NEET-PG 2013 Practice Questions

Q: Which enzyme catalyzes oxidative deamination?

Glutamate dehydrogenase. ***Glutamate dehydrogenase*** - This enzyme catalyzes the conversion of **glutamate** to **α-ketoglutarate** and ammonia (NH₃), which is an oxidative deamination reaction. - It utilizes **NAD⁺ or NADP⁺** as a coenzyme to remove hydrogen atoms during the oxidation process. - Plays a crucial role in both **amino acid catabolism** and anabolism. *Glutaminase* - This enzyme hydrolyzes **glutamine** to glutamate and ammonia, which is a **hydrolytic deamidation** reaction, not an oxidative deamination. - It does not involve the oxidation of the substrate or require NAD⁺/NADP⁺ as cofactors. *Glutamine synthase* - This enzyme synthesizes **glutamine** from glutamate and ammonia, using ATP, which is a **biosynthetic** reaction, not a catabolic deamination. - It is involved in **ammonia detoxification** and amino acid synthesis, functioning in the opposite direction of deamination. *None of the options* - This option is incorrect because **glutamate dehydrogenase** is a valid correct answer. - Glutamate dehydrogenase is the primary enzyme responsible for oxidative deamination in human metabolism.

Q: Which amino acids accumulate in maple syrup urine disease?

All branched-chain amino acids. ***All branched-chain amino acids*** - Maple syrup urine disease (MSUD) is characterized by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the breakdown of branched-chain amino acids (BCAAs). - This deficiency leads to the accumulation of **leucine, isoleucine, and valine**, along with their corresponding alpha-keto acids, in the blood and urine. - The distinctive **maple syrup odor** in the urine is caused by the accumulation of branched-chain keto acids derived from all three BCAAs. *Leucine* - While leucine is one of the BCAAs that accumulates in MSUD, it is not the *only* amino acid involved. - The accumulation of **leucine** is particularly associated with the severe neurological symptoms seen in MSUD, as it is the most neurotoxic of the three BCAAs. *Valine* - Valine is another BCAA that accumulates due to the metabolic block in MSUD. - However, the disease involves the accumulation of all three BCAAs, not just valine in isolation. *Isoleucine* - Isoleucine is the third BCAA that accumulates in MSUD due to the defective enzyme. - Like leucine and valine, isoleucine and its corresponding keto acid accumulate in blood and urine when the branched-chain alpha-keto acid dehydrogenase complex is deficient.

Q: Rate limiting step in pyrimidine synthesis?

Carbamoyl phosphate synthase-II. ***Carbamoyl phosphate synthetase II (CPS-II)*** - **CPS-II** is the **committed and rate-limiting enzyme** in **de novo pyrimidine synthesis** in **mammals (including humans)** - It catalyzes the formation of **carbamoyl phosphate** from glutamine, CO₂, and 2 ATP in the **cytoplasm** - This is the **first committed step** and the main **regulatory checkpoint**, inhibited by UTP (feedback inhibition) and activated by PRPP and ATP - CPS-II is part of the **CAD complex** (carbamoyl phosphate synthetase, aspartate transcarbamoylase, dihydroorotase) in mammals *Aspartate transcarbamoylase (ATCase)* - ATCase catalyzes the **second step**: condensation of carbamoyl phosphate with aspartate to form carbamoyl aspartate - While ATCase is the **rate-limiting step in bacteria** (E. coli), in **mammals** it is part of the CAD complex and **not the primary regulatory step** - This option is incorrect for human/mammalian biochemistry tested in NEET PG *Dihydro-orotase* - The **third enzyme** in the pathway, converting carbamoyl aspartate to dihydroorotate - Part of the CAD complex in mammals but **not the rate-limiting step** *Dihydroorotate dehydrogenase* - Catalyzes the **fourth step**: oxidation of dihydroorotate to orotate - Located on the **outer surface of the inner mitochondrial membrane** (only mitochondrial enzyme in the pathway) - Important enzyme but **not rate-limiting**

Q: Which vitamin is primarily associated with the antioxidant properties of glutathione?

Niacin. ***Niacin*** - **Niacin** (Vitamin B3) is the vitamin most directly associated with glutathione's antioxidant properties - Niacin is a precursor to **NAD+** and **NADP+**, which are converted to **NADPH** - **NADPH is the essential cofactor** for **glutathione reductase**, the primary enzyme that reduces oxidized glutathione (GSSG) back to its active reduced form (GSH) - This NADPH-dependent enzymatic pathway is the **main mechanism** for maintaining the body's glutathione antioxidant system - Without adequate niacin → NADPH, glutathione cannot be efficiently regenerated *Vitamin C* - **Vitamin C** can non-enzymatically reduce GSSG to GSH, providing a **secondary backup mechanism** - While vitamin C does support glutathione regeneration, this is an **indirect, non-enzymatic process** - It acts as an antioxidant itself but is not the primary vitamin associated with glutathione's antioxidant function *Vitamin E* - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from oxidative damage - Works synergistically with other antioxidants but has **no direct role** in glutathione synthesis or regeneration *Vitamin A* - **Vitamin A** (retinol) is crucial for vision, immune function, and cell differentiation - Has some antioxidant properties as a carotenoid derivative but **no direct involvement** in glutathione metabolism

Q: Hereditary orotic aciduria Type-I is due to deficiency of?

UMP synthase. ***UMP synthase*** - Hereditary orotic aciduria Type-I is caused by a deficiency of the **bifunctional enzyme UMP synthase** (also called UMP synthase complex). - UMP synthase catalyzes two sequential reactions in the *de novo* pyrimidine synthesis pathway: 1. **OPRT activity**: Converts orotate → orotidine 5'-monophosphate (OMP) 2. **ODC activity**: Converts OMP → uridine 5'-monophosphate (UMP) - This is the **most precise and complete answer** as it identifies the actual enzyme complex that is deficient. - **Clinical features**: Megaloblastic anemia, growth retardation, immunodeficiency; responds to oral uridine supplementation. *Orotate phosphoribosyl transferase* - This represents only **one of the two catalytic activities** of the UMP synthase enzyme (the first step). - While this activity is indeed deficient in Type-I orotic aciduria, naming only this activity is **incomplete** because the enzyme has two functions. - This would be a **partial answer** rather than the complete enzyme name. *Orotic acid decarboxylase* - This represents only **the second catalytic activity** of the UMP synthase enzyme (converts OMP to UMP). - Like OPRT, this activity is also deficient, but naming only this component is **incomplete**. - **Type II orotic aciduria** (extremely rare) involves isolated ODC deficiency without OPRT deficiency. *All of the options* - While technically both OPRT and ODC activities are affected in Type-I orotic aciduria, the **standard nomenclature** refers to the deficient enzyme as **"UMP synthase"** - the name of the complete bifunctional enzyme. - In medical terminology and examination context, we identify enzyme deficiencies by the **name of the enzyme complex**, not by listing all its individual catalytic activities. - Therefore, **"UMP synthase"** is the single most accurate and complete answer.

Q: Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?

DNA polymerase gamma. ***DNA polymerase gamma*** - **DNA polymerase gamma** is the sole DNA polymerase responsible for replicating and repairing the mitochondrial DNA in eukaryotic cells. - It consists of a large catalytic subunit and two smaller accessory subunits that provide **proofreading** and processivity functions. *DNA polymerase alpha* - **DNA polymerase alpha** is primarily involved in initiating DNA replication on both the leading and lagging strands of nuclear DNA. - It forms a complex with **primase** to synthesize short RNA primers followed by a short stretch of DNA. *DNA polymerase delta* - **DNA polymerase delta** is a key enzyme in nuclear DNA replication, primarily responsible for the **elongation of the lagging strand**. - It also plays a significant role in various DNA repair pathways, including **nucleotide excision repair**. *DNA polymerase beta* - **DNA polymerase beta** is mainly involved in **DNA repair processes**, specifically **base excision repair (BER)** in the nucleus. - It has a low processivity and lacks **proofreading activity**, making it unsuitable for bulk DNA replication.

Q: In eukaryotic cells, where does the majority of functional RNA activity occur?

Cytoplasm. ***Cytoplasm*** - The **cytoplasm** is the cellular compartment where the **majority of functional RNA activity** occurs, including **translation** (protein synthesis) involving mRNA, tRNA, and rRNA. - **Ribosomes** (the sites of translation) are located in the cytoplasm, either free-floating or bound to the endoplasmic reticulum. - Many types of **regulatory RNAs** such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) exert their functions in the cytoplasm by targeting mRNAs for degradation or translational repression. - **mRNA degradation** and **RNA interference pathways** primarily operate in the cytoplasm. - The question asks for the broader location rather than the specific molecular machinery, making cytoplasm the most comprehensive answer. *Nucleus* - While RNA is **transcribed** from DNA and **processed** (capping, polyadenylation, splicing) in the nucleus, these are preparatory steps. - The nucleus is primarily the site of **RNA synthesis**, not where most RNA performs its functional roles. - Only a small fraction of functional RNA activity (like rRNA processing in the nucleolus) occurs here compared to the cytoplasm. *Ribosome* - While **ribosomes are the specific sites of translation** and are composed of rRNA and proteins, they represent molecular machinery rather than a cellular location. - Ribosomes themselves are located **within the cytoplasm**, making cytoplasm the more inclusive answer for where RNA activity occurs. - The question asks "where" in terms of cellular compartment, not which molecular complex. *None of the options* - This is incorrect as the cytoplasm is indeed the primary site where the majority of functional RNA activities occur in eukaryotic cells.

Q: A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?

3. ***3*** - A **frameshift mutation** occurs when nucleotides are inserted or deleted in a number not divisible by three, altering the **reading frame** of the codons. - If insertions or deletions occur in multiples of **three**, the reading frame is restored after the mutation, largely preserving the downstream amino acid sequence. *1* - An insertion or deletion of a single nucleotide (1) definitively causes a **frameshift mutation**. - This alters all subsequent **codons**, leading to a completely different amino acid sequence downstream from the mutation. *2* - An insertion or deletion of two nucleotides (2) also results in a **frameshift mutation**. - This change shifts the **reading frame**, leading to the production of an altered protein or a premature stop codon. *None of the options* - This option is incorrect because a specific number, **three**, can allow for a frameshift mutation to not affect the complete amino acid sequence. - Multiples of three maintain the original **reading frame** (although potentially adding or removing a specific amino acid), whereas other numbers guarantee a frameshift.

Q: The main function of Vitamin C in the body is

Cofactor for hydroxylation reactions in collagen synthesis. ***Cofactor for hydroxylation reactions in collagen synthesis*** - Vitamin C (ascorbic acid) serves as an essential **cofactor** for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes. - These enzymes catalyze the **hydroxylation of proline and lysine** residues in collagen, forming **hydroxyproline** and **hydroxylysine**. - This hydroxylation is crucial for the **stability and cross-linking** of collagen triple helix structure. - Deficiency leads to **scurvy**, characterized by defective collagen synthesis, bleeding gums, poor wound healing, and bone abnormalities. - This is the **primary and main function** of Vitamin C in the human body. *Involvement as antioxidant* - While Vitamin C does act as a **water-soluble antioxidant**, protecting cells from oxidative damage by free radicals, this is a **secondary function**. - It can donate electrons to neutralize reactive oxygen species and regenerate other antioxidants like Vitamin E. - This protective role is important but not the main function compared to its role in collagen synthesis. *Regulation of lipid synthesis* - Vitamin C is **not directly involved** in the primary pathways of lipid synthesis or metabolism. - It may play a minor role in **carnitine synthesis** (needed for fatty acid oxidation), but this is not a major function. - Other nutrients like B vitamins play more significant roles in lipid metabolism regulation. *Inhibition of cell growth* - Vitamin C does **not inhibit normal cell growth**; it is essential for cell health, differentiation, and tissue repair. - While high doses may have some anti-proliferative effects in certain cancer cell lines in vitro, this is not a physiological function in the healthy body.

Q: Which of the following is not a characteristic of Fragile X syndrome?

Large nose. **Large nose** - **Large nose** is generally not considered a characteristic feature of **Fragile X syndrome**. - While individuals with Fragile X syndrome have distinct facial features, a prominent or large nose is not typically among them. *Large head* - **Macrocephaly** (large head circumference) is a recognized physical feature in many individuals with **Fragile X syndrome**. - This characteristic often becomes more apparent in infancy and childhood. *Large ear* - **Large, prominent ears** are a very common and classic physical characteristic observed in individuals with **Fragile X syndrome**. - This feature is often noted during developmental assessments. *Large testis* - **Macro-orchidism** (enlarged testes) is a hallmark physical characteristic of **Fragile X syndrome** in post-pubertal males. - This is a highly specific finding and a key diagnostic pointer for the syndrome in adolescent and adult males. *Long narrow face* - **Long, narrow face** with a prominent forehead and jaw is a typical facial feature of **Fragile X syndrome**. - This characteristic facial appearance is part of the recognizable phenotype of the syndrome.

Question 1

Which enzyme catalyzes oxidative deamination?

Accepted Answer

Glutamate dehydrogenase

Answer

Glutaminase

Answer

Glutamine synthase

Answer

None of the options

Question 2

Which amino acids accumulate in maple syrup urine disease?

Accepted Answer

All branched-chain amino acids

Answer

Valine

Answer

Leucine

Answer

Isoleucine

Question 3

Rate limiting step in pyrimidine synthesis?

Accepted Answer

Carbamoyl phosphate synthase-II

Answer

Aspartate transcarbamoylase (ATCase)

Answer

Dihydroorotate dehydrogenase

Answer

Dihydro-orotase

Question 4

Which vitamin is primarily associated with the antioxidant properties of glutathione?

Accepted Answer

Niacin

Answer

Vitamin E

Answer

Vitamin C

Answer

Vitamin A

Question 5

Hereditary orotic aciduria Type-I is due to deficiency of?

Accepted Answer

UMP synthase

Answer

Orotate phosphoribosyl transferase

Answer

Orotic acid decarboxylase

Answer

All of the options

Question 6

Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?

Accepted Answer

DNA polymerase gamma

Answer

DNA polymerase delta

Answer

DNA polymerase alpha

Answer

DNA polymerase beta

Question 7

In eukaryotic cells, where does the majority of functional RNA activity occur?

Accepted Answer

Cytoplasm

Answer

Nucleus

Answer

Ribosome

Answer

None of the options

Question 8

A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?

Accepted Answer

3

Answer

1

Answer

2

Answer

None of the options

Question 9

The main function of Vitamin C in the body is

Accepted Answer

Cofactor for hydroxylation reactions in collagen synthesis

Answer

Regulation of lipid synthesis

Answer

Involvement as antioxidant

Answer

Inhibition of cell growth

Question 10

Which of the following is not a characteristic of Fragile X syndrome?

Accepted Answer

Large nose

Answer

Large ear

Answer

Large testis

Answer

Large head

Answer

Long narrow face

NEET-PG 2013

Biochemistry

Psychiatry