৪৯তম বিসিএস ⎯ প্রাণ রসায়ন [৬০১]

প্রশ্ন:


সমাধান:

প্রশ্ন: প্রশ্নবোধক স্থানে কোন সংখ্যাটি বসবে?

সমাধান:
(২য় কলাম × ৩য় কলাম) - ১ম কলাম = ৪র্থ কলাম

(6 × 10) - 2 = 60 - 2 = 58
(7 × 11) - 3 = 77 - 3 = 74
(8 × 12) - 4 = 96 - 4 = 92

সুতরাং, প্রশ্নবোধক স্থানে 92 সংখ্যাটি বসবে।

Explanation: (d) While substrate availability (a) plays a role, the key regulatory step is the synthesis of carbamoyl phosphate by CPS I, which is absolutely dependent on its allosteric activator, N-acetylglutamate. NAG synthesis is the primary control point.

Explanation: (b) Histidine decarboxylase removes the carboxyl group from histidine to produce the potent biogenic amine, histamine, which is involved in allergic and inflammatory responses.

Explanation: (b) If the carbon skeleton is degraded to pyruvate or a TCA cycle intermediate (e.g., α-ketoglutarate, oxaloacetate, succinyl-CoA, fumarate), it is glucogenic. If it is degraded to acetyl-CoA or acetoacetate, it is ketogenic.

Explanation: (a) When aspartate leaves the TCA cycle (to donate its nitrogen to the urea cycle), the cycle must be replenished. Pyruvate carboxylase, which converts pyruvate to oxaloacetate, is a crucial anaplerotic reaction that maintains TCA cycle intermediates, ensuring a supply of oxaloacetate for transamination back to aspartate.

Answer: c) -200 J

Explanation: We use the First Law of Thermodynamics: ΔU=Q−W. Here, the system absorbs heat, so Q=+300 J. The system does work, so W=+500 J. Therefore, ΔU=300J−500J=−200J. The internal energy of the system has decreased by 200 J.

Answer: b) State variables

Explanation: The thermodynamic state of a system is described by its macroscopic properties, known as state variables or state functions. These include properties like pressure (P), volume (V), temperature (T), and internal energy (U). These variables are independent of the path taken to reach that state.

Answer: d) Equal to zero

Explanation: The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements in their most stable states under standard conditions (1 bar pressure and a specified temperature, usually 298 K). By definition, the ΔHf∘​ for an element in its most stable form (e.g., O₂(g), C(graphite), Na(s)) is zero, as no change is required to form it from itself.

Correct Answer: খ) Temperature

Explanation: An isothermal process is a thermodynamic process during which the temperature of a system remains constant (ΔT = 0). For an ideal gas undergoing an isothermal process, this means its internal energy (which depends on temperature) is also constant, so any heat added to the system must be converted into work, and any work done on the system must be released as heat. Examples include the melting of ice or boiling of water at their phase change temperatures.

Correct Answer: ক) 420 mmHg

Solution:
Dalton’s Law:

Ptotal=PO₂+PN₂
PN₂=600−180=420 mmHg

Correct Answer: খ) The molecular volume becomes significant.

Explanation:

At high pressures, gas molecules are very close together.
Their actual volume cannot be neglected, leading to deviation from the ideal gas assumption where volume of molecules is considered negligible.

Answer: ঘ) Group 18 (Noble gases)

Explanation: Noble gases show a similar, if small, increase in atomic radius down the group because each new element adds a new principal energy level of electrons, which are further from the nucleus and more effectively shielded by inner-shell electrons. Although they are generally much larger than elements in other groups within the same period, the van der Waals radius makes comparison difficult, and despite their large size, their weak interatomic forces result in low melting and boiling points.

Answer: গ) Cu

Explanation: Transitional elements, or transition metals, are metallic elements in the d-block of the periodic table (Groups 3-12) characterized by incompletely filled d-orbitals, either in their neutral state or in one of their common oxidation states. Key properties include high melting points, variable oxidation states, the ability to form colored compounds, and a wide range of technological applications. Examples include iron, copper, gold, and platinum.

Answer: গ) m

Explanation:

Energy depends on n and l.
mₗ only determines orientation of orbitals, not energy.

Answer: খ) Cl

Explanation:

Chlorine has a slightly higher electron affinity than fluorine.
This is because fluorine’s very small size causes electron-electron repulsion, slightly lowering the energy released when it gains an electron.

The carbylamine reaction, commonly called Hofmann’s isocyanide test, is a chemical test used to identify primary amines. This reaction involves the reaction of primary amine with alcoholic potassium hydroxide and chloroform to form isocyanide.
This isocyande has crooky smell which is unique to this reaction.

Explanation

The correct answer is (a) phenol.

Carbolic acid is the common, historical name for the organic compound phenol (C6​H5​OH).
Phenol consists of a hydroxyl (-OH) group directly attached to a benzene ring.
Although phenol is not a carboxylic acid, it exhibits weak acidity (it is more acidic than typical alcohols) due to the stabilization of the resulting phenoxide ion by resonance in the benzene ring, hence the name "carbolic acid."
It was famously used by Joseph Lister as an antiseptic.

Answer: c) The degree of saturation or unsaturation of a fat or oil.

Explanation: The iodine value is a measure of the amount of iodine that can be absorbed by 100 g of a fat or oil. Iodine reacts with the double bonds of unsaturated fatty acids. Therefore, a higher iodine value indicates a higher degree of unsaturation (more double bonds) and, thus, a more liquid substance (oil).

Answer: B) Sanger sequencing
Explanation:
Sanger sequencing was the workhorse method for HGP. Shotgun sequencing was not the primary method for the Human Genome Project (HGP), which instead used a clone-by-clone method based on bacterial artificial chromosomes (BACs) to organize large pieces of DNA. However, a whole-genome shotgun (WGS) sequencing approach was also applied to the HGP data, involving the random fragmentation of the entire genome into smaller pieces that were sequenced individually and then reassembled by computer algorithms to fill gaps and correct errors. This combination of hierarchical and shotgun methods was crucial for generating a high-quality, complete sequence of the human genome.

Answer: b) A strong base.

Explanation: Saponification is the process of hydrolyzing an ester bond using an alkali (a strong base like sodium hydroxide or potassium hydroxide) to produce an alcohol and a salt of a carboxylic acid (soap). In the context of fats, this yields glycerol and fatty acid salts (soaps).

Answer: c) Carry genetic information from the DNA in the nucleus to the ribosome.

Explanation: mRNA is a linear molecule that is transcribed from a DNA template. It serves as a copy of a gene, carrying the genetic code in the form of codons from the nucleus (in eukaryotes) to the ribosome in the cytoplasm, where it is translated into a protein.

Correct Answer: গ) Cooperative oxygen binding
Explanation: Hemoglobin's four subunits allow for cooperative binding, where binding of one oxygen enhances the binding of others. Hemoglobin is an iron-rich protein in red blood cells that transports oxygen from the lungs to the body's tissues and carries carbon dioxide from the tissues back to the lungs. It gives blood its red color, and a hemoglobin test is a blood test that measures the amount of this protein. Both low hemoglobin (anemia) and high hemoglobin (erythrocytosis) can indicate various health conditions, such as fatigue or, in more severe cases, chest pain.

Answer: a) The presence of a 2'-OH group on the ribose sugar in RNA.

Explanation: The sugar in RNA is ribose, which has a hydroxyl (-OH) group on its 2' carbon. This hydroxyl group makes RNA susceptible to hydrolysis and thus less chemically stable than DNA's deoxyribose sugar, which lacks this group. The lack of the 2'-OH group in DNA is a key reason for its chemical inertness and suitability as a long-term genetic storage molecule.

Answer: a) The ability of one tRNA to recognize multiple codons.

Explanation: The wobble hypothesis, proposed by Francis Crick, states that the first two bases of the mRNA codon form strong Watson-Crick base pairs with the anticodon, but the third base can have more flexibility, or "wobble," allowing for non-standard base pairing. This explains why a single tRNA molecule can sometimes recognize more than one codon, reducing the total number of tRNAs needed.

Answer: c) tRNA.

Explanation: Transfer RNA (tRNA) acts as an adaptor molecule. Each tRNA molecule has a specific anticodon that can bind to a complementary mRNA codon, and it is also covalently attached to the corresponding amino acid. It ensures the correct amino acid is incorporated into the growing polypeptide chain.

Answer: c) Cytosine.

Explanation: Cytosine is one of the three pyrimidine bases (single-ring structures). It is found in both DNA and RNA. The other two pyrimidines are Thymine (T), found only in DNA, and Uracil (U), found only in RNA. Adenine and Guanine are purines (double-ring structures).

Answer: c) Long-term storage of genetic information.

Explanation: DNA (deoxyribonucleic acid) is the primary molecule of heredity in all cellular life. Its stable, double-helical structure and its ability to be replicated faithfully make it the ideal molecule for the long-term storage and transmission of an organism's genetic blueprint.

Answer: c) tRNA.

Explanation: While hairpins and stem-loops can be found in various RNA molecules, they are a fundamental part of the characteristic cloverleaf secondary structure of transfer RNA (tRNA). These loops are crucial for the tRNA's function in recognizing codons and carrying amino acids.

Answer: a) Vitamin B_6 (Pyridoxine).

**Explanation:** Vitamin B_6 is a coenzyme in over 100 metabolic reactions, especially in amino acid metabolism. It is also essential for the synthesis of key neurotransmitters like serotonin and dopamine, and for the synthesis of the heme component of hemoglobin.

Answer: b) Iodine.

Explanation: Goiter is the visible enlargement of the thyroid gland, which results from the gland's attempt to trap more iodine from the blood to synthesize thyroid hormones (T3​ and T4​). Severe and prolonged iodine deficiency leads to this condition.

Answer: b) It is a cofactor for enzymes in amino acid metabolism, such as transaminases.

Explanation: The active form of Vitamin B6​, pyridoxal phosphate (PLP), is a versatile coenzyme for a wide range of enzymes, primarily those involved in amino acid metabolism, including transamination, deamination, and decarboxylation.

Answer: b) Vitamin D.

Explanation: Vitamin D is unique because its active form, calcitriol, functions as a steroid hormone. It binds to intracellular receptors and regulates the expression of genes involved in calcium absorption and bone mineralization.

Answer: b) Vitamin B12 (Cobalamin).

Explanation: Vitamin B12 is essential for DNA synthesis and the formation of red blood cells. It is uniquely found in animal-based foods like meat, eggs, and dairy, making vegetarians and vegans particularly susceptible to deficiency. Vitamin B12 is essential for making healthy red blood cells, white blood cells, and platelets. It is crucial for DNA synthesis, a process vital for rapidly dividing cells like those in the bone marrow that produce blood cells. A deficiency in B12 leads to megaloblastic anemia, where red blood cells are abnormally large and cannot function properly, resulting in reduced oxygen transport and symptoms like extreme tiredness.

Answer: c) Pyruvate dehydrogenase.

Explanation: Thiamine pyrophosphate (TPP) is a crucial coenzyme for several enzymes involved in carbohydrate metabolism, including pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. Without TPP, the breakdown of glucose is halted, leading to a buildup of pyruvate and lactate and causing the neurological and cardiovascular symptoms of beriberi.

Answer: b) Antioxidant.

Explanation: Vitamin E (tocopherols) is a fat-soluble vitamin that protects cell membranes from oxidative damage by neutralizing free radicals. It is particularly important for the protection of polyunsaturated fatty acids.

Answer: d) Iodine.

Explanation: Iodine deficiency is a major cause of preventable intellectual disability and goiter. The universal salt iodization program is a highly effective, low-cost strategy implemented globally to ensure adequate iodine intake in the population.

Answer: b) Ensuring adequate protein, calcium, and vitamin D.

Explanation: Elderly individuals are at risk of sarcopenia (muscle loss) and osteoporosis (bone loss). A diet rich in protein helps maintain muscle mass, while calcium and vitamin D are critical for bone health.

Answer: b) Edema (swelling) and a distended abdomen.

Explanation: Kwashiorkor results from a severe protein deficiency with relatively adequate calorie intake. This leads to hypoalbuminemia (low blood protein), which reduces osmotic pressure and causes fluid to leak into tissues, leading to edema and the characteristic swollen belly.

Answer: c) Sodium.

Explanation:** Sodium is the main cation in the extracellular fluid and is crucial for maintaining fluid balance, blood volume, and nerve and muscle function. However, high sodium intake is a major risk factor for hypertension (high blood pressure).

Answer: c) Vitamin C.

**Explanation:** Vitamin C and Vitamin E work together as a part of the body's antioxidant defense system. After Vitamin E donates an electron to neutralize a free radical, it becomes oxidized. Vitamin C then donates an electron to regenerate the active, reduced form of Vitamin E.

Answer: c) 600 $\mu g$/day.

**Explanation:** The RDA for folic acid is higher for pregnant women (600 $\mu g$/day) compared to non-pregnant adults (400 $\mu g$/day). This increased requirement is critical during the first trimester to support neural tube development in the fetus.

Answer: খ) Ornithine transcarbamoylase

Explanation:
Deficiency of this enzyme prevents citrulline formation, causing ammonia accumulation in the blood.

Answer: ক) Ornithine

Explanation: Ornithine reacts with carbamoyl phosphate to form citrulline, catalyzed by ornithine transcarbamoylase.

The urea cycle is a biochemical pathway, primarily in the liver, that converts toxic ammonia into urea for excretion. This process is vital for mammals and amphibians to remove excess nitrogen, which originates from protein metabolism. Key amino acids involved include ornithine, citrulline, and arginine, along with bicarbonate, forming urea as the final product that is then filtered by the kidneys and excreted in urine.

Answer: গ) Mitochondria

Explanation: The first two steps, including carbamoyl phosphate formation, occur in the mitochondrial matrix.
The first step of the urea cycle occurs in the mitochondria of the liver cells (hepatocytes). In this initial reaction, the enzyme carbamoyl phosphate synthetase I (CPS I) converts ammonia (NH3) and bicarbonate (HCO3-) into carbamoyl phosphate, utilizing energy from two ATP molecules.

Answer: গ) Pyridoxal phosphate (PLP)

Explanation: PLP, derived from vitamin B6, acts as a carrier of amino groups during transamination. Pyridoxal phosphate (PLP) is the active form of vitamin B6, serving as a crucial coenzyme for numerous enzymes in the body. It plays a vital role in amino acid metabolism, neurotransmitter synthesis, and the production of other vital molecules, acting in reactions such as transamination, decarboxylation, and deamination. PLP deficiency can lead to neurological issues like seizures and impaired brain development, highlighting its importance for proper brain function and overall health.

Answer: গ) Uncompetitive

Explanation: An uncompetitive enzyme inhibitor decreases both Vmax and Km. This type of inhibitor binds only to the enzyme-substrate (ES) complex, forming a new ESI complex from which product cannot be released, thus reducing Vmax. The formation of the ESI complex also pulls the equilibrium of enzyme-substrate binding, effectively increasing the apparent affinity of the enzyme for its substrate, which is represented by a decreased Km.

Answer: খ) Sigmoidal kinetics

Explanation:
Allosteric enzymes display sigmoidal (S-shaped) curves because they exhibit cooperative binding, similar to hemoglobin binding oxygen.

Answer: গ) Carbonic anhydrase

Explanation:
Carbonic anhydrase has one of the highest turnover numbers, catalyzing the hydration of CO₂ at nearly 10⁶ reactions per second.

Answer: গ) Kinases

Explanation:
Kinases transfer phosphate groups, often producing ATP during substrate-level phosphorylation, such as pyruvate kinase in glycolysis.

Answer: গ) Apoenzyme plus cofactor

Explanation:
A holoenzyme is a complete, biologically active enzyme formed when an inactive protein component, the apoenzyme, binds to a nonprotein component called a cofactor (either an organic coenzyme or an inorganic metal ion). This binding enables the holoenzyme to catalyze biochemical reactions with the necessary speed and stability provided by the cofactor, which is essential for the enzyme's full catalytic power.

Apoenzyme: Inactive protein part
Cofactor: Non-protein part essential for activity

Answer: খ) They can include metal ions or organic molecules

Explanation:
Cofactors are non-protein molecules that help enzymes function.

Metal ion cofactors: Zn²⁺, Mg²⁺, Fe²⁺
Coenzymes: Organic molecules like NAD⁺, FAD, coenzyme A.

Answer: গ) ECG/EKG
Explanation: Electrocardiogram records electrical impulses of the heart to detect arrhythmias and heart damage.
An ECG (electrocardiogram) is a quick, non-invasive test that records the heart's electrical activity by using sticky patches (electrodes) on the chest, arms, and legs to create a wavy line graph, revealing the heart's rate, rhythm, and any signs of damage or stress. It helps diagnose heart conditions like heart attacks, arrhythmias (irregular heartbeats), or enlargement of the heart.

Answer: খ) Tricuspid valve
Explanation: The tricuspid valve ensures one-way blood flow from the right atrium to the right ventricle.
The tricuspid valve is one of four valves in the heart, located between the right atrium and the right ventricle, and it controls blood flow from the upper right chamber to the lower right chamber of the heart. It consists of three flaps (leaflets) that open to allow unoxygenated blood to move into the right ventricle and then close to prevent blood from flowing backward into the right atrium as the ventricle contracts to send blood to the lungs.

Answer: খ) Hemolysis occurs
Explanation: Anti-B antibodies in type A blood attack B antigens, causing red cell destruction (hemolysis).

Answer: খ) Hematocrit
Explanation: Hematocrit measures the proportion of RBCs to total blood volume, indicating oxygen-carrying capacity.
Hematocrit measures the proportion of red blood cells in your blood, expressed as a percentage. A hematocrit test is used to identify conditions like anemia or polycythemia, and to monitor overall health. Normal levels vary, but generally, males have a slightly higher range than females.

Answer: গ) Alpha and Beta globulins
Explanation:
Alpha and beta globulins act as carrier proteins for lipids, hormones, and fat-soluble vitamins.
Alpha and Beta globulins, produced by the liver, primarily function as transport proteins, carrying lipids (like cholesterol), hormones, and minerals such as iron and copper through the blood. Lipids are transported within lipoproteins (like LDLs and HDLs), which are types of alpha and beta globulins, because they are not water-soluble.

Explanation: (a) The cis double bond at the Δ⁹ position is not in the right configuration or location for the normal enoyl-CoA hydratase enzyme. An isomerase (enoyl-CoA isomerase) converts the cis-Δ³ double bond (formed after three rounds of beta-oxidation) to a trans-Δ² double bond, which is a normal substrate for the hydratase.

Explanation: (d) Prostaglandins are eicosanoids derived from the polyunsaturated fatty acid arachidonic acid. Bile salts, steroid hormones (cortisol, estrogen, testosterone), and vitamin D are all synthesized from cholesterol.
Prostaglandins are hormone-like, physiologically active lipid compounds derived from arachidonic acid that are found in almost every human and animal tissue. They exert potent, localized effects on various bodily functions including blood flow, blood clotting, inflammation, pain perception, smooth muscle contraction, and menstruation. Prostaglandins are synthesized through the cyclooxygenase (COX) pathway, with COX-1 maintaining normal "housekeeping" functions and COX-2 induced by inflammatory stimuli. They have important medical uses, such as in treating glaucoma and inducing labor, and their action is often inhibited by common medications like aspirin and ibuprofen.

Explanation: (a) HMG-CoA reductase converts HMG-CoA to mevalonate. This is the major point of regulation for the entire cholesterol synthesis pathway and is the target of statin drugs.
HMG-CoA reductase is the key enzyme that catalyzes the rate-limiting step in cholesterol and isoprenoid biosynthesis, converting HMG-CoA to mevalonate. It is the primary target for statins, a class of drugs used to lower cholesterol levels by inhibiting this crucial liver enzyme. The enzyme is found in the endoplasmic reticulum and is extensively regulated at the transcriptional, translational, and post-translational levels to maintain cholesterol homeostasis.

Explanation: (c) The brain normally relies on glucose. During prolonged fasting, it can adapt to derive up to 2/3 of its energy from ketone bodies, which helps conserve body protein by reducing the need for glucose. The liver (a) produces but cannot significantly use ketone bodies. Red blood cells (d) lack mitochondria and cannot use any fatty fuels.

Explanation: (b) When beta-oxidation is high (e.g., during starvation/diabetes), large amounts of acetyl-CoA are produced. If oxaloacetate is depleted (for gluconeogenesis), acetyl-CoA cannot enter the TCA cycle and is diverted into the pathway of ketogenesis.

Answer: গ) Creatinine
Explanation: Creatinine levels reflect kidney filtration efficiency (GFR). elevated blood creatinine levels are an important indicator of kidney function in blood tests because creatinine is a waste product filtered by the kidneys. When kidneys aren't working well, they can't filter creatinine effectively, causing its level in the blood to rise, which can signal kidney disease. This blood level is used to calculate the glomerular filtration rate (GFR), a measure of how well the kidneys filter waste.

Answer: ঘ) Sympathetic nervous signals
Explanation: Low blood pressure activates sympathetic nerves, prompting renin release. a decrease in blood pressure or renal perfusion is sensed by the kidney itself, triggering renin release through sympathetic nervous system activation and local signals from the macula densa. Catecholamines, which are hormones from the sympathetic nervous system, and prostaglandin, which are local vasodilators, can indirectly influence renin release by acting on receptors on the kidney's juxtaglomerular cells.

Answer: খ) Glomerulus
Explanation: The glomerulus filters plasma to form initial urine.
The primary function of the glomerulus is to act as a filter, performing glomerular filtration, which is the first step in urine formation. Under high pressure, the glomerulus sifts blood, allowing small molecules like water, salts, and waste products (urea, creatinine) to pass into Bowman's capsule, while larger components such as blood cells and proteins are retained in the bloodstream. This process ensures the elimination of toxins and excess fluids, maintaining the body's balance of essential substances.

Answer: ক) Iron atom in heme group
Explanation: Oxygen binds to the ferrous iron in hemoglobin's heme group.

Answer: গ) Bronchi
Explanation: Bronchi and bronchioles have cilia and mucus to filter dust and microbes. These airways are lined with cells that produce mucus, which traps foreign particles like dust. The cilia then work to sweep the mucus and trapped debris up and out of the airways, where it is coughed up or swallowed.

Answer: c
Explanation: ACTH (adrenocorticotropic hormone) from the anterior pituitary stimulates the adrenal cortex (zona fasciculata) to synthesize and secrete glucocorticoids (cortisol). ACTH is a tropic hormone that travels through the bloodstream to the adrenal glands, where it binds to receptors in the cortex, leading to the release of cortisol.

Answer: c
Explanation: The posterior pituitary (neurohypophysis) releases oxytocin and vasopressin (ADH), which are synthesized in the hypothalamus and transported to the posterior pituitary for release. TSH, ACTH, and prolactin are anterior pituitary hormones.

Answer: c
Explanation: Synergism occurs when combined hormone action is greater than additive; glucagon and epinephrine both stimulate hepatic glycogenolysis and gluconeogenesis and together produce a larger effect.

Answer: c
Explanation: Lipophilic hormones (steroids, thyroid hormones) bind carrier proteins in plasma, which prolongs their half-life and reduces renal clearance. While only the free fraction is bioactive (true), binding extends circulation time; (d) is partially true but incomplete — the bound fraction acts as a reservoir.

Answer: খ) Loop of Henle
Explanation: The loop of Henle establishes the medullary gradient for water reabsorption. The loop of Henle is a part of the kidney's nephron that recovers water and salts from the filtrate, concentrating urine and maintaining the body's hydration. It consists of a descending limb that reabsorbs water and an ascending limb that actively reabsorbs solutes like sodium, creating a salty medullary environment crucial for water balance. This U-shaped structure facilitates a countercurrent mechanism essential for urine concentration and regulating blood volume and blood pressure.

Answer: D) All of the above
Explanation: HGP data has transformed multiple scientific disciplines. The main targets of the Human Genome Project (HGP) were to sequence the three billion base pairs of human DNA, identify all human genes, store this data in publicly accessible databases, and develop new sequencing technologies. Additionally, the HGP aimed to address the Ethical, Legal, and Social Implications (ELSI) of genomic research, develop tools for data analysis, and use the project to advance other areas like comparative genomics and technology transfer to other sectors.

Answer: B) DNA ligase
Explanation:
DNA ligase forms phosphodiester bonds, sealing DNA fragments. DNA ligase is an enzyme that acts like molecular glue, joining two fragments of DNA together by forming a crucial phosphodiester bond in the DNA backbone. It is essential for DNA repair and replication, but it also plays a key role in molecular biology techniques like DNA cloning, where it is used to insert DNA fragments into other DNA molecules, such as plasmids.

Answer: B) Haemophilus influenzae
Explanation: HindIII is a restriction enzyme derived from the bacterium Haemophilus influenzae Rd. This enzyme is used in molecular biology to cut DNA at a specific palindromic sequence, forming sticky ends that are useful for DNA cloning.

Answer: b
Explanation: The two-cell, two-gonadotropin model: Theca interna cells (under LH) produce androgens (androstenedione/testosterone) from cholesterol; these androgens diffuse to granulosa cells where FSH-induced aromatase converts them to estrogens (estradiol).

Answer: a
Explanation: The frequency of GnRH pulses influences gonadotropin secretion: high-frequency pulses favor LH synthesis/secretion, while low-frequency pulses favor FSH. Continuous GnRH suppresses gonadotropin release (used therapeutically).

Answer: b
Explanation: Progesterone produced by the corpus luteum promotes secretory transformation of the endometrium, making it receptive to implantation and maintaining early pregnancy. It also raises basal body temperature slightly.

Answer: c
Explanation: Granulosa cells aromatize androgens (from theca cells) via aromatase to produce estradiol (E2), the primary estrogen during the follicular phase. Progesterone is produced mainly by the corpus luteum in luteal phase.

Answer: b
Explanation: Elevated blood glucose enters β-cells via GLUT2, increasing ATP production which closes KATP channels, depolarizing the membrane, opening voltage-gated Ca²⁺ channels, causing Ca²⁺-dependent insulin exocytosis. Amino acids and incretins also stimulate insulin.

Answer: b
Explanation: PTH acts on osteoblasts/osteocytes to increase expression of RANKL which activates osteoclast differentiation and activity, increasing bone resorption and releasing calcium and phosphate.
The immediate action of parathyroid hormone (PTH) on bone is to directly stimulate osteoblasts, which then indirectly leads to bone resorption by osteoclasts. PTH increases RANKL expression in osteoblasts, a cytokine that promotes the development and activation of osteoclasts, which are the cells responsible for dissolving bone and releasing calcium into the blood.

Answer: b
Explanation: Parathyroid chief cells produce PTH, the main hormone that raises serum calcium by acting on bone, kidney (increase reabsorption), and indirectly increasing intestinal absorption via activation of vitamin D.
The chief cells of the parathyroid glands secrete parathyroid hormone (PTH), also known as parathormone, which regulates calcium and phosphate levels in the blood and is essential for maintaining calcium homeostasis in the body.

Answer: B) Dual regulation (positive and negative)
Explanation: AraC switches roles depending on arabinose availability.
The arabinose operon is a classic example of a positively regulated inducible(negative) operon. In the presence of the inducer arabinose and in the absence of glucose, the AraC protein acts as a positive regulator by interacting with the promoter to facilitate RNA polymerase binding and gene transcription, a mechanism that exemplifies positive control.

Answer: B) cAMP
Explanation: The primary signaling molecule in bacterial catabolite repression is cyclic AMP (cAMP). When glucose is low, cAMP levels rise and bind to the CRP protein, forming a complex that activates the transcription of genes needed to metabolize alternative sugars. Conversely, high glucose levels inhibit adenylate cyclase, lowering cAMP levels and preventing the CRP-cAMP complex from forming, thereby repressing the transcription of genes for alternative catabolism.

Answer: B) Operator and initiator to form a loop
Explanation: In the absence of arabinose, the AraC protein binds to the araO2 and araI1 DNA half-sites within the promoter region of the araBAD operon. This binding causes the DNA to form a loop, which blocks the binding of RNA polymerase, thereby repressing gene expression. This loop prevents transcription when arabinose is absent.

Answer: B) Arabinose metabolism
Explanation: It enables bacteria to utilize arabinose as a carbon source.

Answer: C) Both positive and negative regulation
Explanation: AraC protein acts as both activator and repressor depending on arabinose presence.

Answer: C) Both positive and negative
Explanation: The lac operon exhibits both negative inducible regulation and positive regulation. It is considered negative inducible because a repressor protein normally keeps the operon off, but is inactivated by an inducer (allolactose) when lactose is present, turning the operon on. It also exhibits positive regulation through the cAMP-CAP complex, which enhances transcription when glucose levels are low, ensuring the operon is only active when lactose is available and glucose is absent.
Negative by repressor, positive by CAP-cAMP complex.

Answer: B) Activating lac operon transcription
Explanation: CAP-cAMP binds near the promoter to enhance RNA polymerase binding.

Answer: C) Inducer
Explanation: Allolactose binds to the repressor, inactivating it and inducing transcription.

Answer: B) Permease
Explanation: The lac Y gene encodes lactose permease, a membrane protein that functions as a transporter to facilitate the uptake of lactose into the bacterial cell. This process is essential for the bacterium to utilize lactose as an energy source, as the permease pumps lactose across the cytoplasmic membrane.

Answer: A) Lac Z
Explanation: Lac Z encodes β-galactosidase, which hydrolyzes lactose to glucose and galactose.