NEET-PG 2013 - Biochemistry Practice Questions

Q: Sweaty feet odor in urine is seen in which condition?

Isovaleric acidemia. ***Isovaleric acidemia*** - This condition is characterized by a distinctive "sweaty feet" odor in body fluids, including urine, due to the accumulation of **isovaleric acid**. - It results from a deficiency in the enzyme **isovaleryl-CoA dehydrogenase**, which is crucial for leucine metabolism. *Phenylketonuria* - Patients with **phenylketonuria (PKU)** typically have a "mousy" or "musty" odor in their urine, not a sweaty feet smell. - This is due to the accumulation of **phenylalanine** and its metabolites. *Maple syrup urine disease* - This metabolic disorder is named for the characteristic sweet, maple syrup-like odor of the urine, which is distinctly different from a sweaty feet odor. - It is caused by a defect in the metabolism of **branched-chain amino acids (leucine, isoleucine, and valine)**. *Alkaptonuria* - This condition is known for urine that turns **dark brown or black** upon standing or when exposed to air, due to the oxidation of **homogentisic acid**. - It does not produce a sweaty feet odor.

Q: What does salvage purine synthesis refer to?

Synthesis of purine nucleotides from purine bases. ***Synthesis of purine nucleotides from purine bases*** - **Salvage pathways** recycle pre-existing purine or pyrimidine bases (from nucleic acid degradation) by re-attaching them to a **ribose phosphate** to form a new nucleotide. - This process is energy-efficient as it bypasses several steps of the de novo synthesis pathway, utilizing enzymes like **adenine phosphoribosyltransferase (APRT)** and **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. *Synthesis of purine nucleotides from ribose-5-phosphate.* - While **ribose-5-phosphate** is a precursor, the complete synthesis from this molecule is part of the **de novo pathway**, which starts with PRPP (phosphoribosyl pyrophosphate) formation from ribose-5-phosphate. - This option does not specify the direct reuse of a pre-formed purine base, which is the hallmark of salvage. *Synthesis of purine nucleotides from simple precursors (de novo synthesis).* - **De novo synthesis** is the creation of nucleotides from scratch using simple metabolic precursors like amino acids (glycine, aspartate, glutamine), CO2, and THF derivatives. - This contrasts with salvage pathways, which recycle existing bases. *Synthesis of purine nucleotides from degraded RNA.* - Degraded RNA breaks down into **nucleotides**, which can then be further broken down into **purine bases** and ribose phosphates. - The direct synthesis of purine nucleotides from *degraded RNA* involves recovering the individual bases or nucleosides, then converting them to nucleotides via salvage, not directly using the entire degraded RNA.

Q: Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?

Liver. ***Liver*** - The **liver** is capable of both *de novo* synthesis and the salvage pathway of purine nucleotides, but it primarily utilizes the **de novo pathway** due to its high metabolic capacity and central role in biosynthesis for the entire body. - While salvage pathways exist, the liver's robust *de novo* synthesis allows it to readily produce purines from simple precursors, making it less reliant on salvaging pre-formed bases. *Brain* - The **brain** relies heavily on the **salvage pathway** for purine nucleotide synthesis because it has a limited capacity for *de novo* purine synthesis. - This dependency makes the brain particularly vulnerable to deficiencies in salvage enzymes, such as in **Lesch-Nyhan syndrome** where HGPRT deficiency leads to severe neurological dysfunction. *RBC* - **Red blood cells (RBCs)** are anucleated and lack the machinery for *de novo* purine synthesis, making them entirely dependent on the **salvage pathway** to maintain their purine nucleotide pool. - They salvage pre-formed purine bases and nucleosides from the plasma to synthesize necessary adenine and guanine nucleotides. *Leukocytes* - **Leukocytes**, particularly lymphocytes, have a high turn-over rate and metabolic activity, and they primarily rely on the **salvage pathway** for purine nucleotide synthesis. - The **immune system's rapid proliferation** and response demand efficient nucleotide synthesis, and the salvage pathway offers a quick and energy-efficient way to achieve this.

Q: What is the end product of purine metabolism in humans?

Uric acid. ***Uric acid*** - **Uric acid** is the final breakdown product of **purine metabolism** in humans. - It is formed from the degradation of **adenosine** and **guanosine**, with xanthine oxidase playing a key role in its synthesis. *Allantoin* - **Allantoin** is the end product of **purine metabolism** in most mammals other than primates, as they possess the enzyme **uricase** to further break down uric acid. - Humans lack **uricase**, hence allantoin is not the end product in humans. *Carbon Dioxide* - **Carbon dioxide** is a major end product of **carbohydrate** and **fat metabolism** through cellular respiration. - It is not directly associated with the degradation pathway of purines. *None of the options* - This option is incorrect because **uric acid** is indeed the definitive end product of purine metabolism in humans.

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: 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 of the following molecular interactions are found in the structure of DNA?

All of the options. ***All of the options*** - All three types of molecular interactions listed are present in DNA structure, making this the correct answer. - **Hydrogen bonds** hold together the two strands of the DNA double helix, forming between complementary base pairs (A-T with 2 hydrogen bonds, G-C with 3 hydrogen bonds). - **Glycosidic bonds** (N-glycosidic bonds) link the nitrogenous bases to the C1' carbon of the deoxyribose sugar in each nucleotide. - **Covalent interactions** (phosphodiester bonds) form the strong, stable sugar-phosphate backbone by linking the 3' hydroxyl group of one sugar to the 5' phosphate group of the next. *Hydrogen bond* - This is a **true statement** - hydrogen bonds are essential structural components of DNA. - However, this option alone is **incomplete** as DNA structure also contains glycosidic bonds and covalent phosphodiester bonds. - If only hydrogen bonds were present, there would be no nucleotides or backbone structure. *Glycosidic bond* - This is a **true statement** - glycosidic bonds are present in every nucleotide of DNA. - However, this option alone is **incomplete** as DNA also requires hydrogen bonds for base pairing and phosphodiester bonds for the backbone. - Without other bonds, individual nucleotides could not form a functional double helix. *Covalent interactions* - This is a **true statement** - covalent phosphodiester bonds form the DNA backbone within each strand. - However, this option alone is **incomplete** as it doesn't account for glycosidic bonds (nucleotide formation) or hydrogen bonds (strand pairing). - While the strongest bonds in DNA, they alone cannot create the complete double helix structure.

Q: Which enzyme polymerises Okazaki fragments?

DNA polymerase III. ***DNA polymerase III*** - **DNA polymerase III** is the primary replicative enzyme in **prokaryotes (bacteria)** responsible for synthesizing new DNA strands, including the **polymerization of Okazaki fragments** on the lagging strand. - It possesses high processivity (can add ~500 nucleotides without dissociating), essential for rapid and efficient DNA synthesis during replication, adding nucleotides in a **5' to 3' direction**. - In **eukaryotes**, DNA polymerase δ (delta) performs the analogous function of polymerizing Okazaki fragments. *DNA polymerase I* - **DNA polymerase I** in prokaryotes primarily functions in **removing RNA primers** left by primase and **filling the resulting gaps** with DNA nucleotides. - It has 5' to 3' exonuclease activity for primer removal and polymerase activity for gap filling, but is **not the main enzyme for elongating Okazaki fragments**. - Its role is in **DNA repair and finishing replication**, not the extensive synthesis of Okazaki fragments. *DNA polymerase II* - **DNA polymerase II** in prokaryotes is primarily involved in **DNA repair mechanisms**, particularly in **restarting stalled replication forks** and responding to DNA damage. - It is not the main enzyme responsible for the polymerization of **Okazaki fragments** during normal DNA replication. *RNA polymerase* - **RNA polymerase** (specifically **primase**, a specialized RNA polymerase) synthesizes short **RNA primers** (8-12 nucleotides) during DNA replication, which provide the 3'-OH group necessary to initiate DNA synthesis. - It does not synthesize DNA or polymerize **Okazaki fragments**; its function is to create RNA primers, not extend DNA strands.

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: Which type of RNA contains codons for specific amino acids?

Messenger RNA (mRNA). ***Messenger RNA (mRNA)*** - **mRNA** carries the genetic information from **DNA** in the nucleus to the **ribosomes** in the cytoplasm. - This information is encoded in sequences of three nucleotides called **codons**, each specifying a particular amino acid. *Transfer RNA (tRNA)* - **tRNA** molecules are responsible for **carrying specific amino acids** to the ribosome during protein synthesis. - Each **tRNA** has an **anticodon** that base-pairs with a complementary **codon** on the **mRNA** strand. *Small nuclear RNA (snRNA)* - **snRNA** is primarily involved in **RNA splicing**, a process that removes introns from pre-mRNA. - It forms part of the **spliceosome** complex, which is crucial for mature mRNA formation but does not contain codons itself. *Ribosomal RNA (rRNA)* - **rRNA** is a major component of **ribosomes**, the cellular machinery responsible for protein synthesis. - While it plays a critical structural and catalytic role in translation, it does not carry genetic code in the form of codons.

Question 1

Sweaty feet odor in urine is seen in which condition?

Accepted Answer

Isovaleric acidemia

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Maple syrup urine disease

Question 2

What does salvage purine synthesis refer to?

Accepted Answer

Synthesis of purine nucleotides from purine bases

Answer

Synthesis of purine nucleotides from ribose-5-phosphate

Answer

Synthesis of purine nucleotides from simple precursors (de novo synthesis)

Answer

Synthesis of purine nucleotides from degraded RNA

Question 3

Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?

Accepted Answer

Liver

Answer

RBC

Answer

Leukocytes

Answer

Brain

Question 4

What is the end product of purine metabolism in humans?

Accepted Answer

Uric acid

Answer

Carbon Dioxide

Answer

Allantoin

Answer

None of the options

Question 5

Rate limiting step in pyrimidine synthesis?

Accepted Answer

Carbamoyl phosphate synthase-II

Answer

Aspartate transcarbamoylase (ATCase)

Answer

Dihydroorotate dehydrogenase

Answer

Dihydro-orotase

Question 6

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 7

Which of the following molecular interactions are found in the structure of DNA?

Accepted Answer

All of the options

Answer

Hydrogen bond

Answer

Glycosidic bond

Answer

Covalent interactions

Question 8

Which enzyme polymerises Okazaki fragments?

Accepted Answer

DNA polymerase III

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

RNA polymerase

Question 9

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 10

Which type of RNA contains codons for specific amino acids?

Accepted Answer

Messenger RNA (mRNA)

Answer

Transfer RNA (tRNA)

Answer

Small nuclear RNA (snRNA)

Answer

Ribosomal RNA (rRNA)

NEET-PG 2013 — Biochemistry