Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 241: Which molecule provides nitrogen-9 to the purine ring?

A. Glycine (Correct Answer)
B. Glutamine
C. CO2
D. Aspartate

Explanation: ***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.

Question 242: Which of the following is a function of ribozymes?

A. Peptidyl transferase activity (Correct Answer)
B. Cut DNA at specific site
C. GTPase activity
D. Participate in DNA synthesis

Explanation: ***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.

Question 243: Which condition is associated with defects in pre-mRNA splicing and SMN protein dysfunction?

A. Sickle cell disease
B. Huntington's disease
C. Spinal muscular atrophy (Correct Answer)
D. α-Thalassemia

Explanation: ***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 244: During DNA replication, which bond breaks?

A. Phosphodiester bonds
B. Phosphate bond
C. Hydrogen bonds (Correct Answer)
D. Glycosidic bonds

Explanation: ***Hydrogen bond*** - During DNA replication, the **double helix unwinds** as the two strands separate. - This separation occurs through the breaking of **hydrogen bonds** that link complementary base pairs (adenine with thymine, guanine with cytosine) between the two strands. *Phosphodiester bonds* - **Phosphodiester bonds** form the sugar-phosphate backbone of a single DNA strand. - These bonds are generally stable during replication and are not broken to separate the strands; rather, they hold the individual nucleotides together within each strand. *Phosphate bond* - This term is often used generally, but specifically in DNA, it refers to the **phosphodiester bonds** within the backbone or the high-energy bonds in ATP. - The direct breaking of "phosphate bonds" in this context is not the primary mechanism for separating the DNA strands. *Glycosidic bonds* - **Glycosidic bonds** link the nitrogenous base to the deoxyribose sugar within a nucleotide. - These bonds remain intact during replication, as they are crucial for maintaining the structure of individual nucleotides.

Question 245: The helical structure model of two polynucleotide chains of DNA was proposed by whom?

A. Watson and Crick (Correct Answer)
B. Craig Venter
C. Linus Pauling
D. Michael Bishop and Harold Varmus

Explanation: ***Watson and Crick*** - **James Watson** and **Francis Crick** are credited with proposing the **double helix structure** of DNA in 1953, based on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins. - Their model elucidated the complementary base pairing and antiparallel nature of the two polynucleotide strands. *Linus Pauling* - **Linus Pauling** proposed a **triple-helical structure for DNA** in 1953, which was later found to be incorrect. - He was a prominent American chemist who made significant contributions to the fields of quantum chemistry and molecular biology, but his DNA model was not accurate. *Craig Venter* - **J. Craig Venter** is known for his work on the **Human Genome Project** and for being a pioneer in synthetic genomics. - He later founded Celera Genomics to compete with the publicly funded Human Genome Project in sequencing the human genome. *Michael Bishop and Harold Varmus* - **Michael Bishop** and **Harold Varmus** were awarded the Nobel Prize in Medicine in 1989 for their discovery that **oncogenes can arise from normal cellular genes (proto-oncogenes)**. - Their work focused on the genetic basis of cancer, specifically the role of retroviruses in transforming host cells.

Question 246: Sumoylation of histone proteins is associated with

A. Activation of gene transcription
B. Condensation of chromosome
C. Transcription repression (Correct Answer)
D. DNA replication

Explanation: ***Transcription repression*** - **Sumoylation** is a post-translational modification involving the covalent attachment of **Small Ubiquitin-like Modifier (SUMO) proteins** to target proteins, which leads to transcriptional repression. - When histones are sumoylated, it alters chromatin structure and recruits **transcriptional corepressors**, making the DNA less accessible for transcription factors. - This is the **primary and well-established function** of histone sumoylation in gene regulation. *Activation of gene transcription* - **Histone acetylation** and specific methylation patterns (e.g., H3K4me3, H3K36me3) are associated with **transcriptional activation**, not sumoylation. - Sumoylation typically creates a repressive chromatin environment, hindering gene expression. *Condensation of chromosome* - While sumoylation can influence chromatin structure, **chromosome condensation** during cell division is primarily regulated by **condensins** and **cohesins**. - Sumoylation's role in condensation is indirect and not its primary function. *DNA replication* - DNA replication is a separate process from transcriptional regulation and involves DNA polymerases and replication machinery. - Histone sumoylation specifically affects **gene transcription**, not DNA replication.

Question 247: Which of the following statements about RNA capping is INCORRECT?

A. RNA capping occurs in the cytoplasm after transcription is complete. (Correct Answer)
B. RNA capping occurs in the nucleus and is co-transcriptional.
C. RNA capping involves the addition of a cap structure at the 5' end of the RNA molecule.
D. S-adenosyl methionine (SAM) acts as a methyl donor

Explanation: ***RNA capping occurs in the cytoplasm after transcription is complete.*** - **RNA capping** occurs in the **nucleus**, not the cytoplasm, making this statement incorrect. - The process is **co-transcriptional**, occurring during transcription rather than after it is complete. *RNA capping occurs in the nucleus and is co-transcriptional.* - This is a **correct statement** - RNA capping takes place in the **nucleus** of eukaryotic cells. - It occurs **co-transcriptionally**, beginning when the nascent RNA chain is about **20-40 nucleotides** long. *RNA capping involves the addition of a cap structure at the 5' end of the RNA molecule.* - This is a **correct statement** - the **7-methylguanosine cap** is indeed added to the **5' end** of mRNA. - The cap structure is linked via a **5' to 5' triphosphate bridge** to protect against degradation. *S-adenosyl methionine (SAM) acts as a methyl donor* - This is a **correct statement** - **SAM** serves as the **methyl donor** for cap methylation reactions. - **SAM** provides methyl groups for the **N7-methylguanosine** and **2'-O-methylation** of the first transcribed nucleotide.

Question 248: The poly(A) tail of eukaryotic mRNA primarily functions to:

A. Enhancing mRNA stability (Correct Answer)
B. Facilitating ribosome binding
C. All of the above
D. Promoting nuclear export

Explanation: ***Enhancing mRNA stability*** - The **primary function** of the poly(A) tail is to **protect mRNA from degradation** by 3' to 5' exonucleases - **Poly(A)-binding proteins (PABPs)** bind to the tail and shield it from enzymatic degradation - The **length of the poly(A) tail** directly correlates with mRNA half-life - shorter tails lead to faster degradation - This is the most fundamental and universally recognized function of the poly(A) tail in all eukaryotic cells - According to standard biochemistry references (Harper's, Lehninger), mRNA stability is the primary role *Facilitating ribosome binding* - The poly(A) tail does contribute to translation efficiency through the **"closed-loop" model** - PABPs interact with translation initiation factors (eIF4G) to circularize mRNA - However, this is considered a **secondary function** that depends on the tail's presence for stability - This role is indirect and contingent on the mRNA being stable enough to be translated *Promoting nuclear export* - The poly(A) tail is required for **mRNA nuclear export** as PABPs interact with export machinery - This is an important but **tertiary function** - a prerequisite for cytoplasmic localization - Once in the cytoplasm, the tail's primary ongoing role is maintaining stability *All of the above* - While the poly(A) tail does contribute to all these functions, the question asks for its **PRIMARY function** - In molecular biology, the primary function is defined as the most direct, fundamental, and universally critical role - **Enhancing mRNA stability** is the primary function, while others are secondary or supportive roles

Question 249: The Watson-Crick double helix model of DNA is

A. Right-handed anti-parallel (Correct Answer)
B. Left-handed anti-parallel
C. Left-handed parallel structure
D. Right-handed parallel structure

Explanation: ***Right-handed anti-parallel*** - The **Watson-Crick model** describes DNA as a double helix that twists in a **right-handed direction**. - The two strands are **anti-parallel**, meaning they run in opposite 5' to 3' directions relative to each other. *Left-handed anti-parallel* - While DNA strands are **anti-parallel**, the primary form of DNA, **B-DNA**, is **right-handed**, not left-handed. - A less common form, **Z-DNA**, is left-handed, but it's not the canonical Watson-Crick model. *Left-handed parallel structure* - The **Watson-Crick DNA model** is **right-handed** and its strands are **anti-parallel**, not parallel. - A **parallel DNA structure** would have both strands oriented in the same 5' to 3' direction, which is not found in nature for the double helix. *Right-handed parallel structure* - Although **DNA is right-handed**, the two strands are **anti-parallel**, not parallel. - A **parallel DNA structure** is a theoretical arrangement and does not represent the biologically functional form of the Watson-Crick double helix.

Question 250: Most common physiological form of DNA is

A. A-form
B. B-form (Correct Answer)
C. Z-form
D. C-form

Explanation: ***B-form*** - **B-DNA** is the most common and stable form of DNA under **physiological conditions** (high humidity, aqueous environment). - Its structure is a **right-handed double helix**, characterized by 10 base pairs per turn, a wider diameter, and a deep major groove along with a shallow minor groove. *A-form* - **A-DNA** is a **right-handed double helix** that forms under conditions of **low humidity** or desiccation. - It is shorter and wider than B-DNA, with 11 base pairs per turn, and a much shallower major groove and a deeper minor groove. *Z-form* - **Z-DNA** is a **left-handed double helix** that can form in regions with alternating purine-pyrimidine sequences (e.g., GCGCGC). - It has a more irregular, zigzag backbone and is thought to play a role in gene regulation, but it is not the most common physiological form. *C-form* - **C-DNA** is another **right-handed double helical** form of DNA, which is observed under even lower humidity and in the presence of certain ions, such as lithium ions. - It has 9.3 base pairs per turn, making it slightly less compact than B-DNA, but it is not the predominant physiological form.

Practice by Chapter

Nucleotide Structure and Function

Practice Questions

DNA Structure and Replication

Practice Questions

RNA Structure and Types

Practice Questions

Transcription: RNA Synthesis

Practice Questions

Post-Transcriptional Modifications

Practice Questions

Translation: Protein Synthesis

Practice Questions

Genetic Code and Codon Usage

Practice Questions

Regulation of Gene Expression

Practice Questions

Mutations and DNA Repair

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Purine Metabolism and Disorders

Practice Questions

Pyrimidine Metabolism and Disorders

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Nucleotide Degradation and Salvage Pathways

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