RNA Structure and Types — MCQs
Which of the following is not a component of a nucleotide?
Which of the following is a primarily RNA based technique?
If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?
Which of the following is a ribozyme?
What is the most stabilizing force for nucleic acids?
Apolipoprotein B-48 is made by which process?
Which of the following is not associated with post-transcription modification?
Which type of bonds are represented by the dotted lines in the image? (AIIMS Nov 2017)
Which of the following is a function of ribozymes?
Which amino acid is used in the synthesis of purines?
RNA Structure and Types Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is not a component of a nucleotide?
- A. Sugar
- B. Fatty acid (Correct Answer)
- C. Base
- D. Phosphate
Explanation: ***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 2: Which of the following is a primarily RNA based technique?
- A. Western blotting
- B. Northern blotting (Correct Answer)
- C. Southern blotting
- D. Sanger's technique
Explanation: ***Northern blotting*** - **Northern blotting** is a molecular biology technique used to study **gene expression** by detecting specific **RNA molecules** (mRNA) in a sample. - It involves separating RNA fragments by **gel electrophoresis**, transferring them to a membrane, and then detecting specific sequences using **labeled probes**. *Western blotting* - **Western blotting** is a technique used to detect specific **proteins** in a sample. - It involves separating proteins by **gel electrophoresis**, transferring them to a membrane, and then detecting specific proteins using labeled **antibodies**. *Southern blotting* - **Southern blotting** is a molecular biology method used for the detection of **specific DNA sequences** in DNA samples. - It involves separating **DNA fragments** by **gel electrophoresis**, transferring them to a membrane, and then hybridizing with a labeled probe. *Sanger's technique* - **Sanger sequencing**, or the **dideoxy chain-termination method**, is a widely used method for **DNA sequencing**. - It uses **dideoxynucleotides** to terminate DNA synthesis at specific bases, allowing the determination of the **DNA sequence**.
Question 3: If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?
- A. 4
- B. 64
- C. 16
- D. 256 (Correct Answer)
Explanation: ***256*** - With **4 distinct nucleotides** and a code sequence of **4 nucleotides** per amino acid, the number of possible unique combinations is calculated as 4^4. - This results in 4 × 4 × 4 × 4 = **256 theoretically possible amino acids**. - This is a mathematical combinatorics calculation: with 4 choices at each of 4 positions, total combinations = 4^4 = 256. *64* - This number represents the combinations if **3 nucleotides** coded for one amino acid (4^3 = 64), which is the actual case in the **standard genetic code** (triplet codons). - However, the question specifies a hypothetical sequence of **4 nucleotides** per amino acid, making this option incorrect. *16* - This number would be correct if **2 nucleotides** coded for one amino acid (4^2 = 16). - The problem explicitly states that **4 nucleotides** code for each amino acid in this theoretical scenario. *4* - This would only be the case if each **single nucleotide** coded for one amino acid (4^1 = 4). - Given **4 distinct nucleotides** and a sequence length of 4, the potential for combinations is much higher.
Question 4: Which of the following is a ribozyme?
- A. Peptidyl transferase (Correct Answer)
- B. Elongation factor 2
- C. Primase
- D. RNA polymerase
Explanation: ***Peptidyl transferase*** - This enzyme is an integral part of the **large ribosomal subunit** and is responsible for catalyzing the formation of peptide bonds during protein synthesis. - While historically thought to be purely proteinaceous, it is now known that the **catalytic activity** of peptidyl transferase comes from its **rRNA component**, specifically the 23S rRNA in prokaryotes and 28S rRNA in eukaryotes, making it a ribozyme. *Elongation factor 2* - **Elongation Factor 2 (EF2)** is a **GTPase** that facilitates the translocation of the ribosome along the mRNA during protein synthesis. - It is a **protein**, not an RNA molecule, and thus does not possess catalytic activity as a ribozyme. *Primase* - **Primase** is an **RNA polymerase** that synthesizes short RNA primers required for the initiation of DNA replication. - It is a **protein enzyme** and not an RNA molecule with catalytic activity. *RNA polymerase* - **RNA polymerase** is a **protein enzyme** responsible for synthesizing RNA from a DNA template during transcription. - It uses a DNA template to produce an RNA strand, but its own catalytic activity is derived from its **protein structure**, not from an RNA component.
Question 5: What is the most stabilizing force for nucleic acids?
- A. Conformational entropy
- B. Hydrogen bonds (Correct Answer)
- C. Covalent bond
- D. Van der Waals interaction
Explanation: ***Hydrogen bonds*** - **Hydrogen bonds** between complementary base pairs (A-T/U and G-C) are the **primary stabilizing force** in nucleic acid double helix structures. - These bonds hold the two strands together and provide **specificity** in base pairing, which is fundamental to DNA/RNA structure and function. - The cumulative effect of multiple hydrogen bonds along the helix provides substantial stability to the overall structure. *Van der Waals interaction* - Van der Waals forces contribute to **base stacking interactions** within the double helix. - While base stacking is important for stability, it is considered a **secondary stabilizing force** compared to the hydrogen bonds that directly hold complementary strands together. - These interactions contribute to the hydrophobic core of the helix. *Covalent bond* - **Covalent bonds** form the **sugar-phosphate backbone** of each DNA or RNA strand, linking nucleotides together. - While essential for strand integrity, they do not stabilize the double-helical structure by holding two strands together. - They maintain the primary structure but not the secondary structure stability. *Conformational entropy* - **Conformational entropy** refers to the degree of disorder or freedom of movement within a molecule. - The formation of a stable, ordered double helix represents a **decrease** in conformational entropy. - This is an unfavorable thermodynamic factor that opposes helix formation, not a stabilizing force.
Question 6: Apolipoprotein B-48 is made by which process?
- A. DNA editing
- B. RNA editing (Correct Answer)
- C. RNA alternate splicing
- D. RNA interference
Explanation: ***RNA editing*** - Apolipoprotein B-48 is synthesized from ApoB-100 mRNA through a process called **RNA editing** (specifically ApoB mRNA editing) - This involves a **cytidine deaminase enzyme (APOBEC-1)** that converts cytidine to uridine at position 6666, changing a glutamine codon (CAA) to a premature stop codon (UAA) in the small intestine - This results in a truncated protein that is 48% the length of ApoB-100 - ApoB-48 is produced in the **intestine**, while ApoB-100 (unedited) is produced in the **liver** *DNA editing* - DNA editing refers to permanent modifications in the DNA sequence itself - The ApoB gene remains unchanged; only the mRNA transcript is edited in intestinal cells - This is not the mechanism for producing ApoB-48 *RNA alternate splicing* - Alternative splicing involves selecting different combinations of exons from pre-mRNA to produce multiple mRNA isoforms - This process creates different protein variants through exon inclusion/exclusion - ApoB-48 production does not involve alternative splicing but rather direct nucleotide modification (C to U) within the coding sequence *RNA interference* - RNA interference (RNAi) is a biological process involving small RNA molecules (siRNA, miRNA) that silence gene expression - RNAi typically degrades mRNA or blocks translation - This process is not involved in generating a truncated protein like ApoB-48 from the same mRNA transcript
Question 7: Which of the following is not associated with post-transcription modification?
- A. Methylation
- B. Glycosylation (Correct Answer)
- C. 5' capping
- D. Endonuclease cleavage
Explanation: ***Glycosylation*** - **Glycosylation** is a type of **post-translational modification** where carbohydrates are added to proteins, not directly to RNA during post-transcriptional processing. - This process is crucial for **protein folding, stability, cell recognition**, and other cellular functions. *Methylation* - **Methylation** of ribosomal RNA, transfer RNA, and messenger RNA is a common **post-transcriptional modification** that influences RNA stability and function. - For example, methylation at the N6 position of adenosine (m6A) in mRNA plays a role in **mRNA splicing, stability, and translation**. *5' capping* - **5' capping** is a critical **post-transcriptional modification** of eukaryotic mRNA, involving the addition of a 7-methylguanosine cap to the 5' end. - This cap is essential for **mRNA stability, transport out of the nucleus, and efficient translation initiation**. *Endonuclease cleavage* - **Endonuclease cleavage** is a fundamental aspect of **post-transcriptional modification**, particularly in the processing of precursor RNA molecules (pre-mRNA, pre-rRNA, pre-tRNA). - This process involves enzymes called **endonucleases** that cut phosphodiester bonds within RNA strands, crucial for producing mature functional RNA molecules, such as removing introns during **splicing** or generating fragments during **miRNA processing**.
Question 8: Which type of bonds are represented by the dotted lines in the image? (AIIMS Nov 2017)
- A. Hydrogen bond (Correct Answer)
- B. Covalent bond
- C. Ionic bond
- D. Phosphodiester
Explanation: ***Hydrogen bond*** - The dotted lines in the image represent the weak, non-covalent interactions between the **nitrogenous bases** on opposite strands of the DNA double helix. - Specifically, these are **hydrogen bonds** formed between complementary base pairs (Adenine with Thymine via two hydrogen bonds, and Guanine with Cytosine via three hydrogen bonds). *Covalent bond* - **Covalent bonds** involve the sharing of electron pairs between atoms and are much stronger than hydrogen bonds. - In DNA, covalent bonds form the **sugar-phosphate backbone** of each strand and link the nitrogenous bases to the deoxyribose sugars. *Ionic bond* - **Ionic bonds** involve the electrostatic attraction between oppositely charged ions, formed by the complete transfer of electrons. - While ions (like magnesium or sodium) interact with DNA, the dotted lines specifically represent the inter-strand forces between bases, which are not ionic. *Phosphodiester* - A **phosphodiester bond** is a specific type of covalent bond that links the 3' carbon of one deoxyribose sugar to the 5' carbon of the next deoxyribose sugar via a phosphate group, forming the backbone of a single DNA strand. - The dotted lines are between the two DNA strands, not within a single strand's backbone.
Question 9: 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 10: Which amino acid is used in the synthesis of purines?
- A. Glycine (Correct Answer)
- B. Ornithine
- C. Alanine
- D. Threonine
Explanation: **Explanation:** Purine ring synthesis is a complex process that occurs primarily in the liver. The purine nucleus (adenine and guanine) is constructed "piece by piece" onto a ribose-5-phosphate backbone. **Why Glycine is Correct:** **Glycine** is a fundamental building block of the purine ring. It contributes three specific atoms: **C4, C5, and N7**. During the second step of de novo synthesis, glycine reacts with phosphoribosylamine to form glycinamide ribotide (GAR). It is the only amino acid that is incorporated into the ring in its entirety. **Analysis of Incorrect Options:** * **B. Ornithine:** This is an intermediate in the Urea Cycle. It is not involved in nucleotide synthesis. * **C. Alanine:** While a common glucogenic amino acid, it does not contribute atoms to the purine or pyrimidine rings. * **D. Threonine:** This is an essential amino acid but plays no role in the structural assembly of nucleic acids. **High-Yield NEET-PG Pearls:** To master purine synthesis, remember the "Sources of Atoms" mnemonic: 1. **Glycine:** C4, C5, N7 (The entire molecule). 2. **Aspartate:** N1. 3. **Glutamine (Amide N):** N3 and N9. 4. **Tetrahydrofolate (N10-formyl THF):** C2 and C8. 5. **CO₂ (Respiratory):** C6. * **Clinical Correlation:** The rate-limiting enzyme of this pathway is **PRPP Glutamyl Amidotransferase**. Drugs like **Methotrexate** inhibit purine synthesis by depleting the folate pool, while **Azathioprine** (converted to 6-mercaptopurine) acts as a purine analogue to inhibit de novo synthesis.
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