla regulates heart rate, the cerebellum coordinates movement, and the occipital/temporal lobes process sensory information.
This question belongs to: Science Biology
Question #1348
Which of the following hormones is secreted by the pineal gland and plays a key role in regulating the circadian rhythm (sleep-wake cycle)?
A. Melatonin
B. Oxytocin
C. Thymosin
D. Calcitonin
Discuss
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Correct Answer: Option A

Explanation:
Melatonin is a hormone secreted by the pineal gland, a small endocrine gland located in the brain. Its secretion is highly sensitive to light and dark cycles, being produced in higher amounts at night. Melatonin is crucial for regulating the circadian rhythm (sleep-wake cycle), maintaining biological clocks, and influencing seasonal reproductive functions in some animals. Thymosin is from the thymus, oxytocin from the posterior pituitary.
This question belongs to: Science Biology
Question #1349
The process of spermiogenesis refers to:
A. The release of spermatozoa from the seminiferous tubules into the lumen
B. The transformation of spermatids into mature, motile spermatozoa
C. The formation of spermatogonia from stem cells
D. The meiotic division of primary spermatocytes to form spermatids
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Correct Answer: Option B

Explanation:
Spermiogenesis is the final stage of spermatogenesis, where the spherical, non-motile spermatids undergo a complex morphological transformation to become highly specialized, motile spermatozoa (sperm). This involves the formation of the acrosome, condensation of the nucleus, development of the flagellum (tail), and the shedding of excess cytoplasm. Spermiation is the subsequent release of these mature sperm into the tubule lumen.
This question belongs to: Science Biology
Question #1350
During the menstrual cycle, the surge of which hormone directly triggers ovulation?
A. Progesterone
B. Estrogen
C. Luteinizing Hormone (LH)
D. Follicle Stimulating Hormone (FSH)
Discuss
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Correct Answer: Option C

Explanation:
Ovulation is triggered by a sudden and massive surge in Luteinizing Hormone (LH) from the anterior pituitary, typically around the 14th day of a 28-day cycle. This LH surge causes the mature Graafian follicle to rupture and release the secondary oocyte. FSH stimulates follicular growth earlier in the cycle. Estrogen peaks just before the LH surge, and progesterone peaks after ovulation during the luteal phase.
This question belongs to: Science Biology
Question #1351
The placenta secretes several hormones essential for maintaining pregnancy. Which of the following is NOT primarily secreted by the placenta?
A. Human Chorionic Gonadotropin (hCG)
B. Prolactin
C. Progesterone
D. Human Placental Lactogen (hPL)
Discuss
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Correct Answer: Option B

Explanation:
The placenta acts as a temporary endocrine gland, secreting hCG (which maintains the corpus luteum), hPL (which alters maternal metabolism for fetal growth), and high levels of estrogen and progesterone (which maintain the uterine lining and suppress further ovulation). Prolactin, however, is primarily secreted by the anterior pituitary gland, although its levels rise significantly during pregnancy to prepare the mammary glands for lactation.
This question belongs to: Science Biology
Question #1352
Which of the following bacterial diseases is transmitted through the bite of an infected rat flea (Xenopsylla cheopis)?
A. Leprosy
B. Tuberculosis
C. Tetanus
D. Plague
Discuss
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Correct Answer: Option D

Explanation:
Plague is a severe zoonotic bacterial disease caused by Yersinia pestis. It is primarily transmitted to humans through the bite of infected rat fleas, most commonly Xenopsylla cheopis. The disease has three main forms: bubonic (swollen lymph nodes), pneumonic (lungs), and septicemic (blood). Tuberculosis is airborne, leprosy is transmitted via prolonged close contact, and tetanus enters through deep wounds.
This question belongs to: Science Biology
Question #1353
The causative agent of Kala-azar (visceral leishmaniasis) is:
A. Leishmania donovani
B. Trypanosoma brucei
C. Entamoeba histolytica
D. Plasmodium falciparum
Discuss
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Correct Answer: Option A

Explanation:
Kala-azar, also known as visceral leishmaniasis, is caused by the protozoan parasite Leishmania donovani. It is transmitted by the bite of the infected female sandfly (Phlebotomus). The disease affects internal organs, particularly the liver, spleen, and bone marrow, causing prolonged fever, weight loss, and enlargement of the liver and spleen. Trypanosoma brucei causes sleeping sickness, and Plasmodium causes malaria.
This question belongs to: Science Biology
Question #1354
Which type of antibody is predominantly found in mucosal secretions, such as saliva, tears, and breast milk, providing localized immunity?
A. IgE
B. IgA
C. IgG
D. IgM
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Correct Answer: Option B

Explanation:
Immunoglobulin A (IgA) is the primary antibody found in mucosal secretions, including saliva, tears, nasal secretions, breast milk (colostrum), and the respiratory and gastrointestinal tracts. It plays a crucial role in mucosal immunity by preventing the attachment of pathogens to epithelial cells, a process known as immune exclusion. IgG is the most abundant in blood, IgM is the first to be produced in an infection, and IgE is involved in allergies.
This question belongs to: Science Biology
Question #1355
Graves' disease is an autoimmune disorder characterized by hyperthyroidism. It is caused by the production of autoantibodies that:
A. Block the TSH receptors on the thyroid gland
B. Mimic TSH and continuously stimulate the TSH receptors
C. Destroy the thyroid follicular cells
D. Target the parathyroid glands
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Correct Answer: Option B

Explanation:
Graves' disease is an autoimmune condition where the immune system produces thyroid-stimulating immunoglobulins (TSIs). These autoantibodies bind to and mimic the action of Thyroid Stimulating Hormone (TSH) on the TSH receptors of the thyroid gland. This continuous, unregulated stimulation causes the thyroid to overproduce thyroid hormones (T3 and T4), leading to hyperthyroidism, goiter, and often exophthalmos (bulging eyes).
This question belongs to: Science Biology
Question #1356
Which of the following vitamins is a water-soluble vitamin that acts as a coenzyme in the metabolism of amino acids and fatty acids?
A. Vitamin K
B. Vitamin D
C. Vitamin A
D. Vitamin B6 (Pyridoxine)
Discuss
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Correct Answer: Option D

Explanation:
Vitamin B6, in its active form pyridoxal phosphate (PLP), is a crucial water-soluble coenzyme involved in over 100 enzymatic reactions, primarily in amino acid metabolism (transamination, decarboxylation) and fatty acid metabolism. It is also essential for neurotransmitter synthesis and hemoglobin formation. Vitamins A, D, and K are fat-soluble vitamins. Exam tip: B-complex and C are water-soluble; A, D, E, K are fat-soluble.
This question belongs to: Science Biology
Question #1357
Pellagra is a disease characterized by the '4 Ds': dermatitis, diarrhea, dementia, and death. It is caused by the deficiency of:
A. Vitamin B3 (Niacin)
B. Vitamin B1 (Thiamine)
C. Vitamin B12 (Cobalamin)
D. Vitamin B2 (Riboflavin)
Discuss
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Correct Answer: Option A

Explanation:
Pellagra is caused by a severe deficiency of Vitamin B3 (Niacin) or its precursor, the amino acid tryptophan. The classic symptoms are the '4 Ds': dermatitis (symmetrical, pigmented rash on sun-exposed skin), diarrhea, dementia (neurological symptoms), and eventually death if untreated. It is historically associated with diets heavily reliant on untreated corn (maize), which has low bioavailable niacin and tryptophan.
This question belongs to: Science Biology
Question #1358
The essential fatty acids that cannot be synthesized by the human body and must be obtained from the diet are:
A. Butyric acid and caproic acid
B. Linoleic acid and alpha-linolenic acid
C. Oleic acid and arachidonic acid
D. Palmitic acid and stearic acid
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Correct Answer: Option B

Explanation:
Essential fatty acids are polyunsaturated fats that the human body lacks the enzymes to synthesize, so they must be consumed in the diet. The two primary essential fatty acids are linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid). They are precursors for other important lipids like arachidonic acid and are crucial for cell membrane structure and eicosanoid synthesis. Palmitic and stearic are saturated fats.
This question belongs to: Science Biology
Question #1359
In a dihybrid cross between two heterozygous pea plants (RrYy x RrYy), if the two genes are completely linked, the phenotypic ratio in the F2 generation will be:
A. 1:2:1
B. 1:1:1:1
C. 3:1
D. 9:3:3:1
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Correct Answer: Option C

Explanation:
If two genes are completely linked, they are located very close together on the same chromosome and do not undergo independent assortment or crossing over. They are inherited together as a single unit. A dihybrid cross (RrYy x RrYy) with complete linkage behaves like a monohybrid cross. The F1 produces only parental gametes (RY and ry). The F2 generation will have a genotypic ratio of 1 RRYY : 2 RrYy : 1 rryy, resulting in a phenotypic ratio of 3:1.
This question belongs to: Science Biology
Question #1360
The phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits is known as:
A. Incomplete dominance
B. Epistasis
C. Polygenic inheritance
D. Pleiotropy
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Correct Answer: Option D

Explanation:
Pleiotropy occurs when a single gene affects two or more distinct phenotypic traits. This happens because the gene product (protein) may be used in multiple biological pathways or expressed in different tissues. Examples include phenylketonuria (PKU), sickle cell anemia, and Marfan syndrome. Epistasis involves the interaction of different genes, polygenic inheritance involves multiple genes affecting one trait, and incomplete dominance is a single gene interaction.
This question belongs to: Science Biology

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