Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 311: The DNA model described by Watson and Crick was:

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

Explanation: ### Explanation The DNA model proposed by Watson and Crick (1953) describes the **B-DNA** form, which is the most common physiological form of DNA found in living cells. **1. Why "Right-handed Antiparallel" is Correct:** * **Right-handed Helix:** The two polynucleotide chains are coiled around a common axis in a clockwise direction (right-handed). * **Antiparallel Orientation:** The two strands run in opposite directions. One strand is oriented in the **5' → 3'** direction, while the complementary strand is oriented in the **3' → 5'** direction. This orientation is essential for the formation of hydrogen bonds between complementary nitrogenous bases (A=T and G≡C). **2. Analysis of Incorrect Options:** * **A & D (Right-handed parallel):** While the helix is right-handed, the strands are never parallel. Parallel strands would prevent proper base pairing and the stability of the double helix. * **B (Left-handed antiparallel):** This describes **Z-DNA**. Z-DNA is a rare, zig-zag form of DNA that occurs in regions with alternating purine-pyrimidine sequences (e.g., CGCGCG) and is associated with gene expression regulation. **3. NEET-PG High-Yield Clinical Pearls:** * **B-DNA Dimensions:** 10 base pairs per turn; pitch of 3.4 nm; diameter of 2 nm. * **Z-DNA:** The only **left-handed** helix; contains 12 base pairs per turn. * **A-DNA:** Right-handed, produced by dehydration of B-DNA; contains 11 base pairs per turn. * **Chargaff’s Rule:** In double-stranded DNA, the amount of Adenine equals Thymine, and Guanine equals Cytosine (A+G = T+C). * **Denaturation:** The separation of strands (melting) occurs at the **Tm (Melting Temperature)**; DNA with high G-C content has a higher Tm due to triple hydrogen bonds.

Question 312: What is essential for the Polymerase Chain Reaction (PCR)?

A. A thermostable enzyme is needed. (Correct Answer)
B. Multiple primer annealing steps are required.
C. Non-specific primer binding occurs.
D. A thermolabile enzyme is used.

Explanation: **Explanation:** **Polymerase Chain Reaction (PCR)** is an *in vitro* technique used to amplify specific DNA sequences. The process involves repeated cycles of high-temperature denaturation, primer annealing, and extension. **Why Option A is Correct:** The most critical component of PCR is a **thermostable DNA polymerase**, typically **Taq Polymerase** (derived from the bacterium *Thermus aquaticus*). During the denaturation step, the reaction mixture is heated to approximately **94–96°C** to separate the double-stranded DNA. A standard human DNA polymerase (which is thermolabile) would denature and lose function at these temperatures. Taq polymerase remains stable and active through multiple heating cycles, allowing the reaction to proceed without adding new enzymes at every step. **Analysis of Incorrect Options:** * **Option B:** In a standard PCR cycle, there is only **one specific primer annealing step** per cycle (usually at 50–65°C). Multiple annealing steps are not a standard requirement. * **Option C:** PCR relies on **high specificity**. Primers are designed to be complementary to the target sequence. Non-specific binding leads to "noise" or amplification of unintended DNA fragments, which is a technical error, not a requirement. * **Option D:** A **thermolabile** enzyme (like *E. coli* DNA polymerase) would be destroyed during the first denaturation step, making the process inefficient and manually intensive. **High-Yield Clinical Pearls for NEET-PG:** * **Components of PCR:** Template DNA, Primers (forward and reverse), dNTPs, Mg²⁺ (cofactor), and Taq Polymerase. * **RT-PCR:** Used for RNA viruses (e.g., SARS-CoV-2); involves **Reverse Transcriptase** to convert RNA to cDNA before amplification. * **Real-Time PCR (qPCR):** Allows quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green). * **Applications:** Diagnosis of genetic mutations, viral load monitoring (HIV, HBV), and forensic medicine.

Question 313: What does the term ‘sequence’ mean in genetics?

A. Order of nucleotides within DNA (Correct Answer)
B. Order of amino acids in tRNA
C. None
D. None

Explanation: **Explanation:** In genetics, the term **‘sequence’** refers to the specific linear arrangement of nitrogenous bases (Adenine, Guanine, Cytosine, and Thymine) along a DNA strand. This order of nucleotides constitutes the genetic code, providing the instructions necessary for the synthesis of RNA and proteins. **1. Why Option A is correct:** DNA is a polymer of nucleotides. The unique sequence of these nucleotides (e.g., ATGC...) determines the hereditary information passed from parents to offspring. In the central dogma, the DNA sequence is transcribed into mRNA, which is then translated into a protein. Therefore, the "sequence" is the fundamental blueprint of life. **2. Why other options are incorrect:** * **Option B:** The order of amino acids refers to the **primary structure of a protein**, not a genetic sequence. While tRNA carries amino acids, it does not consist of them; tRNA itself is composed of a ribonucleotide sequence. * **Options C & D:** These are distractors and do not define the biological term. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sanger Sequencing:** The "Gold Standard" method for determining the nucleotide sequence of DNA. * **Next-Generation Sequencing (NGS):** A high-throughput method used clinically to detect multiple gene mutations simultaneously (e.g., in oncology or rare genetic disorders). * **Mutations:** Any change in the DNA sequence (e.g., point mutations, insertions, or deletions) can lead to dysfunctional proteins, resulting in diseases like Sickle Cell Anemia (a single nucleotide substitution in the β-globin gene). * **Chargaff’s Rule:** In a double-stranded DNA sequence, the amount of A = T and G = C.

Question 314: Which of the following conditions is associated with defective DNA repair?

A. Albinism
B. Xeroderma pigmentosum (Correct Answer)
C. Both of the above
D. None of the above

Explanation: ### Explanation **Correct Answer: B. Xeroderma pigmentosum** **1. Why Xeroderma Pigmentosum (XP) is correct:** Xeroderma pigmentosum is the classic example of a defect in **Nucleotide Excision Repair (NER)**. In healthy individuals, NER enzymes (specifically UV-specific endonucleases) identify and remove pyrimidine dimers (usually thymine dimers) caused by UV light exposure. In XP patients, these mutations remain unrepaired, leading to extreme photosensitivity, severe skin damage, and a 1000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **2. Why the other options are incorrect:** * **Albinism:** This is a disorder of **melanin synthesis**, not DNA repair. It is most commonly caused by a deficiency of the enzyme **tyrosinase**, which converts tyrosine to DOPA and subsequently to melanin. While albinos are also sensitive to UV light due to lack of protective pigment, their underlying DNA repair machinery is intact. * **Option C and D:** Since only XP involves a DNA repair defect, these options are mathematically incorrect. **3. High-Yield Clinical Pearls for NEET-PG:** * **Other DNA Repair Defect Disorders:** * **Lynch Syndrome (HNPCC):** Defect in Mismatch Repair (MMR). * **Ataxia-Telangiectasia:** Defect in repair of double-strand DNA breaks (ATM gene). * **Fanconi Anemia:** Defect in homologous recombination/interstrand cross-link repair. * **Bloom Syndrome:** Defect in DNA Helicase (*BLM* gene). * **Cockayne Syndrome:** Defect in transcription-coupled DNA repair. * **Key Enzyme in XP:** UV-specific endonuclease (also known as XP proteins A through G). * **Clinical Presentation:** "Sunburn on first exposure," freckling in sun-exposed areas, and early-onset skin malignancies.

Question 315: All of the following can be used to detect mutations, except:

A. Single strand conformational polymorphism
B. Ligase chain reaction (Correct Answer)
C. Polymerase chain reaction
D. DNA Sequencing

Explanation: The correct answer is **Ligase Chain Reaction (LCR)**. ### **Explanation of the Correct Answer** While LCR can be used to detect known point mutations, its primary and most common clinical application is the **amplification of DNA** to detect the presence of specific pathogens (e.g., *Chlamydia trachomatis* and *Neisseria gonorrhoeae*). In the context of standard molecular techniques used primarily for **mutation screening and discovery**, LCR is categorized as an amplification tool rather than a primary mutation detection method like the other options. ### **Analysis of Incorrect Options** * **A. Single Strand Conformational Polymorphism (SSCP):** This is a classic mutation screening method. It relies on the principle that single-stranded DNA folds into specific 3D conformations based on its sequence. A single base change alters this shape, resulting in different migration speeds during non-denaturing electrophoresis. * **C. Polymerase Chain Reaction (PCR):** PCR is the "gold standard" starting point for mutation detection. Specific variants like **AS-PCR (Allele-Specific PCR)** are designed specifically to identify known mutations by using primers that only bind to mutant sequences. * **D. DNA Sequencing:** This is the **definitive "Gold Standard"** for mutation detection. It identifies the exact nucleotide sequence, allowing for the discovery of both known and unknown mutations. ### **Clinical Pearls for NEET-PG** * **Gold Standard for Mutation Detection:** DNA Sequencing (Sanger or Next-Generation). * **SSCP vs. RFLP:** SSCP is used for screening unknown mutations, while RFLP (Restriction Fragment Length Polymorphism) is used to detect known mutations that alter a restriction site. * **LCR Mechanism:** It uses two sets of complementary probes and **DNA Ligase** to join them. It is highly specific for detecting small amounts of target DNA. * **High-Yield Fact:** If the question asks for the best method to detect a **known** single-base mutation in a clinical setting, **ARMS-PCR** (Amplification Refractory Mutation System) is a frequent correct answer.

Question 316: Formation of DNA using an RNA template is performed by which enzyme?

A. DNA dependent RNA polymerase
B. Reverse transcriptase (Correct Answer)
C. DNA polymerase
D. RNA polymerase

Explanation: ### Explanation The central dogma of molecular biology typically follows the flow: **DNA → RNA → Protein**. However, the formation of DNA from an RNA template reverses this flow, a process known as **Reverse Transcription**. **1. Why Reverse Transcriptase is correct:** Reverse transcriptase (also known as **RNA-dependent DNA polymerase**) is the enzyme responsible for synthesizing complementary DNA (cDNA) using an RNA strand as a template. This enzyme is essential for the replication of retroviruses and the maintenance of chromosomal ends. **2. Analysis of Incorrect Options:** * **DNA-dependent RNA polymerase (Option A):** This is the standard enzyme used in **Transcription**, where an RNA strand is synthesized using a DNA template (e.g., synthesis of mRNA). * **DNA polymerase (Option C):** This enzyme is used in **DNA Replication**. It is a DNA-dependent DNA polymerase, meaning it synthesizes a new DNA strand using an existing DNA template. * **RNA polymerase (Option D):** A general term usually referring to DNA-dependent RNA polymerase (involved in transcription) or RNA-dependent RNA polymerase (found in certain RNA viruses like Poliovirus). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Retroviruses:** HIV uses reverse transcriptase to integrate its viral genome into the host cell's DNA. This enzyme is the target of **NRTI/NNRTI** drugs (e.g., Zidovudine, Efavirenz). * **Telomerase:** A specialized reverse transcriptase that carries its own RNA template to extend the ends of chromosomes (telomeres). It is highly active in cancer cells and germ cells. * **Laboratory Use:** Reverse transcriptase is a critical component of **RT-PCR**, used to detect RNA viruses (like SARS-CoV-2) by first converting viral RNA into DNA. * **Discovery:** Discovered by Howard Temin and David Baltimore (Teminism).

Question 317: Which enzyme forms the RNA primer in eukaryotes?

A. Ligase
B. RNA polymerase (Correct Answer)
C. Topoisomerase
D. Helicase

Explanation: ### Explanation **Correct Answer: B. RNA polymerase** In eukaryotic DNA replication, DNA polymerase cannot initiate the synthesis of a new strand *de novo*; it requires a free 3'-OH group. This is provided by a short sequence of RNA known as a **primer**. The enzyme responsible for synthesizing this primer is **DNA-dependent RNA polymerase**, specifically a specialized form called **DNA Primase**. In eukaryotes, Primase exists in a complex with **DNA Polymerase α**. The Primase subunit synthesizes a short RNA stretch (approx. 10 nucleotides), which is then extended by Polymerase α with a short segment of DNA before the "polymerase switching" occurs to Polymerase δ or ε. **Why other options are incorrect:** * **A. Ligase:** Its role is to join DNA fragments (like Okazaki fragments) by catalyzing the formation of phosphodiester bonds. It does not synthesize new strands. * **C. Topoisomerase:** These enzymes (Type I and II) relieve torsional strain and supercoiling ahead of the replication fork by creating transient breaks in the DNA backbone. * **D. Helicase:** This enzyme (MCM complex in eukaryotes) uses ATP to unwind the DNA double helix into single strands; it has no synthetic activity. --- ### High-Yield Clinical Pearls for NEET-PG * **Polymerase Switching:** In eukaryotes, **Pol α** initiates (Primase activity), **Pol δ** synthesizes the lagging strand, and **Pol ε** synthesizes the leading strand. * **Prokaryotic Equivalent:** In *E. coli*, the primase is the **DnaG** protein. * **Directionality:** Primers are always synthesized in the **5' to 3' direction**. * **Drug Link:** Many antiviral and anticancer drugs (like Cytarabine) target DNA polymerases, inhibiting the elongation phase after the primer is formed.

Question 318: What is the primary use of Polymerase Chain Reaction (PCR)?

A. To detect specific plasmids
B. To amplify small quantities of DNA (Correct Answer)
C. To ligate DNA fragments
D. To cut bacterial plasmids

Explanation: **Explanation:** **1. Why Option B is Correct:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic technique used to produce millions of copies of a specific DNA segment from a very small initial sample. It utilizes a heat-stable DNA polymerase (typically **Taq polymerase**) and specific primers to synthesize complementary strands through repeated cycles of denaturation, annealing, and extension. This "molecular photocopying" is essential in medicine for detecting pathogens (like HIV or SARS-CoV-2) or genetic mutations where the initial DNA concentration is too low for standard analysis. **2. Why Other Options are Incorrect:** * **Option A:** While PCR can be used to *identify* the presence of a plasmid, its primary function is amplification. Specific plasmids are typically detected using techniques like **Southern Blotting** or gel electrophoresis after isolation. * **Option C:** The joining or ligation of DNA fragments is performed by the enzyme **DNA Ligase**, not PCR. * **Option D:** Cutting DNA at specific sequences is the function of **Restriction Endonucleases** (molecular scissors), which are used in recombinant DNA technology. **3. High-Yield Clinical Pearls for NEET-PG:** * **Components:** Requires Template DNA, Primers, dNTPs, and Taq Polymerase (derived from *Thermus aquaticus*). * **Steps & Temperatures:** 1. Denaturation (~94°C) 2. Annealing (~55°C) 3. Extension (~72°C). * **RT-PCR:** Used to amplify **RNA** (e.g., COVID-19 testing) by first converting it to cDNA using the enzyme **Reverse Transcriptase**. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes.

Question 319: Which enzyme synthesizes the RNA primer?

A. Topoisomerase
B. Helicase
C. DNA primase (Correct Answer)
D. DNA ligase

Explanation: **Explanation:** **1. Why DNA Primase is Correct:** DNA replication cannot be initiated *de novo* because DNA polymerases require a free **3'-OH group** to begin adding nucleotides. To overcome this, **DNA primase** (a specialized RNA polymerase) synthesizes a short sequence of RNA (approximately 10 nucleotides long) known as the **RNA primer**. This primer provides the necessary 3'-OH terminus that DNA Polymerase α (in eukaryotes) or DNA Polymerase III (in prokaryotes) uses to start DNA synthesis. **2. Why the Other Options are Incorrect:** * **Topoisomerase:** These enzymes (e.g., DNA Gyrase) relieve the **torsional strain** and supercoiling caused by the unwinding of the DNA double helix. * **Helicase:** This enzyme uses ATP to break hydrogen bonds between nitrogenous bases, effectively **unzipping** the double-stranded DNA at the replication fork. * **DNA Ligase:** This enzyme acts as "molecular glue." It catalyzes the formation of phosphodiester bonds to join **Okazaki fragments** on the lagging strand or to seal nicks during DNA repair. **3. High-Yield Clinical Pearls for NEET-PG:** * **Primosome:** This is a functional complex consisting of **DNA Helicase + DNA Primase**. * **Directionality:** RNA primers are synthesized in the **5' to 3' direction**. * **Removal:** In eukaryotes, RNA primers are removed by **RNase H** and **FEN1**; in prokaryotes, they are removed by the **5'→3' exonuclease activity of DNA Polymerase I**. * **Drug Target:** Fluoroquinolones (e.g., Ciprofloxacin) inhibit bacterial DNA Gyrase (Topoisomerase II) and Topoisomerase IV, preventing replication.

Question 320: What is the staining material used for the production of insulin from bacteria?

A. Genomic DNA from beta pancreatic cells of human
B. Genomic DNA from lymphocytes of human
C. mRNA from beta pancreatic cells of human (Correct Answer)
D. mRNA from lymphocytes of human

Explanation: **Explanation:** The production of recombinant human insulin (Humulin) involves the insertion of human genetic material into a bacterial vector (usually *E. coli*). **Why mRNA is the correct choice:** Human **Genomic DNA** contains both **exons** (coding regions) and **introns** (non-coding regions). Bacteria are prokaryotes and lack the cellular machinery (spliceosomes) required to remove introns. If genomic DNA were used, the bacteria would translate the introns, resulting in a non-functional protein. To bypass this, scientists extract **mRNA** from the **beta cells of the Islets of Langerhans** in the pancreas. This mRNA has already undergone post-transcriptional splicing. Using the enzyme **Reverse Transcriptase**, this mRNA is converted into **complementary DNA (cDNA)**, which contains only the coding sequences. This cDNA is then inserted into the bacterial plasmid for expression. **Analysis of Incorrect Options:** * **Options A & B (Genomic DNA):** Incorrect because genomic DNA contains introns, which bacteria cannot process, leading to defective protein synthesis. * **Option D (mRNA from lymphocytes):** Incorrect because, although all cells contain the same genome, the insulin gene is only "expressed" (transcribed into mRNA) in the specialized beta cells of the pancreas. Lymphocytes do not produce insulin mRNA. **High-Yield NEET-PG Pearls:** * **Eli Lilly (1983):** The first company to produce recombinant insulin. * **Reverse Transcriptase:** Also known as RNA-dependent DNA polymerase; essential for creating cDNA libraries. * **Humulin Structure:** Human insulin consists of two chains (A and B) linked by disulfide bonds. In recombinant technology, these chains are often produced separately and then chemically joined. * **Proinsulin vs. Insulin:** Bacteria cannot perform the post-translational cleavage of the C-peptide from proinsulin; hence, cDNA for chains A and B is expressed independently.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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