Nucleic Acid Biochemistry Practice Questions

Q: Which of the following statements is FALSE regarding gout?

It is due to increased metabolism of pyrimidines.. ### Explanation **1. Why Option A is the correct (False) statement:** Gout is a disorder of **purine metabolism**, not pyrimidine metabolism. The end product of purine (Adenine and Guanine) catabolism in humans is **uric acid**. In contrast, the end products of pyrimidine (Cytosine, Thymine, and Uracil) catabolism are highly soluble molecules like **$\beta$-alanine** and **$\beta$-aminoisobutyrate**, which are easily excreted in urine and do not cause crystal deposition or gout. **2. Analysis of other options:** * **Option B:** This is true. Hyperuricemia, the precursor to gout, results from either the overproduction of purines (e.g., Lesch-Nyhan syndrome, PRPP synthetase overactivity) or decreased renal excretion of uric acid. * **Option C:** This is true. While hyperuricemia is the hallmark, serum uric acid levels can be **normal or even low during an acute attack** of gout because the urate crystals have shifted from the blood into the joint space. * **Option D:** This is true. The first metatarsophalangeal joint (great toe) is the most common site of the initial attack, a clinical condition known as **Podagra**. **3. NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** Identification of **negatively birefringent, needle-shaped** monosodium urate crystals under polarized light microscopy. * **Enzyme Defects:** Gout is associated with **Von Gierke’s disease** (due to increased G6P entering the HMP shunt, increasing ribose-5-phosphate and PRPP). * **Drug of Choice:** **NSAIDs** are first-line for acute attacks; **Allopurinol** (a xanthine oxidase inhibitor) is used for chronic management but should never be started during an acute flare. * **Dietary triggers:** High-protein diets (red meat), seafood, and alcohol (especially beer) exacerbate gout.

Q: If the molar percentage of guanine in a human DNA is 30%, what is the molar percentage of adenine in that molecule?

20%. ### Explanation The correct answer is **20%**. This question is based on **Chargaff’s Rules**, which are fundamental to understanding the structure of double-stranded DNA (dsDNA). #### 1. Why the Correct Answer is Right According to Chargaff’s Rule of Base Pairing: * The amount of Purines equals the amount of Pyrimidines ($A+G = T+C$). * Specifically, **Adenine (A) always pairs with Thymine (T)**, and **Guanine (G) always pairs with Cytosine (C)**. Therefore, $\%A = \%T$ and $\%G = \%C$. **Step-by-step Calculation:** 1. If **G = 30%**, then its partner **C must also be 30%**. 2. Total G + C = $30\% + 30\% = 60\%$. 3. The remaining percentage for A + T is $100\% - 60\% = 40\%$. 4. Since A and T must be equal, **A = 40% / 2 = 20%** (and T = 20%). #### 2. Why Incorrect Options are Wrong * **Option A (10%):** This would imply G+C equals 80%, which contradicts the given data. * **Option C (30%):** This assumes A = G, which is not a rule. A only equals G if the GC content is exactly 50%. * **Option D (40%):** This represents the total sum of A+T, not the individual percentage of Adenine. #### 3. Clinical Pearls & High-Yield Facts for NEET-PG * **Applicability:** Chargaff’s rules apply **only to double-stranded DNA**. They do not apply to single-stranded DNA (ssDNA) or RNA (e.g., Parvovirus B19 or HIV genome). * **Bonding:** A-T pairs are held by **2 hydrogen bonds**, while G-C pairs are held by **3 hydrogen bonds**. * **Melting Temperature ($T_m$):** DNA with higher G-C content has a higher $T_m$ because triple hydrogen bonds require more energy to denature. * **Human Genome:** The human genome is generally "AT-rich" (~59-60% A+T and ~40-41% G+C).

Q: Deamination of methylated cytosine will produce which of the following?

Thymine. ### Explanation **1. Why Thymine is Correct:** Cytosine undergoes spontaneous deamination to form **Uracil**. However, in DNA, cytosine is frequently methylated at the 5th carbon (5-methylcytosine) by DNA methyltransferase, a process essential for gene silencing and epigenetic regulation. When **5-methylcytosine** undergoes deamination, the amino group is replaced by a carbonyl group, directly converting it into **Thymine** (5-methyluracil). **2. Why Other Options are Incorrect:** * **Adenine & Guanine:** These are purines. Deamination of cytosine (a pyrimidine) cannot result in a purine structure. * **Xanthine:** This is an intermediate in purine catabolism produced by the deamination of **Guanine**. (Note: Deamination of Adenine produces Hypoxanthine). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Mutation Hotspot":** Because thymine is a natural base in DNA, the repair enzyme *Uracil DNA Glycosylase* cannot recognize this specific deamination product as "damaged." Consequently, C → T transitions at methylated CpG islands are the most common cause of single-point mutations in humans. * **Uracil in DNA:** Spontaneous deamination of *unmethylated* cytosine produces Uracil. This is why DNA uses Thymine instead of Uracil—it allows the cell to identify deaminated cytosine as "foreign" and initiate Base Excision Repair (BER). * **Summary of Deamination:** * Cytosine → Uracil * 5-Methylcytosine → **Thymine** * Adenine → Hypoxanthine * Guanine → Xanthine

Q: What is the complementary sequence to 5' GCACC 3'?

5' GGTGC 3'. ### Explanation **1. Understanding the Concept** To find the complementary sequence of DNA, two fundamental rules of molecular biology must be applied: * **Base Pairing Rule:** Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). * **Antiparallel Orientation:** DNA strands run in opposite directions. If the template is **5' to 3'**, the complement must be **3' to 5'**. **Step-by-step derivation:** 1. **Template:** 5' G-C-A-C-C 3' 2. **Complement (Antiparallel):** 3' C-G-T-G-G 5' 3. **Standard Notation:** By convention, DNA sequences are always written in the **5' to 3' direction** unless specified otherwise. Flipping 3' CGTGG 5' gives us **5' GGTGC 3'**. **2. Analysis of Options** * **Option D (Correct):** Correct base pairing and correctly reversed to the 5' → 3' orientation. * **Option A:** Incorrect; it simply reverses the original sequence without applying base-pairing rules. * **Option B:** Incorrect; while it applies base pairing, it maintains the same polarity (5' to 3'), violating the antiparallel rule. * **Option C:** Incorrect; it uses the correct bases but labels the ends incorrectly (it is actually the 5' to 3' sequence). **3. NEET-PG High-Yield Clinical Pearls** * **Chargaff’s Rule:** In any double-stranded DNA, %A = %T and %G = %C. Therefore, Purines (A+G) = Pyrimidines (T+C). * **Bond Strength:** G-C pairs have **three hydrogen bonds**, while A-T pairs have **two**. Sequences with high G-C content have a higher melting temperature (Tm). * **Clinical Relevance:** Understanding complementarity is essential for **PCR (Polymerase Chain Reaction)**, where primers must be complementary to the 3' end of the target DNA strand to initiate synthesis.

Q: Which of the following is a disorder of purine metabolism?

Gout. **Explanation:** **Gout** is the correct answer because it is a clinical syndrome resulting from the deposition of monosodium urate crystals in joints and tissues. This occurs due to **hyperuricemia**, which is an elevation of uric acid—the final breakdown product of **purine metabolism** in humans. Key enzymes involved in purine pathways, such as PRPP synthetase (overactivity) or HGPRT (deficiency in Lesch-Nyhan syndrome), are frequently implicated in secondary gout. **Analysis of Incorrect Options:** * **Hyperammonemia (A):** This is a disorder of the **Urea Cycle** (protein/amino acid metabolism), resulting from the body's inability to detoxify ammonia into urea. * **Orotic aciduria (B):** This is a disorder of **pyrimidine metabolism**. It typically results from a deficiency of the bifunctional enzyme UMP synthase, leading to a buildup of orotic acid and megaloblastic anemia. * **Hepatolenticular degeneration (D):** Also known as Wilson’s disease, this is a disorder of **copper metabolism** caused by mutations in the *ATP7B* gene, leading to copper accumulation in the liver and brain. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step of Purine Synthesis:** Glutamine PRPP amidotransferase. * **Lesch-Nyhan Syndrome:** An X-linked recessive deficiency of **HGPRT** (salvage pathway), characterized by hyperuricemia, self-mutilation, and mental retardation. * **Von Gierke’s Disease (G6Pase deficiency):** Often presents with secondary gout due to increased shunting of G6P into the Pentose Phosphate Pathway, increasing Ribose-5-Phosphate and subsequent purine synthesis. * **Drug of Choice:** **Allopurinol** (a suicide inhibitor of Xanthine Oxidase) is used for chronic gout to lower uric acid levels.

Q: Which form of DNA is predominantly seen in living cells under physiological conditions?

B. ***B*** - **B-form DNA** is the most stable conformation under **physiological conditions** with normal hydration levels and **Watson-Crick base pairing**. - It features a **right-handed double helix** with **10.5 base pairs per turn** and **major and minor grooves** that are well-defined for protein binding. *A* - **A-form DNA** occurs under **dehydrated conditions** or in **RNA-DNA hybrids**, making it less common in living cells. - It has a **wider, shorter helix** with **11 base pairs per turn** and is predominantly found in laboratory conditions, not physiological environments. *C* - **C-form DNA** is an **intermediate form** that exists at **low humidity** and specific salt conditions. - This form is **rarely observed** in biological systems and represents a **transitional state** between A and B forms. *Z* - **Z-form DNA** is a **left-handed double helix** that forms under **high salt concentrations** and specific base sequences (GC-rich). - While it may exist **transiently** in some cellular processes, it represents less than **1%** of cellular DNA under normal physiological conditions.

Q: DNA labeling is done by using which of the following isotopes?

Phosphorus-32 (P-32). **Explanation:** The correct answer is **Phosphorus-32 (P-32)**. This is based on the fundamental biochemical structure of nucleic acids. DNA and RNA consist of a sugar-phosphate backbone; therefore, radioactive phosphorus can be incorporated directly into the phosphate groups during DNA synthesis or via enzymatic labeling (e.g., nick translation). * **Why P-32 is correct:** P-32 is a high-energy **pure beta emitter**. Its chemical identity with natural phosphorus allows it to substitute for stable phosphorus in the DNA molecule without altering its biological properties. It is widely used in molecular biology for Northern/Southern blotting and DNA sequencing. **Analysis of Incorrect Options:** * **Technetium-99m (Tc-99m):** The most common isotope in diagnostic nuclear medicine (gamma emitter). It is used for organ imaging (bone, thyroid, kidney) but does not naturally incorporate into the DNA structure. * **Iodine-131 (I-131):** Primarily used for the diagnosis and treatment of thyroid disorders (Grave’s disease, Thyroid CA). While iodine can label proteins (tyrosine residues), it is not a constituent of DNA. * **Cobalt-60 (Co-60):** A gamma emitter used in external beam radiotherapy (Telecobalt units) and sterilization of medical equipment, not for molecular DNA labeling. **High-Yield Clinical Pearls for NEET-PG:** * **Therapeutic use of P-32:** Historically used to treat **Polycythemia Vera** and essential thrombocythemia due to its ability to suppress bone marrow. * **Pure Beta Emitters:** Remember the mnemonic **"SPY"** (Strontium-89, Phosphorus-32, Yttrium-90). These are used for therapy, not imaging. * **Tritium (H-3):** Another isotope used for DNA labeling (specifically tritiated thymidine), often used in cell proliferation studies.

Q: Which of the following is NOT an enzyme deficiency responsible for gout?

Enzyme responsible for pseudogout. **Explanation** Gout is a disorder of purine metabolism characterized by hyperuricemia and the deposition of monosodium urate crystals in joints. The correct answer is **B (Enzyme responsible for pseudogout)** because pseudogout is caused by the deposition of **calcium pyrophosphate dihydrate (CPPD)** crystals, not uric acid. It is typically associated with abnormalities in pyrophosphate metabolism (e.g., mutations in the ANKH gene) or hyperparathyroidism, rather than the purine pathway. **Analysis of Incorrect Options:** * **PRPP Synthase (Option A):** Overactivity (gain-of-function mutation) of this enzyme leads to increased production of PRPP, which accelerates de novo purine synthesis, resulting in hyperuricemia and gout. * **5-phosphoribosyl pyrophosphate amidotransferase (Option C):** This is the rate-limiting enzyme of purine synthesis. Loss of feedback inhibition or overactivity leads to excessive purine production and subsequent gout. * **Glucose-6-phosphatase (Option D):** Deficiency causes **Von Gierke Disease (GSD Type I)**. This leads to gout via two mechanisms: (1) Shunting of G6P into the Pentose Phosphate Pathway, increasing PRPP levels, and (2) Lactic acidosis, which competes with uric acid for excretion in the kidneys. **High-Yield Clinical Pearls for NEET-PG:** * **Lesch-Nyhan Syndrome:** Deficiency of HGPRT (salvage pathway) leads to severe gout, self-mutilation, and intellectual disability. * **Crystal Morphology:** Gout shows **needle-shaped**, negatively birefringent crystals (yellow when parallel). Pseudogout shows **rhomboid-shaped**, positively birefringent crystals (blue when parallel). * **Drug of Choice:** NSAIDs for acute gout; Allopurinol (Xanthine Oxidase inhibitor) for chronic management.

Q: A patient presents with increased levels of Hypoxanthine and Xanthine in the blood, along with hypouricemia. Which enzyme deficiency is responsible for this condition?

Xanthine oxidase. **Explanation:** The correct answer is **Xanthine oxidase**. **1. Why Xanthine Oxidase is correct:** Xanthine oxidase (XO) is the key enzyme in the final stages of purine catabolism. It catalyzes two critical steps: the conversion of **Hypoxanthine to Xanthine** and the subsequent conversion of **Xanthine to Uric Acid**. A deficiency in this enzyme (Xanthinuria) leads to a metabolic block. Consequently, Hypoxanthine and Xanthine cannot be converted into uric acid, leading to their accumulation in the blood and urine, while serum uric acid levels drop significantly (**Hypouricemia**). **2. Why the other options are incorrect:** * **HGPRTase:** Deficiency causes **Lesch-Nyhan Syndrome**. This leads to a failure of the purine salvage pathway, causing an *increase* in uric acid production (Hyperuricemia) and gout-like symptoms, not hypouricemia. * **Adenosine deaminase (ADA):** Deficiency leads to **Severe Combined Immunodeficiency (SCID)** due to the toxic accumulation of dATP in lymphocytes. It does not typically present with isolated hypouricemia. * **APRTase:** Deficiency leads to the formation of **2,8-dihydroxyadenine renal stones**. It affects adenine metabolism but does not cause the specific triad of high hypoxanthine/xanthine and low uric acid. **Clinical Pearls for NEET-PG:** * **Xanthinuria:** Often asymptomatic but can lead to **Xanthine stones** (radiolucent) in the urinary tract. * **Allopurinol:** A drug used in gout that acts as a suicide inhibitor of Xanthine Oxidase, mimicking this deficiency to lower uric acid levels. * **Hypouricemia Differential:** Consider Xanthine oxidase deficiency, Fanconi syndrome (renal loss), or severe liver disease.

Q: Which type of RNA molecule is known to contain modified nucleotides?

tRNA. **Explanation:** **tRNA (Transfer RNA)** is the correct answer because it contains the highest percentage of modified nucleotides (approximately 10–15% of its residues). These modifications are essential for its unique "cloverleaf" secondary structure and "L-shaped" tertiary structure. Common modified bases include **Pseudouridine (ψ)**, **Dihydrouridine (D)**, **Ribothymidine (T)**, and **Inosine**. These modifications stabilize the molecule, facilitate proper folding, and ensure accurate codon-anticodon base pairing (wobble hypothesis). **Why other options are incorrect:** * **rRNA (Ribosomal RNA):** While rRNA does undergo some modifications (like methylation), the extent and variety are significantly lower than in tRNA. * **hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA). While it undergoes processing (capping, tailing, splicing), it does not characteristically feature the diverse modified bases seen in tRNA. * **mRNA (Messenger RNA):** In eukaryotes, mRNA has a 7-methylguanosine cap, but it lacks the extensive internal base modifications (like D or ψ) that define tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **TψC Loop:** Contains Ribothymidine and Pseudouridine; it is responsible for binding the tRNA to the ribosomal surface. * **DHU Loop:** Contains Dihydrouridine; it serves as the recognition site for the specific enzyme **Aminoacyl tRNA synthetase**. * **Smallest RNA:** tRNA is the smallest (4S), often called "Soluble RNA." * **Inosine:** Often found in the anticodon loop, allowing a single tRNA to recognize multiple codons (Wobble phenomenon).

Question 1

Which of the following statements is FALSE regarding gout?

Accepted Answer

It is due to increased metabolism of pyrimidines.

Answer

It is due to increased metabolism of purines.

Answer

Uric acid levels may not be elevated in all cases.

Answer

It has a predilection for the great toe.

Question 2

If the molar percentage of guanine in a human DNA is 30%, what is the molar percentage of adenine in that molecule?

Accepted Answer

20%

Answer

10%

Answer

30%

Answer

40%

Question 3

Deamination of methylated cytosine will produce which of the following?

Accepted Answer

Thymine

Answer

Adenine

Answer

Xanthine

Answer

Guanine

Question 4

What is the complementary sequence to 5' GCACC 3'?

Accepted Answer

5' GGTGC 3'

Answer

5' CCACG 3'

Answer

5' CGTGG 3'

Answer

3' GGTGC 5'

Question 5

Which of the following is a disorder of purine metabolism?

Accepted Answer

Gout

Answer

Hyperammonemia

Answer

Orotic aciduria

Answer

Hepatolenticular degeneration (Wilson's disease)

Question 6

Which form of DNA is predominantly seen in living cells under physiological conditions?

Accepted Answer

B

Answer

A

Answer

C

Answer

Z

Question 7

DNA labeling is done by using which of the following isotopes?

Accepted Answer

Phosphorus-32 (P-32)

Answer

Technetium-99m (Tc-99m)

Answer

Iodine-131 (I-131)

Answer

Cobalt-60 (Co-60)

Question 8

Which of the following is NOT an enzyme deficiency responsible for gout?

Accepted Answer

Enzyme responsible for pseudogout

Answer

PRPP synthase

Answer

5-phosphoribosyl pyrophosphate amidotransferase

Answer

Glucose-6-phosphatase

Question 9

A patient presents with increased levels of Hypoxanthine and Xanthine in the blood, along with hypouricemia. Which enzyme deficiency is responsible for this condition?

Accepted Answer

Xanthine oxidase

Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPrtase)

Answer

Adenosine deaminase

Answer

Acyclovir resistance protein (APRTase)

Question 10

Which type of RNA molecule is known to contain modified nucleotides?

Accepted Answer

tRNA

Answer

rRNA

Answer

hnRNA

Answer

mRNA

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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