Nucleic Acid Biochemistry Practice Questions

Q: What is the role of adenine phosphoribosyl transferase (APRT) in purine metabolism?

Salvage pathway of purine nucleotide synthesis. ***Salvage pathway of purine nucleotide synthesis*** - **Adenine phosphoribosyl transferase (APRT)** catalyzes the reaction of **adenine** with **5-phosphoribosyl-1-pyrophosphate (PRPP)** to form **adenosine monophosphate (AMP)**. - This reaction is a crucial step in the **purine salvage pathway**, which reclaims pre-formed purine bases and converts them back into nucleotides, conserving energy. *Breakdown of purines* - The breakdown of purines (catabolism) typically involves enzymes like **adenosine deaminase** and **xanthine oxidase**, leading to the formation of **uric acid**. - APRT is involved in synthesizing nucleotides, not their degradation. *Not involved in purine metabolism* - APRT is an enzyme specifically involved in the **anabolic processes** of purine metabolism, as it contributes to the formation of purine nucleotides. - Its role is well-established within the **salvage pathway**. *De novo synthesis of purines* - The **de novo synthesis pathway** builds purine nucleotides from simpler precursors like **amino acids**, **CO2**, and **THF derivatives**. - While both pathways produce purine nucleotides, APRT is exclusively part of the **salvage pathway**, which recycles existing purine bases.

Q: What does Chargaff's rule state regarding the base pairing in DNA?

A=T, G=C. ***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.

Q: Uric acid is formed by?

Catabolism of purines. ***Catabolism of purines*** - **Uric acid** is the final breakdown product of **purine metabolism** in humans. - **Purines** (adenine and guanine) are components of nucleic acids (DNA and RNA) and are broken down through a series of enzymatic steps, ultimately forming uric acid. *Catabolism of proteins* - The catabolism of proteins leads to the formation of **amino acids**, which are then further broken down to produce **urea** (via the urea cycle) as the main nitrogenous waste product, not uric acid. - Protein breakdown primarily provides energy or building blocks for new proteins. *Catabolism of ketones* - The catabolism of ketones occurs when the body uses **fatty acids** for energy, often during fasting or uncontrolled diabetes. - Key products are **acetyl-CoA** and energy, not uric acid. *Catabolism of pyrimidines* - The catabolism of pyrimidines (cytosine, thymine, and uracil) produces compounds like **beta-alanine** and **ammonia**. - Unlike purines, pyrimidine breakdown does not result in uric acid formation.

Q: Which of the following is not a precursor in the synthesis of pyrimidines?

Thymidine. ***Thymidine*** - **Thymidine** is a *nucleoside* consisting of deoxyribose and thymine. It is a *product* and a component of DNA, not a precursor in the *de novo synthesis* of pyrimidine bases. - While it can be incorporated into DNA via the *salvage pathway*, it does not serve as an initial building block for the pyrimidine ring itself. *Glutamine* - **Glutamine** provides the **nitrogen atoms** crucial for the formation of the pyrimidine ring, specifically N3 in the pyrimidine base. - It is a key donor of *amino groups* in various anabolic pathways, including nucleotide synthesis. *Carbon dioxide (CO2)* - **Carbon dioxide (CO2)** contributes one of the carbon atoms (C2) to the pyrimidine ring. - It combines with **ammonia** (derived from glutamine) to form **carbamoyl phosphate**, an essential intermediate. *Aspartic acid* - **Aspartic acid** provides four atoms (N1, C4, C5, C6) of the pyrimidine ring. - Its carbon skeleton and amino group are directly incorporated into the pyrimidine structure during the *de novo synthesis* pathway.

Q: Which amino acids are involved in the formation of purines?

Aspartate, glycine, glutamine. ***Aspartate, glycine, glutamine*** - **Glycine** provides the largest single contribution to the purine ring structure: carbons at positions 4 and 5, and nitrogen at position 7 (the entire glycine molecule is incorporated). - **Aspartate** contributes the nitrogen atom at position 1 of the purine ring. - **Glutamine** donates two nitrogen atoms to the purine ring, specifically at positions 3 and 9. *Aspartate, glycine* - While both **aspartate** and **glycine** are crucial, **glutamine** is also required for the complete synthesis of the purine ring, contributing two nitrogen atoms. - Omission of glutamine makes this option incomplete for the full complement of amino acids involved. *Aspartate, glutamate* - **Glutamate** is not directly involved as an amino acid precursor contributing atoms to the purine ring structure. - **Glutamine**, an amide of glutamate, is the active donor of nitrogen atoms for purine synthesis. *Aspartic acid, glycine, uric acid* - **Aspartic acid** is another name for aspartate, and **glycine** is correctly identified as a precursor. - However, **uric acid** is the end product of purine catabolism, not an amino acid involved in its synthesis.

Q: Beta-alanine is derived from ?

Uracil. ***Correct Option: Uracil*** - **Uracil**, a pyrimidine base found in RNA, is the primary source of **β-alanine** through its catabolic pathway. - The degradation sequence: **Uracil** → **Dihydrouracil** → **β-Ureidopropionate** → **β-Alanine** + CO₂ + NH₃ - This pathway is catalyzed by enzymes including dihydropyrimidine dehydrogenase and β-ureidopropionase. - **β-Alanine** is also obtained from dietary sources and is a component of carnosine and pantothenic acid (Vitamin B5). *Incorrect Option: Thymine* - **Thymine** is a pyrimidine base in DNA with a similar catabolic pathway to uracil. - However, thymine produces **β-aminoisobutyrate** (NOT β-alanine) as its final product. - The pathway: **Thymine** → **Dihydrothymine** → **β-Ureidoisobutyrate** → **β-Aminoisobutyrate** - The extra methyl group on thymine (compared to uracil) results in a different end product. *Incorrect Option: Adenosine* - **Adenosine** is a purine nucleoside composed of adenine and ribose. - Purine catabolism leads to **uric acid** formation, not β-alanine. - It follows the pathway: Adenosine → Inosine → Hypoxanthine → Xanthine → Uric acid. *Incorrect Option: Guanosine* - **Guanosine** is a purine nucleoside consisting of guanine and ribose. - Like adenosine, it is catabolized to **uric acid** via xanthine. - It does not participate in β-alanine synthesis.

Q: What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?

3' TAC 5'. ***3' TAC 5'*** - The **start codon** for protein synthesis on **mRNA** is **5'-AUG-3'**, which codes for **methionine** (or N-formylmethionine in prokaryotes) and signals the initiation of translation. - To produce an mRNA codon of **5'-AUG-3'**, the complementary sequence on the **template DNA strand** must be **3'-TAC-5'** (adenine pairs with uracil/thymine, guanine pairs with cytosine, and the strands are antiparallel). - During transcription, RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes mRNA in the 5' to 3' direction. *3' TAG 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUC-3'**, which codes for **isoleucine**, not methionine. - Therefore, this sequence does not correspond to the first amino acid inserted into a protein. *3' TAA 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUU-3'**, which also codes for **isoleucine**, not methionine. - This is not the initiation codon sequence. *3' ATG 5'* - While **ATG** appears in this sequence, when presented as the **template strand** in the 3' to 5' orientation, it would be transcribed to produce mRNA **5'-UAC-3'**, which codes for **tyrosine**, not methionine. - The sequence **ATG** on the **coding strand** (non-template strand) corresponds to the start codon, but this option incorrectly presents it as the template strand sequence.

Q: Which molecule provides nitrogen-9 to the purine ring?

Glycine. ***Glycine*** - Glycine provides the **amino nitrogen at position 9** (N-9) of the purine ring. - It also contributes **carbon atoms at positions 4 and 5** (C-4, C-5) and the **nitrogen at position 7** (N-7). - Glycine is incorporated as an intact molecule early in purine synthesis, forming the **glycinamide ribonucleotide** intermediate. *Glutamine* - The **amide nitrogen** of glutamine donates nitrogen atoms at **positions 3 only** (N-3), not position 9. - This donation occurs during the formation of **formylglycinamide ribonucleotide** (FGAR) from FGAM. - Glutamine contributes two amide nitrogen atoms during purine synthesis, but N-9 is not one of them. *Aspartate* - Aspartate contributes the **nitrogen atom at position 1** (N-1) of the purine ring. - It is incorporated into the intermediate **5-aminoimidazole-4-(N-succinylcarboxamide) ribonucleotide** (SACAIR). - The nitrogen from aspartate remains after fumarate is released. *CO2* - Carbon dioxide (CO2) contributes the **carbon atom at position 6** (C-6) of the purine ring. - It provides a **carbon source** for ring formation, not nitrogen atoms. - CO2 is incorporated during the carboxylation step in the purine biosynthetic pathway.

Q: Which of the following is a function of ribozymes?

Peptidyl transferase activity. ***Peptidyl transferase activity*** - The **ribosome's large subunit**, which contains **ribosomal RNA (rRNA)**, catalyzes the formation of peptide bonds during protein synthesis. - This **rRNA enzyme**, known as a **ribozyme**, exhibits **peptidyl transferase activity**. *Cut DNA at specific site* - This function is primarily carried out by **restriction enzymes**, which are **proteins**, not ribozymes. - **Ribozymes** are **RNA molecules** with catalytic activity and do not typically cleave DNA. *Participate in DNA synthesis* - **DNA synthesis** is mediated by **DNA polymerases** and other **protein enzymes**, not ribozymes. - Ribozymes' primary roles involve **RNA processing** and **peptide bond formation**. *GTPase activity* - **GTPase activity** is characteristic of **G-proteins**, which are **protein enzymes** involved in signal transduction and cell regulation. - While some ribosomal activities are **GTP-dependent**, the **GTPase itself is a protein**, not the ribozyme component.

Q: Which condition is associated with defects in pre-mRNA splicing and SMN protein dysfunction?

Spinal muscular atrophy. ***Spinal muscular atrophy*** - **Spinal muscular atrophy (SMA)** is primarily caused by mutations in the **SMN1 gene**, leading to insufficient production of the **survival motor neuron (SMN) protein**. - Without adequate SMN protein, defects occur in the **pre-mRNA splicing** of motor neuron genes, leading to the degeneration of **alpha motor neurons** in the spinal cord. *Sickle cell disease* - **Sickle cell disease** is an inherited **hemoglobinopathy** caused by a point mutation in the beta-globin gene, leading to the production of abnormal **hemoglobin S**. - This condition does not involve defects in pre-mRNA splicing or SMN protein dysfunction, but rather the **polymerization of hemoglobin S** under low oxygen conditions. *Huntington's disease* - **Huntington's disease** (formerly called Huntington chorea) is a neurodegenerative disorder caused by an **expanded CAG trinucleotide repeat** in the huntingtin gene. - Huntington's disease involves protein misfolding and aggregation, but not primary defects in pre-mRNA splicing or SMN protein dysfunction. *α-Thalassemia* - **α-Thalassemia** is a group of inherited blood disorders characterized by reduced or absent production of **alpha-globin chains**, typically due to **gene deletions** on chromosome 16. - This condition affects the assembly of hemoglobin and does not involve pre-mRNA splicing defects or SMN protein dysfunction.

Question 1

What is the role of adenine phosphoribosyl transferase (APRT) in purine metabolism?

Accepted Answer

Salvage pathway of purine nucleotide synthesis

Answer

Breakdown of purines

Answer

Not involved in purine metabolism

Answer

De novo synthesis of purines

Question 2

What does Chargaff's rule state regarding the base pairing in DNA?

Accepted Answer

A=T, G=C

Answer

A=G, T=C

Answer

A=C, G=T

Answer

Any combination possible

Question 3

Uric acid is formed by?

Accepted Answer

Catabolism of purines

Answer

Catabolism of proteins

Answer

Catabolism of ketones

Answer

Catabolism of pyrimidines

Question 4

Which of the following is not a precursor in the synthesis of pyrimidines?

Accepted Answer

Thymidine

Answer

Glutamine

Answer

Carbon dioxide (CO2)

Answer

Aspartic acid

Question 5

Which amino acids are involved in the formation of purines?

Accepted Answer

Aspartate, glycine, glutamine

Answer

Aspartate, glycine

Answer

Aspartate, glutamate

Answer

Aspartic acid, glycine, uric acid

Question 6

Beta-alanine is derived from ?

Accepted Answer

Uracil

Answer

Adenosine

Answer

Guanosine

Answer

Thymine

Question 7

What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?

Accepted Answer

3' TAC 5'

Answer

3' TAG 5'

Answer

3' TAA 5'

Answer

3' ATG 5'

Question 8

Which molecule provides nitrogen-9 to the purine ring?

Accepted Answer

Glycine

Answer

Glutamine

Answer

CO2

Answer

Aspartate

Question 9

Which of the following is a function of ribozymes?

Accepted Answer

Peptidyl transferase activity

Answer

Cut DNA at specific site

Answer

GTPase activity

Answer

Participate in DNA synthesis

Question 10

Which condition is associated with defects in pre-mRNA splicing and SMN protein dysfunction?

Accepted Answer

Spinal muscular atrophy

Answer

Sickle cell disease

Answer

Huntington's disease

Answer

α-Thalassemia

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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