Which enzyme is deficient in Isovaleric acidemia?
Why is the citric acid cycle called an amphibolic pathway?
Lipogenesis is stimulated by?
In which condition is the utilization of pyruvate in tissues decreased?
Which of the following is not a component of a nucleotide?
The Central Dogma of molecular biology includes all except?
At which step in glycolysis is NADH produced during the oxidation of glyceraldehyde-3-phosphate?
Which enzyme is required for cutting the DNA strand during synthesis?
DNA microarrays allow detection of gene mutations through which process?
Protein glycosylation occurs in:
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 111: Which enzyme is deficient in Isovaleric acidemia?
- A. Isovaleryl CoA dehydrogenase (Correct Answer)
- B. Phenylalanine hydroxylase
- C. Arginase
- D. Methylmalonyl CoA mutase
Explanation: ***Isovaleryl CoA dehydrogenase*** - **Isovaleric acidemia** is an **autosomal recessive** metabolic disorder caused by a deficiency in the enzyme **isovaleryl-CoA dehydrogenase** - This enzyme is crucial for the metabolism of **leucine**, a branched-chain amino acid, leading to the accumulation of toxic byproducts like **isovaleryl-CoA** and **isovaleric acid** - Characteristic **sweaty feet odor** due to isovaleric acid accumulation *Phenylalanine hydroxylase* - A deficiency in **phenylalanine hydroxylase** is responsible for **phenylketonuria (PKU)**, a different metabolic disorder involving the metabolism of **phenylalanine** - This enzyme converts **phenylalanine to tyrosine**, and its deficiency leads to the accumulation of phenylalanine and its metabolites, causing neurological damage if untreated *Arginase* - A deficiency in **arginase** causes **argininemia (hyperargininemia)**, which is a disorder of the **urea cycle** - This enzyme converts **arginine into urea and ornithine**, and its deficiency leads to the buildup of arginine and ammonia in the blood, causing neurological symptoms and developmental delay *Methylmalonyl CoA mutase* - A deficiency in **methylmalonyl CoA mutase** causes **methylmalonic acidemia**, another organic acidemia distinct from isovaleric acidemia - This disorder involves **propionate metabolism** and can present with metabolic acidosis, but affects a different metabolic pathway than leucine catabolism
Question 112: Why is the citric acid cycle called an amphibolic pathway?
- A. Both exergonic and endergonic reactions take place
- B. Metabolites are utilized in other pathways. (Correct Answer)
- C. It can proceed in both forward and backward directions.
- D. The same enzymes can be used in reverse directions.
Explanation: ***Metabolites are utilized in other pathways.*** - The citric acid cycle is termed **amphibolic** because it serves both catabolic (breakdown) and anabolic (synthetic) functions. - Its intermediates are constantly drawn off for biosynthesis of molecules like **amino acids**, **heme**, and **glucose**, meaning it's not solely degradative. *Both exergonic and endergonic reactions take place* - While both types of reactions do occur in many metabolic pathways, this is a general characteristic of metabolism and not specific to the definition of an **amphibolic pathway**. - The amphibolic nature specifically refers to the dual role in both **catabolism** and **anabolism**. *It can proceed in both forward and backward directions.* - This statement typically describes a **reversible pathway** or individual reversible reactions, not necessarily an amphibolic pathway. - The citric acid cycle is primarily an oxidative cycle that proceeds in a forward, cyclic direction under aerobic conditions. *The same enzymes can be used in reverse directions.* - While some individual enzymes within metabolic pathways can catalyze reversible reactions, this is not the defining characteristic of an **amphibolic pathway**. - The amphibolic designation refers to the overall pathway's contribution to both breakdown and synthesis of molecules.
Question 113: Lipogenesis is stimulated by?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. Epinephrine
- D. Corticosteroids
Explanation: ***Insulin*** - **Insulin** is a key anabolic hormone that promotes the synthesis and storage of fat (lipogenesis) by increasing the uptake of glucose into adipose tissue and stimulating enzymes involved in fatty acid synthesis. - It enhances the conversion of excess carbohydrates into **triglycerides** for storage. *Glucagon* - **Glucagon** is a catabolic hormone that primarily promotes the breakdown of glycogen (glycogenolysis) and fat (lipolysis) to release glucose and fatty acids into the bloodstream, especially during fasting. - It generally **inhibits** lipogenesis and stimulates **gluconeogenesis**. *Epinephrine* - **Epinephrine** (adrenaline) is a stress hormone that promotes the breakdown of fat (lipolysis) to provide energy during acute stress or exercise. - It would **inhibit** lipogenesis, as its primary role is to mobilize energy stores. *Corticosteroids* - While **corticosteroids** can influence fat metabolism, their effect on lipogenesis is complex and often indirect. High levels can lead to fat redistribution (e.g., central obesity) rather than direct stimulation of overall lipogenesis. - Corticosteroids generally promote **lipolysis** in the extremities and can contribute to insulin resistance, which would hinder lipogenesis in some tissues.
Question 114: In which condition is the utilization of pyruvate in tissues decreased?
- A. Pernicious anemia
- B. Scurvy
- C. Beriberi (Correct Answer)
- D. Pellagra
Explanation: ***Beriberi*** - Beriberi is caused by **thiamine (vitamin B1) deficiency**, which is a crucial cofactor for the **pyruvate dehydrogenase complex (PDH)**. - A dysfunctional PDH enzyme leads to a decreased conversion of **pyruvate to acetyl-CoA**, thus **decreasing pyruvate utilization** and causing its accumulation. *Pernicious anemia* - This condition is caused by a deficiency in **vitamin B12 (cobalamin)**, typically due to a lack of intrinsic factor, leading to **megaloblastic anemia**. - While vitamin B12 is essential for various metabolic pathways, it does not directly impair the utilization of **pyruvate** by PDH. *Scurvy* - Scurvy results from **vitamin C (ascorbic acid) deficiency**, which is essential for collagen synthesis and acts as an antioxidant. - Vitamin C deficiency does not directly impact the activity of the **pyruvate dehydrogenase complex** or the utilization of pyruvate. *Pellagra* - Pellagra is caused by a deficiency in **niacin (vitamin B3)**, or its precursor, tryptophan. - Niacin is a component of **NAD+ and NADP+**, which are crucial coenzymes in many metabolic reactions, but its deficiency does not primarily manifest as decreased **pyruvate utilization**.
Question 115: Which of the following is not a component of a nucleotide?
- A. Sugar
- B. Fatty acid (Correct Answer)
- C. Base
- D. Phosphate
Explanation: ***Fatty acid*** - A **fatty acid** is a component of **lipids**, such as triglycerides and phospholipids, which are structurally and functionally distinct from **nucleotides**. - **Nucleotides** are the building blocks of nucleic acids (DNA and RNA), whereas fatty acids are essential for cell membranes and energy storage. *Sugar* - A **pentose sugar** (either **deoxyribose** in DNA or **ribose** in RNA) is a fundamental component of every nucleotide. - This sugar forms the backbone of the nucleic acid strand, covalently linked to the phosphate group and the nitrogenous base. *Phosphate* - A **phosphate group** is a crucial component of a nucleotide, providing the negative charge and forming the phosphodiester bonds that link nucleotides together into a nucleic acid chain. - The number of phosphate groups (mono-, di-, or triphosphate) determines the nucleotide's energy state and function. *Base* - A **nitrogenous base** (adenine, guanine, cytosine, thymine, or uracil) is an essential component of a nucleotide, responsible for genetic information storage and pairing. - This base is attached to the pentose sugar and determines the specific identity of the nucleotide within the DNA or RNA sequence.
Question 116: The Central Dogma of molecular biology includes all except?
- A. Reverse transcription (Correct Answer)
- B. Transcription
- C. RNA replication
- D. Translation
Explanation: ***Reverse transcription*** - The **original Central Dogma** as proposed by Francis Crick described the unidirectional flow of genetic information: **DNA → RNA → Protein**. - **Reverse transcription** (RNA → DNA) was **not part of the original Central Dogma** and was only discovered later with the identification of **reverse transcriptase enzyme** in retroviruses by Baltimore and Temin (1970). - While reverse transcription is now recognized as an important biological process, it represents an **exception to the classical Central Dogma** rather than a core component. *Transcription* - **Transcription** (DNA → RNA) is a **fundamental process** within the Central Dogma. - It involves synthesis of RNA from a DNA template and is the first step in gene expression in all living organisms. *Translation* - **Translation** (RNA → Protein) is the **second major step** of the Central Dogma. - This process occurs on ribosomes where mRNA is decoded into a specific sequence of amino acids to form proteins. *RNA replication* - **RNA replication** (RNA → RNA), while primarily seen in RNA viruses, is considered part of the **expanded Central Dogma**. - It represents one of the possible flows of genetic information in biological systems, though not universal to all organisms.
Question 117: At which step in glycolysis is NADH produced during the oxidation of glyceraldehyde-3-phosphate?
- A. Pyruvate kinase
- B. Enolase
- C. PFK-1
- D. Glyceraldehyde-3-phosphate dehydrogenase (Correct Answer)
Explanation: ***Glyceraldehyde-3-phosphate dehydrogenase*** - This enzyme catalyzes the oxidation and **phosphorylation** of glyceraldehyde-3-phosphate, producing **1,3-bisphosphoglycerate**. - During this reaction, **NAD+ is reduced to NADH**, which is a crucial step for energy production. *Pyruvate kinase* - This enzyme catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate** to ADP, forming ATP and pyruvate. - This step involves **substrate-level phosphorylation** for ATP production, not NADH. *Enolase* - This enzyme catalyzes the dehydration of **2-phosphoglycerate** to form **phosphoenolpyruvate (PEP)**. - This reaction involves the removal of a water molecule and does not produce NADH. *PFK-1* - **Phosphofructokinase-1 (PFK-1)** catalyzes the phosphorylation of fructose-6-phosphate to **fructose-1,6-bisphosphate**. - This is an ATP-consuming and a crucial regulatory step in glycolysis, but it does not involve NADH production.
Question 118: Which enzyme is required for cutting the DNA strand during synthesis?
- A. DNA polymerase
- B. DNA ligase
- C. Topoisomerase (Correct Answer)
- D. Helicase
Explanation: ***Topoisomerase*** - **Topoisomerases** are enzymes essential for DNA replication; they induce temporary **single- or double-strand breaks** in DNA to relieve **supercoiling** ahead of the replication fork. - This cutting and rejoining activity prevents the DNA from becoming excessively tangled and facilitates the unwinding process required for synthesis. *DNA polymerase* - **DNA polymerase** is responsible for **synthesizing new DNA strands** by adding nucleotides, not for cutting the DNA backbone. - It works by moving along the template strand, reading the bases, and then adding complementary nucleotides to the growing DNA strand. *DNA ligase* - **DNA ligase** functions to **join DNA fragments** together by forming phosphodiester bonds, especially in sealing Okazaki fragments during lagging strand synthesis. - Its role is to ligate (join) cut strands, not to initiate cuts in the DNA. *Helicase* - **Helicase** unwinds the DNA double helix into single strands using ATP hydrolysis; it **separates the two strands** but does not cut the phosphodiester backbone. - This enzyme creates the replication fork by disrupting hydrogen bonds between base pairs, making the DNA accessible for replication machinery.
Question 119: DNA microarrays allow detection of gene mutations through which process?
- A. Polymerase Chain Reaction
- B. Cloning
- C. Southern Blotting
- D. Hybridization (Correct Answer)
Explanation: ***Hybridization*** - DNA microarrays detect mutations by **hybridizing labeled patient DNA** to **thousands of oligonucleotide probes** containing known DNA sequences immobilized on a solid surface. - A mismatch between the patient's DNA and the probe results in **reduced or absent hybridization**, indicating a **mutation or genetic variation**. - This principle allows **high-throughput screening** of multiple genes simultaneously. *Polymerase Chain Reaction* - PCR is used to **amplify specific DNA sequences exponentially**, creating millions of copies from minimal starting material. - While PCR may be used to **prepare DNA samples** before microarray analysis, it is **not the detection mechanism** on the chip itself. *Cloning* - Cloning involves creating **identical copies of DNA fragments, cells, or organisms** using vectors and host cells. - It's a method for **producing large quantities** of specific DNA sequences but **not a detection technique** for mutations on microarrays. *Southern Blotting* - Southern blotting detects specific DNA sequences through **gel electrophoresis, membrane transfer, and probe hybridization**. - While it also uses hybridization, it is a **low-throughput technique** analyzing one sample at a time, unlike the **high-throughput parallel analysis** of DNA microarrays.
Question 120: Protein glycosylation occurs in:
- A. Peroxisomes
- B. Endoplasmic reticulum (ER) (Correct Answer)
- C. Mitochondria
- D. Golgi bodies
Explanation: ***ER*** - **N-linked glycosylation**, the most common type of protein glycosylation, initiates in the **endoplasmic reticulum (ER)**, where an oligosaccharide precursor is transferred to asparagine residues of newly synthesized proteins. - The ER environment facilitates protein folding and quality control, ensuring correctly folded glycoproteins are transported further. *Golgi bodies* - While **further modification and processing** of glycosylated proteins occur in the Golgi apparatus, the initial step of N-linked glycosylation begins in the ER. - The Golgi is responsible for trimming and adding different sugar residues to complete the **glycan chains** and for sorting the glycoproteins to their final destinations. *Mitochondria* - Mitochondria are primarily involved in **cellular respiration** and **ATP production**. - They do not play a significant role in protein glycosylation; most mitochondrial proteins are imported from the cytoplasm in an unglycosylated state. *Peroxisomes* - Peroxisomes are involved in various **metabolic processes**, including fatty acid oxidation and detoxification. - They are not known to be sites of protein glycosylation.