Nucleotide Structure and Function — MCQs
Which coenzyme is required by 6-phosphogluconate dehydrogenase?
What is a key similarity between the processes of replication and transcription?
Most important amino acid which acts as a methyl group donor?
The helical structure model of two polynucleotide chains of DNA was proposed by whom?
False statements are:
If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?
Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?
What is the most stabilizing force for nucleic acids?
If the percentage of thymine residues in DNA is 28%. What is the percentage of cytosine?
If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?
Nucleotide Structure and Function Indian Medical PG Practice Questions and MCQs
Question 1: Which coenzyme is required by 6-phosphogluconate dehydrogenase?
- A. FAD
- B. NADP (Correct Answer)
- C. FMN
- D. NAD
Explanation: ***Correct NADP*** - **6-phosphogluconate dehydrogenase** is a key enzyme in the **oxidative phase of the pentose phosphate pathway**, which generates **NADPH**. - **NADP+** acts as an electron acceptor, getting reduced to **NADPH**, which is crucial for reductive biosynthesis and **antioxidant defense**. *Incorrect FAD* - **Flavin adenine dinucleotide (FAD)** is a derivative of **riboflavin (vitamin B2)** and is typically involved in reactions transferring two electrons, often found in **dehydrogenases** that form carbon-carbon double bonds. - Enzymes like **succinate dehydrogenase** (in the TCA cycle) utilize **FAD**, which is not the case for 6-phosphogluconate dehydrogenase. *Incorrect FMN* - **Flavin mononucleotide (FMN)**, also derived from **riboflavin**, is primarily found in **flavoproteins** and acts as a prosthetic group in various **redox reactions**. - It is a component of **Complex I (NADH dehydrogenase)** in the electron transport chain, but not involved in the pentose phosphate pathway's 6-phosphogluconate dehydrogenase reaction. *Incorrect NAD* - **Nicotinamide adenine dinucleotide (NAD+)** is a coenzyme primarily involved in catabolic reactions where it accepts electrons and is reduced to **NADH**, typically in glycolysis, the TCA cycle, and fatty acid oxidation. - While structurally similar to NADP+, **NAD+** is involved in ATP production, whereas **NADP+** is used in anabolic processes.
Question 2: What is a key similarity between the processes of replication and transcription?
- A. Use RNA primers for initiation.
- B. Use ribonucleotides as precursors.
- C. Are semi-conservative events.
- D. Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction. (Correct Answer)
Explanation: ***Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction.*** - Both DNA replication and RNA transcription synthesize nucleic acid polymers by forming **phosphodiester bonds** between incoming nucleotides. - The new strand in both processes is always elongated in the **5' to 3' direction**, as new nucleotides are added to the 3' hydroxyl group of the growing strand. *Use RNA primers for initiation.* - **DNA replication** requires **RNA primers** to initiate synthesis of new DNA strands, as DNA polymerase cannot start a new strand *de novo*. - **Transcription (RNA synthesis)** does not require a primer; **RNA polymerase** can initiate transcription *de novo* at a promoter sequence. *Use ribonucleotides as precursors.* - **Transcription** uses **ribonucleotides** (ATP, UTP, CTP, GTP) as precursors to synthesize RNA. - **Replication** primarily uses **deoxyribonucleotides** (dATP, dTTP, dCTP, dGTP) to synthesize DNA, although it temporarily uses ribonucleotides for RNA primers. *Are semi-conservative events.* - **DNA replication** is a **semi-conservative process**, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. - **Transcription** is **not semi-conservative**; it involves synthesizing an RNA molecule from a DNA template, leaving the original DNA template unchanged.
Question 3: Most important amino acid which acts as a methyl group donor?
- A. Tryptophan
- B. Methionine (Correct Answer)
- C. Cysteine
- D. Tyrosine
Explanation: ***Methionine*** - **Methionine** is the precursor to **S-adenosylmethionine (SAM)**, which serves as the primary and most important **methyl group donor** in various biochemical reactions, including DNA methylation and neurotransmitter synthesis. - The methyl group of methionine is transferred to a wide range of acceptors via SAM, playing a crucial role in metabolism and gene regulation. *Tryptophan* - **Tryptophan** is an essential amino acid primarily known as a precursor for the synthesis of **serotonin** and **niacin**. - While it has various metabolic roles, it does not directly act as a significant methyl group donor. *Cysteine* - **Cysteine** is important for maintaining protein structure through **disulfide bonds** and is a precursor for **glutathione** synthesis. - It contains a sulfur-containing thiol group but does not donate methyl groups. *Tyrosine* - **Tyrosine** is a non-essential amino acid that is a precursor for **thyroid hormones**, **catecholamines** (dopamine, norepinephrine, epinephrine), and **melanin**. - It does not function as a methyl group donor in biochemical pathways.
Question 4: 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 5: False statements are:
- A. DNA replication proceeds in one direction
- B. Lagging strand is synthesized by RNA primase
- C. All of the options (Correct Answer)
- D. Bacteria have multiple origins of replication
Explanation: ***All of the options*** - All statements are **false**. DNA replication proceeds **bidirectionally**, bacteria typically have a **single origin of replication**, and the lagging strand is synthesized by **DNA polymerase** after an RNA primer is laid down by **RNA primase**. *DNA replication proceeds in one direction* - This statement is **false** because **DNA replication** is a **bidirectional process**, meaning it proceeds in both directions from the origin of replication. - Replication forks move away from the **origin** on both sides, unraveling the DNA and synthesizing new strands. *Bacteria have multiple origins of replication* - This statement is **false**. Most **bacteria** (prokaryotes) have a **single origin of replication** (oriC) on their circular chromosome. - In contrast, **eukaryotes** have **multiple origins of replication** on their linear chromosomes to replicate their much larger genomes efficiently. - While rare exceptions exist in some bacterial species, the general rule for bacterial DNA replication is a single origin. *Lagging strand is synthesized by RNA primase* - This statement is **false**. The **lagging strand** is primarily synthesized by **DNA polymerase III** (in prokaryotes) or **DNA polymerase δ** (in eukaryotes). - **RNA primase** is responsible for synthesizing short **RNA primers** that provide a starting point for DNA polymerase, but it does not synthesize the entire lagging strand itself.
Question 6: 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 7: Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?
- A. RBC
- B. Leukocytes
- C. Liver (Correct Answer)
- D. Brain
Explanation: ***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.
Question 8: 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 9: If the percentage of thymine residues in DNA is 28%. What is the percentage of cytosine?
- A. 36%
- B. 44%
- C. 22% (Correct Answer)
- D. 28%
Explanation: ***22%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If thymine (T) is 28%, then adenine (A) is also 28%, making a total of 56% for A+T. The remaining 44% (100% - 56%) is split equally between guanine and cytosine, so cytosine (C) is 22%. *36%* - This percentage would be plausible if the sum of adenine and thymine were 28%, which is incorrect as A and T are equal and their sum would thus be 56%. - This value does not adhere to the principle of **base pairing complementarity** and the total percentage of all bases summing to 100%. *44%* - This would be the combined percentage of guanine and cytosine, not the percentage of cytosine alone. - If cytosine were 44%, then guanine would also be 44%, leading to a total of 88% for G+C, which is inconsistent with T being 28%. *28%* - This is the percentage of thymine, and by **Chargaff's rules**, it would also be the percentage of adenine, not cytosine. - Cytosine percentages are derived from the remaining proportion of bases after accounting for adenine and thymine.
Question 10: If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?
- A. 15%
- B. 85%
- C. 70%
- D. 35% (Correct Answer)
Explanation: ***35%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If A = 15%, then T must also be 15%. This means A + T = 30%. Since the total percentage of all bases is 100%, G + C must be 100% - 30% = 70%. As G = C, then G = 70% / 2 = 35%. *15%* - This would only be correct if guanine paired with adenine, which it does not; guanine pairs with **cytosine**. - This answer incorrectly assumes that all four bases are present in equal proportions, or that G equals A, which violates **Chargaff's rules**. *85%* - This percentage would imply an incorrect base pairing or an imbalanced ratio of purines and pyrimidines, violating the fundamental structure of DNA. - An 85% guanine content would mean that G + C far exceeds 100% or that T is extremely low, which is biologically impossible. *70%* - This represents the combined percentage of **guanine and cytosine**, not guanine alone. - While it correctly acknowledges the remaining proportion of bases, it fails to divide this sum between the two equal components, **G and C**.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free