What does Chargaff's rule state regarding the base pairing in DNA?
Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?
What is the rate-limiting enzyme in heme synthesis?
The primary defect which leads to sickle cell anemia is:
The cofactor vitamin B12 is required for the following conversion:
Which vitamin is considered the most potent antioxidant?
Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
What is the primary biochemical defect in alkaptonuria?
Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?
Which one of the following statements concerning gluconeogenesis is correct?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 121: What does Chargaff's rule state regarding the base pairing in DNA?
- A. A=T, G=C (Correct Answer)
- B. A=G, T=C
- C. A=C, G=T
- D. Any combination possible
Explanation: ***A=T, G=C*** - **Chargaff's rules** state that in any double-stranded DNA, the amount of **adenine (A)** is approximately equal to the amount of **thymine (T)**, and the amount of **guanine (G)** is approximately equal to the amount of **cytosine (C)**. - This equivalency reflects the specific **base pairing** in the DNA double helix, where A always pairs with T, and G always pairs with C. *A=G, T=C* - This statement is incorrect as it proposes an atypical and biologically inaccurate pairing between a **purine (A)** and another **purine (G)**, and a **pyrimidine (T)** with a **pyrimidine (C)**. - This combination would disrupt the uniform diameter of the DNA double helix required for its structural stability. *A=C, G=T* - This option is incorrect because it suggests pairing a purine (A) with a pyrimidine (C) and a purine (G) with a pyrimidine (T) in a way that is not observed in natural DNA. - Such pairings would also lead to an irregular width of the DNA molecule, destabilizing its structure. *Any combination possible* - This statement is false; base pairing in DNA is **highly specific** and not random due to chemical and structural constraints. - The specific pairing rules (**A with T, G with C**) are crucial for maintaining the consistent structure of the DNA double helix and for accurate DNA replication and transcription.
Question 122: Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?
- A. Transcribed from nuclear DNA.
- B. Has a lower molecular weight than hn-RNA. (Correct Answer)
- C. Contains uracil instead of thymine.
- D. Sugar is ribose.
Explanation: ***Has a lower molecular weight than hn-RNA.*** - **Mature mRNA** undergoes **splicing**, which removes **introns** (non-coding regions) from the heterogeneous nuclear RNA (hnRNA) precursor. - The removal of these introns results in a **shorter, more compact molecule** with a lower molecular weight compared to the original hnRNA. *Transcribed from nuclear DNA.* - While mRNA is indeed **transcribed from DNA**, this statement is true for **all types of RNA (rRNA, tRNA, and mRNA)**, not just mature cytoplasmic mRNA specifically, and does not differentiate it. - The initial transcript is **hnRNA**, which is then processed into mature mRNA. *Contains uracil instead of thymine.* - This is a characteristic of **all RNA molecules**, not just mature cytoplasmic mRNA, and is a fundamental difference between RNA and DNA. - DNA contains **thymine**, while RNA contains **uracil**. *Sugar is ribose.* - This is a distinguishing feature of **all RNA molecules**, indicating that the sugar component of its nucleotides is **ribose**, whereas DNA contains **deoxyribose**. - This statement is not unique to mature cytoplasmic mRNA.
Question 123: What is the rate-limiting enzyme in heme synthesis?
- A. ALA synthase (Correct Answer)
- B. HMG CoA reductase
- C. ALA dehydratase
- D. Uroporphyrinogen 1 synthase
Explanation: ***ALA synthase*** - **Aminolevulinate synthase** (ALA synthase) is the first and **rate-limiting enzyme** in the heme synthesis pathway. - Its activity is tightly regulated, and its overexpression or deficiency can lead to disorders like **acute intermittent porphyria**. *Hmg coa reductase* - **HMG-CoA reductase** is the **rate-limiting enzyme** in the **cholesterol biosynthesis pathway**, not heme synthesis. - It is the target enzyme for statin medications, which lower cholesterol levels. *ALA dehydratase* - **ALA dehydratase** (also known as porphobilinogen synthase) is the second enzyme in the heme synthesis pathway, responsible for converting two molecules of **ALA to porphobilinogen**. - While critical, it is not the rate-limiting step; inhibition of this enzyme can lead to **lead poisoning**. *Uroporphyrinogen 1 synthase* - **Uroporphyrinogen I synthase** (also called hydroxymethylbilane synthase or porphobilinogen deaminase) catalyzes the formation of **hydroxymethylbilane** from four molecules of **porphobilinogen**. - A deficiency in this enzyme is associated with **acute intermittent porphyria**, but it is not the rate-limiting enzyme of the overall pathway.
Question 124: The primary defect which leads to sickle cell anemia is:
- A. An abnormality in porphyrin part of hemoglobin
- B. A nonsense mutation in the β-chain of HbA
- C. Substitution of valine by glutamate in the α-chain of HbA
- D. Replacement of glutamate by valine in β-chain of HbA (Correct Answer)
Explanation: ***Replacement of glutamate by valine in β-chain of HbA*** - The primary defect in sickle cell anemia is a **point mutation** that leads to the replacement of **glutamic acid** with **valine** in the **sixth position** of the β-globin chain [1]. - This mutation causes the hemoglobin (HbS) to polymerize under low oxygen conditions, resulting in the characteristic **sickle-shaped red blood cells** [1]. *A nonsence mutation in the I3-chain of HbA* - A nonsense mutation leads to a **premature stop codon**, which is not the mechanism behind sickle cell anemia. - This mutation does not involve the β-globin chain, which is critical in this specific disorder. *Substitution of valine by glutamate in the a-chain of HbA* - This statement is incorrect as sickle cell anemia specifically involves the **β-chain**, not the **α-chain**. - Substituting valine with glutamate does not cause sickling but rather the opposite of the actual defect observed in this condition. *An abnormality in porphyrin part of hemoglobin* - Sickle cell anemia does not arise from **porphyrin metabolism issues**, which are related to conditions like **porphyrias**. - The primary defect is a change in the amino acid sequence, not in the porphyrin structure of hemoglobin. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 598-599.
Question 125: The cofactor vitamin B12 is required for the following conversion:
- A. Dopamine to Norepinephrine
- B. Propionyl CoA to methyl malonyl CoA
- C. Methyl malonyl CoA to succinyl CoA (Correct Answer)
- D. Homocysteine to cysteine
Explanation: ***Methyl malonyl CoA to succinyl CoA*** - **Vitamin B12**, in its active form **adenosylcobalamin**, is a crucial cofactor for the enzyme **methylmalonyl-CoA mutase**, which catalyzes the isomerization of **methylmalonyl-CoA to succinyl-CoA**. - This conversion is vital for the metabolism of **odd-chain fatty acids** and certain **amino acids**, allowing their entry into the **Krebs cycle**. *Dopamine to Norepinephrine* - This conversion is catalyzed by **dopamine beta-hydroxylase**, which requires **vitamin C** (ascorbate) and **copper** as cofactors, not vitamin B12. - It is a key step in the synthesis of **catecholamines** within the nervous system. *Propionyl CoA to methyl malonyl CoA* - This conversion is catalyzed by **propionyl-CoA carboxylase** and requires **biotin** as a cofactor, not vitamin B12. - This reaction is the first step in the metabolic pathway that leads to succinyl-CoA from odd-chain fatty acids. *Homocysteine to cysteine* - This conversion occurs via the **transsulfuration pathway** and requires **vitamin B6** (pyridoxal phosphate) as a cofactor, not vitamin B12. - The enzymes involved are **cystathionine β-synthase** and **cystathionine γ-lyase**, both B6-dependent. - Vitamin B12 is involved in the **remethylation** of homocysteine to methionine (not in transsulfuration to cysteine).
Question 126: Which vitamin is considered the most potent antioxidant?
- A. Vit A
- B. Vit K
- C. Vit E (Correct Answer)
- D. Vit C
Explanation: ***Vit E*** - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from **oxidative damage** by scavenging free radicals. - Its ability to interrupt **lipid peroxidation** makes it highly effective in protecting tissues rich in polyunsaturated fatty acids, such as cell membranes. *Vit A* - **Vitamin A**, particularly in its carotenoid forms like **beta-carotene**, is an antioxidant, but its primary role is in **vision** and **immune function**. - While it can quench **singlet oxygen** and trap free radicals, it is generally considered less potent than vitamin E in protecting against lipid peroxidation. *Vit K* - **Vitamin K** is crucial for **blood coagulation** and **bone metabolism**, but it does not have significant antioxidant properties. - Its primary biological functions are unrelated to scavenging **free radicals** or preventing oxidative stress. *Vit C* - **Vitamin C** is a potent **water-soluble antioxidant** that works in aqueous environments, such as the cytoplasm and extracellular fluid. - While it can neutralize **reactive oxygen species** and regenerate other antioxidants like vitamin E, its solubility limits its direct activity in protecting lipid membranes, making vitamin E more potent in that specific context.
Question 127: Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
- A. NAD+ (Correct Answer)
- B. FAD
- C. FMN
- D. FADH2
Explanation: ***NAD+*** - Glutamate dehydrogenase catalyzes the oxidative deamination of **glutamate** to **α-ketoglutarate** and ammonia, and this reaction primarily uses **NAD+** as an electron acceptor. - In some organisms and contexts, it can also use **NADP+**, but **NAD+** is the more common and significant cofactor for its catabolic role. *FAD* - **FAD (flavin adenine dinucleotide)** is typically associated with **flavoproteins** and enzymes involved in oxidation-reduction reactions, such as those in the **electron transport chain** and the **Krebs cycle**. - Enzymes like **succinate dehydrogenase** use FAD, not glutamate dehydrogenase. *FMN* - **FMN (flavin mononucleotide)** is another flavin coenzyme, similar to FAD, and is found in various **flavoproteins** and enzymes of the **electron transport chain**, such as **NADH dehydrogenase (Complex I)**. - It does not serve as a primary cofactor for **glutamate dehydrogenase** activity. *FADH2* - **FADH2** is the reduced form of **FAD**, carrying high-energy electrons to the **electron transport chain** for ATP synthesis. - It's a product or reactant of various metabolic pathways, but not a direct cofactor for **glutamate dehydrogenase**.
Question 128: What is the primary biochemical defect in alkaptonuria?
- A. FeCl3 test is negative
- B. Urine turns black immediately upon voiding
- C. Benedict's test is diagnostic for alkaptonuria
- D. Deficiency of homogentisate 1,2-dioxygenase (Correct Answer)
Explanation: ***Deficiency of homogentisate 1,2-dioxygenase*** - **Alkaptonuria** is an autosomal recessive disorder caused by the deficiency of **homogentisate 1,2-dioxygenase**, an enzyme in the **tyrosine degradation pathway**. - This deficiency leads to the accumulation of **homogentisic acid** in the body, which is excreted in urine and deposited in connective tissues. *Urine turns black immediately upon voiding* - While urine in alkaptonuria does **turn black**, it typically darkens upon **standing** and exposure to air, not immediately upon voiding. - The darkening is due to the oxidation of accumulated **homogentisic acid**. *FeCl3 test is negative* - The **ferric chloride (FeCl3) test** typically yields a **positive result** (transient green color) in the presence of homogentisic acid in the urine. - Therefore, a negative result would argue against a diagnosis of alkaptonuria. *Benedict's test is diagnostic for alkaptonuria* - **Benedict's test** is used to detect reducing sugars like glucose in urine and would not be used to diagnose alkaptonuria. - A positive Benedict's test in alkaptonuria is due to the reducing properties of homogentisic acid, but it is not specific or diagnostic.
Question 129: Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?
- A. Glucose-6-phosphate (Correct Answer)
- B. Uridine diphosphoglucose
- C. Fructose-6-phosphate
- D. Galactose-1-phosphate
Explanation: ***Glucose-6-phosphate*** - In **glycogenolysis**, **glycogen phosphorylase** breaks down glycogen into **glucose-1-phosphate**, which is then converted into **glucose-6-phosphate** by **phosphoglucomutase**. - In **glycolysis**, **glucose-6-phosphate** is isomerized to **fructose-6-phosphate** by **phosphoglucose isomerase**, committing it to the glycolytic pathway. - In **gluconeogenesis**, **glucose-6-phosphate** is the final product formed from other precursors; it can then be dephosphorylated to free glucose by **glucose-6-phosphatase**. *Galactose-1-phosphate* - This is an intermediate specifically in **galactose metabolism**, not directly involved in the central common pathways of glycogenolysis, glycolysis, or gluconeogenesis. - It is converted to **glucose-1-phosphate** via the **Leloir pathway** (involving **galactose-1-phosphate uridylyltransferase**), which can then enter glycogen metabolism. *Uridine diphosphoglucose* - **UDP-glucose** is crucial for **glycogen synthesis** (**glycogenesis**), serving as the activated glucose donor. - It is not directly a metabolite in the catabolic process of glycogenolysis, nor is it a direct intermediate in glycolysis or gluconeogenesis. *Fructose-6-phosphate* - **Fructose-6-phosphate** is a key intermediate in **glycolysis** and **gluconeogenesis**, specifically downstream from **glucose-6-phosphate**. - However, it is not directly produced from glycogenolysis; **glucose-6-phosphate** is the direct link between glycogenolysis and glycolysis.
Question 130: Which one of the following statements concerning gluconeogenesis is correct?
- A. It occurs primarily in the liver.
- B. It is stimulated by elevated levels of acetyl CoA.
- C. It is important in maintaining blood glucose during the normal overnight fast. (Correct Answer)
- D. It is primarily inhibited by insulin.
Explanation: ***It is important in maintaining blood glucose during the normal overnight fast.*** - **This is the BEST answer** as it emphasizes the **primary physiological role** of gluconeogenesis in human metabolism. - During the **overnight fast** (8-12 hours), hepatic glycogen stores become depleted, making gluconeogenesis the **critical mechanism** to maintain blood glucose for glucose-dependent tissues like the **brain** (requires ~120g glucose/day) and **red blood cells**. - Without gluconeogenesis, blood glucose would drop dangerously during fasting, leading to hypoglycemia and neurological dysfunction. *It occurs primarily in the liver.* - This statement is **technically correct** - the liver accounts for approximately **90%** of total gluconeogenesis under normal conditions. - However, the **kidney cortex** also contributes significantly (10% normally, up to 40% during prolonged fasting), and the **intestine** plays a minor role. - While true, this is more of a **anatomical fact** rather than highlighting the critical physiological importance of the pathway, making it a less comprehensive answer than Option 1. *It is stimulated by elevated levels of acetyl CoA.* - This statement is **biochemically correct** - **Acetyl-CoA** is an important **allosteric activator** of **pyruvate carboxylase**, the first committed enzyme of gluconeogenesis. - However, this represents just **one regulatory mechanism** at the enzymatic level, not the overall physiological significance. - Primary regulation occurs through **hormones** (glucagon, cortisol, epinephrine) that coordinate the entire pathway, making this a narrower answer than Option 1. *It is primarily inhibited by insulin.* - This statement is also **correct** - **Insulin** is the primary hormonal **inhibitor** of gluconeogenesis. - Insulin suppresses gluconeogenesis by inhibiting key enzymes (PEPCK, glucose-6-phosphatase) and decreasing transcription of gluconeogenic genes. - However, this describes **inhibition** rather than the positive physiological role, making it less representative of gluconeogenesis's essential function than Option 1. **Note:** All four statements are technically correct, but Option 1 best captures the **essential physiological importance** of gluconeogenesis in human metabolism, which is why it is the preferred answer for this question.