Nucleic Acid Biochemistry Practice Questions

Q: C4, C5, and N7 in the purine ring are derived from which of the following?

Glycine. ***Glycine*** - The entire **glycine molecule** contributes C4, C5, and N7 to the purine ring structure. - This amino acid provides a significant portion of the backbone to the imidazole ring within the purine. *Aspartate* - **Aspartate** contributes N1 to the purine ring. - It does not involve C4, C5, or N7, which are distinct atoms within the purine molecule. *CO₂* - **CO₂** contributes C6 to the purine ring through a carboxylation step. - It is not involved in providing the atoms at positions C4, C5, or N7. *Glutamine* - The nitrogen atoms N3 and N9 in the purine ring are derived from the **amide nitrogen of glutamine**. - Glutamine's contributions are different from the carbons and nitrogen provided by glycine.

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: 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 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 type of RNA is most commonly associated with pseudouridine?

transfer RNA (tRNA). ***Transfer RNA (tRNA)*** - **Pseudouridine (ψ)** is one of the most abundant modified nucleosides in RNA, and **tRNA contains the highest proportion** of pseudouridine modifications among all RNA types. - **tRNA molecules can contain up to 10-15% modified bases**, with pseudouridine being particularly abundant in the **TψC arm** (thymine-pseudouridine-cytosine loop). - These modifications are critical for **tRNA stability, proper folding, and accurate codon-anticodon recognition** during translation. - Pseudouridine enhances base stacking and stabilizes RNA structure through additional hydrogen bonding capability. *Ribosomal RNA (rRNA)* - While rRNA does contain pseudouridine modifications, they are present in **lower proportions compared to tRNA**. - rRNA pseudouridine modifications do play important roles in **ribosomal assembly and function**, but tRNA remains the RNA type most commonly associated with this modification. *Messenger RNA (mRNA)* - **mRNA is generally much less modified** than tRNA or rRNA. - Pseudouridine modifications in mRNA are relatively rare in prokaryotes and were only recently discovered to be more common in eukaryotic mRNA. - When present, they may affect **mRNA stability and translation efficiency**. *DNA* - **DNA does not contain pseudouridine** as this is an RNA-specific modification. - Pseudouridine is formed by **post-transcriptional isomerization** of uridine residues in RNA.

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: The anticodon region is an important part of which type of RNA?

t-RNA. **t-RNA** - The **anticodon region** is a critical component of **transfer RNA (tRNA)**, responsible for recognizing and binding to the complementary codon on mRNA during protein synthesis. - This interaction ensures that the correct **amino acid** is delivered to the growing polypeptide chain according to the genetic code. *r-RNA* - **Ribosomal RNA (rRNA)** is a structural and enzymatic component of **ribosomes**, which are the cellular machinery for protein synthesis. - While rRNA plays a crucial role in forming **peptide bonds** and facilitating translation, it does not possess an anticodon region. *m-RNA* - **Messenger RNA (mRNA)** carries the **genetic code** from DNA to the ribosomes in the form of codons, which specify the sequence of amino acids for protein synthesis. - mRNA molecules have codons, but they do not have an **anticodon region**; instead, they are read by the anticodons of tRNA. *hn-RNA* - **Heterogeneous nuclear RNA (hnRNA)** is a precursor to mRNA in eukaryotic cells, containing both exons and introns. - It undergoes extensive processing, including **splicing**, to become mature mRNA, but it does not have an **anticodon region**.

Q: The Watson-Crick model is for which type of DNA?

B DNA. ***B DNA*** - The **Watson-Crick model** describes the most common and stable form of DNA found in living organisms under physiological conditions. - **B-DNA** is a right-handed double helix, characterized by a **major and minor groove**, with approximately 10-10.5 base pairs per turn. *A DNA* - **A-DNA** is a **right-handed double helix** that forms under dehydrating conditions and is shorter and wider than B-DNA. - It has a more tilted base pair arrangement and is not the primary form described by the Watson-Crick model. *C DNA* - **C-DNA** is a less common **right-handed double helix** that forms under even lower hydration conditions and in the presence of certain ions. - It has fewer base pairs per turn than B-DNA, typically around 9.3 base pairs. *Z DNA* - **Z-DNA** is unique because it is a **left-handed double helix**, unlike the right-handed forms of A, B, and C DNA. - It is transiently formed in regions with a high concentration of **GC base pairs** and has a zigzag backbone, hence its name.

Q: Which of the following is not a component of a nucleotide?

Fatty acid. ***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 1

C4, C5, and N7 in the purine ring are derived from which of the following?

Accepted Answer

Glycine

Answer

CO₂

Answer

Aspartate

Answer

Glutamine

Question 2

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 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 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 5

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 6

Which type of RNA is most commonly associated with pseudouridine?

Accepted Answer

transfer RNA (tRNA)

Answer

messenger RNA (mRNA)

Answer

ribosomal RNA (rRNA)

Answer

DNA

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

The anticodon region is an important part of which type of RNA?

Accepted Answer

t-RNA

Answer

r-RNA

Answer

m-RNA

Answer

hn-RNA

Question 9

The Watson-Crick model is for which type of DNA?

Accepted Answer

B DNA

Answer

A DNA

Answer

C DNA

Answer

Z DNA

Question 10

Which of the following is not a component of a nucleotide?

Accepted Answer

Fatty acid

Answer

Sugar

Answer

Base

Answer

Phosphate

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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