Nucleic Acid Structure and Function — MCQs
Which type of RNA contains codons for specific amino acids?
If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?
Beta-alanine is derived from ?
What is the most stabilizing force for nucleic acids?
What is Northern blot used to detect?
Nucleic acid is not found in which of the following?
Nucleic acid is not found in -
A sex worker presents with purulent urethral discharge and gram-negative diplococci in culture. What is the most likely diagnosis?
Which type of DNase produced by Streptococcus is known to be the most antigenic in humans?
All are true about prions EXCEPT:
Nucleic Acid Structure and Function Indian Medical PG Practice Questions and MCQs
Question 1: Which type of RNA contains codons for specific amino acids?
- A. Transfer RNA (tRNA)
- B. Messenger RNA (mRNA) (Correct Answer)
- C. Small nuclear RNA (snRNA)
- D. Ribosomal RNA (rRNA)
Explanation: ***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 2: 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 3: Beta-alanine is derived from ?
- A. Adenosine
- B. Uracil (Correct Answer)
- C. Guanosine
- D. Thymine
Explanation: ***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.
Question 4: 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 5: What is Northern blot used to detect?
- A. Protein
- B. Immunoglobulin
- C. RNA (Correct Answer)
- D. DNA
Explanation: ***RNA*** - **Northern blot** is a laboratory technique used to detect specific **RNA** molecules among a mixture of RNA. - It involves separating RNA fragments by **gel electrophoresis**, transferring them to a membrane, and then probing with a labeled complementary sequence. *Protein* - **Proteins** are typically detected using a **Western blot**, which involves similar separation and transfer techniques but uses **antibodies** as probes. - While RNA codes for proteins, Northern blot *directly* detects RNA transcripts, not the resulting protein products. *Immunoglobulin* - **Immunoglobulins** (antibodies) are a type of protein, and their detection usually falls under **Western blot** or specific immunological assays like **ELISA**. - Northern blot is specifically designed for nucleic acid analysis, not protein detection. *DNA* - **DNA** is detected using a **Southern blot** technique, which also involves electrophoresis, transfer to a membrane, and hybridization with a complementary probe. - The name "Northern blot" was coined as a play on "Southern blot" because it uses similar methodology but for RNA instead of DNA.
Question 6: Nucleic acid is not found in which of the following?
- A. Virus
- B. Fungus
- C. Prions (Correct Answer)
- D. Bacteria
Explanation: ***Prions*** - **Prions** are infectious protein particles composed solely of misfolded proteins, lacking any **nucleic acid** (DNA or RNA). - They replicate by inducing normal cellular proteins to misfold into the abnormal, pathogenic prion form. *Virus* - **Viruses** are obligate intracellular parasites that contain either **DNA or RNA** as their genetic material, enclosed within a protein coat. - This nucleic acid is essential for directing the synthesis of new viral particles within a host cell. *Bacteria* - **Bacteria** are prokaryotic organisms that contain **DNA** as their genetic material, typically in a single circular chromosome located in the nucleoid region. - They also contain plasmids, which are small, extrachromosomal DNA molecules. *Fungus* - **Fungi** are eukaryotic organisms that contain **DNA** organized into multiple chromosomes within a membrane-bound nucleus. - Their genetic material directs all cellular activities and reproduction.
Question 7: Nucleic acid is not found in -
- A. Bacteria
- B. Fungus
- C. Prions (Correct Answer)
- D. Virus
Explanation: ***Prions*** - **Prions** are infectious protein particles that lack **nucleic acids (DNA or RNA)**. - They cause transmissible spongiform encephalopathies by inducing abnormal folding of normal cellular proteins. *Bacteria* - **Bacteria** are prokaryotic organisms that contain **double-stranded DNA** as their genetic material, organized in a circular chromosome. - They also rely on **RNA** for protein synthesis and gene regulation. *Fungus* - **Fungi** are eukaryotic organisms that possess genetic material in the form of **DNA**, organized into chromosomes within a nucleus. - They utilize various types of **RNA** for essential cellular processes including transcription and translation. *Virus* - **Viruses** are obligate intracellular parasites that contain either **DNA or RNA** as their genetic material. - This nucleic acid is enclosed within a protein coat (capsid) and is essential for viral replication.
Question 8: A sex worker presents with purulent urethral discharge and gram-negative diplococci in culture. What is the most likely diagnosis?
- A. Treponema pallidum
- B. Haemophilus ducreyi
- C. Chlamydia trachomatis
- D. Neisseria gonorrhoeae (Correct Answer)
Explanation: ***Neisseria gonorrhoeae*** - **Purulent urethral discharge** and identification of **Gram-negative diplococci** on microscopy are classic diagnostic features of gonococcal urethritis [1]. - This sexually transmitted infection is common among sexually active individuals, including **sex workers** [1]. *Treponema pallidum* - This bacterium causes **syphilis**, which is characterized by **chancres** in the primary stage, and widespread rashes or lesions in later stages [2]. - It would not typically present with Gram-negative diplococci or purulent urethral discharge [2]. *Haemophilus ducreyi* - This organism is responsible for **chancroid**, a sexually transmitted infection that causes painful **genital ulcers** with ragged borders and often associated with lymphadenopathy [3]. - It would not lead to purulent urethral discharge, and while Gram-negative, it is typically seen as pleomorphic rods in chains. *Chlamydia trachomatis* - **Chlamydia** infection often presents with mucopurulent urethral discharge, but it is typically less purulent than gonorrhea and may be **asymptomatic** [1]. - **Chlamydia** is an **obligate intracellular bacterium** and would not be visualized as Gram-negative diplococci on a Gram stain [1].
Question 9: Which type of DNase produced by Streptococcus is known to be the most antigenic in humans?
- A. DNase C
- B. DNase D
- C. DNase A
- D. DNase B (Correct Answer)
Explanation: ***DNase B*** - **DNase B** is the most **antigenic** of the four DNases (A, B, C, D) produced by *Streptococcus pyogenes*. - Antibodies against DNase B are commonly measured as anti-DNase B titers in the diagnosis of **streptococcal infections** and their sequelae, like **acute rheumatic fever** and **post-streptococcal glomerulonephritis**, especially when ASO titers are negative. *DNase A* - While *Streptococcus pyogenes* produces **DNase A**, it is not considered the most antigenically potent or clinically relevant for antibody testing in humans. - Its antigenicity is generally **lower** than that of DNase B in response to streptococcal infection. *DNase C* - **DNase C** is another deoxyribonuclease produced by *Streptococcus pyogenes*. - Similar to DNase A, it does not elicit as strong or as consistent an **antibody response** in human infections compared to DNase B. *DNase D* - **DNase D** is the fourth type of deoxyribonuclease produced by *Streptococcus pyogenes*. - Its **antigenicity** in humans is less pronounced and less diagnostically significant than that of **DNase B**.
Question 10: All are true about prions EXCEPT:
- A. Contain nucleic acid (Correct Answer)
- B. Protease resistant
- C. Not affected by radiation
- D. Cause spongiform changes
Explanation: ***Contain nucleic acid*** - This statement is **FALSE** - prions do NOT contain nucleic acid, making this the correct answer for an EXCEPT question. - Prions are unique infectious agents composed solely of **abnormally folded proteins (PrPSc)**, completely lacking any genetic material such as **DNA or RNA**. - This fundamental characteristic differentiates them from all conventional pathogens including **viruses, bacteria, fungi, and parasites**. *Protease resistant* - This statement is TRUE about prions. - The **abnormal folding** of prion proteins (β-pleated sheet conformation) renders them highly **resistant to degradation** by proteases. - This resistance contributes to their **accumulation in neurological tissue** and the progressive pathogenesis of transmissible spongiform encephalopathies. *Not affected by radiation* - This statement is TRUE about prions. - Because prions lack nucleic acid, they are **resistant to inactivation** by treatments like **ultraviolet (UV) radiation** and **ionizing radiation**. - These forms of radiation primarily damage genetic material (DNA/RNA), which prions completely lack. - Prions require **autoclaving at 134°C for extended periods** or treatment with strong alkalis for effective inactivation. *Cause spongiform changes* - This statement is TRUE about prions. - Prion diseases are characterized by **vacuolation** of brain tissue, giving it a distinctive **spongy appearance** on microscopic examination. - These spongiform changes are pathognomonic features of prion diseases such as **Creutzfeldt-Jakob disease (CJD)**, **kuru**, **Gerstmann-Sträussler-Scheinker syndrome**, and **bovine spongiform encephalopathy (BSE)**.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free