Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 201: Which of the following is not common to the synthesis of both leading and lagging strands?

A. RNA primer is needed
B. Nucleoside monophosphates are added in the 5' to 3' direction along the growing DNA chain
C. DNA polymerase III synthesizes DNA
D. DNA ligase is repeatedly used to join fragments of DNA along the growing strand (Correct Answer)

Explanation: **Explanation** DNA replication is a semi-discontinuous process occurring at the replication fork. While both strands are synthesized simultaneously, their mechanisms differ due to the antiparallel nature of DNA and the fact that DNA polymerase can only synthesize in the **5' to 3' direction**. **Why Option D is the Correct Answer:** * **Leading Strand:** Synthesis is **continuous** in the direction of the replication fork. It requires only a single RNA primer at the origin, and DNA ligase is generally not required during the elongation phase. * **Lagging Strand:** Synthesis is **discontinuous**, occurring away from the replication fork in short segments called **Okazaki fragments**. Each fragment requires a new RNA primer. **DNA ligase** is essential and **repeatedly used** here to join these fragments by forming phosphodiester bonds after the RNA primers are removed and replaced by DNA. **Analysis of Incorrect Options:** * **A. RNA primer is needed:** Both strands require a primase-catalyzed RNA primer to provide a free 3'-OH group for DNA polymerase to initiate synthesis. * **B. Nucleoside monophosphates are added in the 5' to 3' direction:** This is a universal rule for all DNA synthesis. Deoxyribonucleoside triphosphates (dNTPs) are added, releasing pyrophosphate, leaving a monophosphate in the chain. * **C. DNA polymerase III:** In prokaryotes, this is the primary enzyme responsible for the elongation of both the leading and lagging strands. **High-Yield NEET-PG Pearls:** * **DNA Polymerase I:** Has 5' to 3' exonuclease activity; it removes RNA primers and fills the gaps (especially on the lagging strand). * **Topoisomerases:** Relieve torsional strain (supercoiling) ahead of the fork. Fluoroquinolones inhibit DNA Gyrase (Topoisomerase II) in bacteria. * **Telomerase:** A reverse transcriptase that maintains the ends of linear chromosomes (eukaryotes), preventing loss of genetic material during lagging strand synthesis.

Question 202: What is the typical size of a microsatellite repeat sequence?

A. Less than 1 kilobase (kb)
B. 2-6 base pairs (bp) (Correct Answer)
C. 1-3 kilobases (kb)
D. Greater than 3 kilobases (kb)

Explanation: **Explanation:** The correct answer is **B (2-6 base pairs)**. This question tests the classification of repetitive DNA sequences, specifically **Short Tandem Repeats (STRs)**, also known as **Microsatellites**. Microsatellites are tracts of repetitive DNA where a short motif, typically **2 to 6 base pairs** in length, is repeated 5 to 50 times. They are highly polymorphic and distributed throughout the human genome. Their high degree of variability between individuals makes them the "gold standard" marker for DNA profiling and linkage analysis. **Analysis of Options:** * **Option A (< 1 kb):** While the *entire* microsatellite locus is usually less than 1 kb, the question asks for the size of the **repeat sequence (motif)** itself, which is much smaller (2-6 bp). * **Options C & D (1-3 kb and > 3 kb):** These sizes are characteristic of **Minisatellites** (Variable Number Tandem Repeats or VNTRs), where the repeat unit is typically 10-60 bp, and the total array can span several kilobases. **High-Yield Clinical Pearls for NEET-PG:** 1. **Microsatellite Instability (MSI):** This is a critical diagnostic marker for **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). It occurs due to mutations in **Mismatch Repair (MMR) genes** (e.g., *MLH1, MSH2*), leading to uncontrolled expansion or contraction of these repeats. 2. **Trinucleotide Repeat Disorders:** Many neurological diseases are caused by microsatellite expansions (e.g., **Huntington’s disease** [CAG], **Fragile X syndrome** [CGG]). 3. **Forensics:** STRs are the primary tools used in **DNA Fingerprinting** because the number of repeats at specific loci varies significantly between individuals.

Question 203: Which of the following processes is involved in the conversion of DNA to RNA?

A. Conjugation
B. Transduction
C. Translocation
D. Transcription (Correct Answer)

Explanation: **Explanation:** **Transcription** is the fundamental process of gene expression where a specific segment of DNA is used as a template to synthesize a complementary RNA molecule (mRNA, tRNA, or rRNA). This process is catalyzed by the enzyme **RNA Polymerase**. In eukaryotes, this occurs in the nucleus, and the resulting primary transcript undergoes post-transcriptional modifications before entering the cytoplasm for translation. **Analysis of Incorrect Options:** * **Conjugation:** A mechanism of horizontal gene transfer in bacteria involving direct cell-to-cell contact via a sex pilus to transfer genetic material (plasmids). * **Transduction:** The process by which foreign DNA is introduced into a cell by a virus or viral vector (bacteriophage). * **Translocation:** This term has two meanings in genetics: (1) In **translation**, it is the movement of the ribosome along the mRNA; (2) In **cytogenetics**, it is a chromosomal abnormality where a segment of one chromosome breaks off and attaches to another. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** RNA synthesis always proceeds in the **5' to 3' direction**, reading the template DNA strand in the 3' to 5' direction. * **Inhibitors:** **Rifampicin** inhibits bacterial RNA polymerase (used in Tuberculosis), while **Actinomycin D** inhibits transcription in both prokaryotes and eukaryotes (used in chemotherapy). * **Alpha-amanitin:** Found in *Amanita phalloides* (death cap mushroom), it specifically inhibits **RNA Polymerase II**, leading to severe liver failure. * **Reverse Transcription:** The conversion of RNA back to DNA, catalyzed by Reverse Transcriptase (seen in Retroviruses like HIV).

Question 204: What is the rate-limiting enzyme of purine nucleotide synthesis?

A. Xanthine oxidase
B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
C. Adenosine deaminase (ADA)
D. PRPP glutamyl amido-transferase (Correct Answer)

Explanation: **Explanation:** The synthesis of purine nucleotides occurs via the **De Novo pathway**, where the purine ring is built upon a ribose-5-phosphate foundation. **1. Why PRPP glutamyl amido-transferase is correct:** The committed and rate-limiting step of purine synthesis is the conversion of **5-Phosphoribosyl-1-pyrophosphate (PRPP)** to **5-Phosphoribosylamine**. This reaction is catalyzed by **PRPP glutamyl amido-transferase**. * **Mechanism:** It involves the displacement of the pyrophosphate group from PRPP by an amide group from Glutamine. * **Regulation:** This enzyme is strictly regulated via feedback inhibition by the end products: AMP, GMP, and IMP. High levels of PRPP act as a feed-forward activator. **2. Why other options are incorrect:** * **Xanthine oxidase:** This is an enzyme involved in the **catabolism** (breakdown) of purines, converting hypoxanthine to xanthine and xanthine to uric acid. It is the target of Allopurinol. * **HGPRT:** This is the key enzyme of the **Purine Salvage Pathway**, not de novo synthesis. Its deficiency leads to **Lesch-Nyhan Syndrome**. * **Adenosine deaminase (ADA):** This enzyme is involved in purine degradation (converting adenosine to inosine). Its deficiency leads to **Severe Combined Immunodeficiency (SCID)**. **Clinical Pearls for NEET-PG:** * **Source of Atoms:** Remember the contributors to the purine ring: **CO₂** (C6), **Glycine** (C4, C5, N7), **Aspartate** (N1), **Glutamine** (N3, N9), and **Formyl-THF** (C2, C8). * **Rate-limiting step of Pyrimidine synthesis:** Carbamoyl phosphate synthetase II (CPS-II). * **Inhibitor:** The drug **Azathioprine** (immunosuppressant) is converted to 6-mercaptopurine, which inhibits PRPP glutamyl amido-transferase.

Question 205: What is true about genes?

A. Smallest functional unit of the genome (Correct Answer)
B. Not capable of independent expression
C. Promoter and enhancer regions are typical examples
D. A cistron is a single functional unit

Explanation: **Explanation:** **Why Option A is Correct:** A **gene** is defined as the basic physical and functional unit of heredity. In molecular biology, it is the smallest segment of DNA that contains the specific instructions required to produce a functional product—either a polypeptide chain (protein) or a functional RNA molecule (like tRNA or rRNA). It represents the fundamental unit of the genome because it is the smallest sequence capable of encoding biological information that results in a phenotype. **Analysis of Incorrect Options:** * **Option B:** Genes **are** capable of independent expression. Through the processes of transcription and translation, a gene can be expressed to form its functional product independently of other genes, provided the necessary cellular machinery (polymerases, ribosomes) is present. * **Option C:** Promoter and enhancer regions are **regulatory elements**, not the genes themselves. While they are essential for controlling gene expression, the "gene" typically refers to the transcribed region (exons and introns). * **Option D:** While a cistron is often used synonymously with a gene, the statement is technically incomplete or misleading in this context. In eukaryotes, genes are typically **monocistronic** (one gene = one protein), but in prokaryotes, they are often **polycistronic** (one promoter controls multiple functional units/cistrons). Therefore, "a cistron" is a structural definition, whereas "gene" is the broader functional definition. **High-Yield Clinical Pearls for NEET-PG:** * **Exons vs. Introns:** Exons are the coding sequences (expressed), while introns are non-coding intervening sequences removed during splicing. * **Pseudogenes:** These are genomic DNA sequences that resemble functional genes but are non-functional due to mutations (e.g., GLO gene in humans). * **Housekeeping Genes:** Genes expressed constitutively in all cells to maintain basic cellular functions (e.g., Actin, GAPDH). * **TATA Box:** A key promoter element located ~25-30 base pairs upstream of the transcription start site in eukaryotes.

Question 206: Which of the following is regarded as the outward expression of a gene?

A. Phenotype (Correct Answer)
B. Proteomics
C. Genotype
D. Anticipation

Explanation: ### Explanation **Correct Answer: A. Phenotype** The **phenotype** refers to the observable physical, biochemical, or physiological characteristics of an organism. It is the "outward expression" of an individual’s genetic makeup (genotype) as it interacts with the environment. In molecular terms, this involves the flow of genetic information (Central Dogma) from DNA to RNA to functional proteins, which ultimately manifest as traits like height, eye color, or the presence of a specific enzyme deficiency. **Why other options are incorrect:** * **B. Proteomics:** This is the large-scale study of the entire set of proteins expressed by a genome, cell, or tissue. While proteins contribute to the phenotype, proteomics is a field of study/methodology rather than the expression itself. * **C. Genotype:** This represents the internal genetic constitution or the specific set of alleles (e.g., *HbA/HbS*) an individual carries. It is the "blueprint," not the outward expression. * **D. Anticipation:** This is a genetic phenomenon where a disease (often triplet repeat disorders like Huntington’s or Fragile X) becomes more severe or appears at an earlier age in succeeding generations. **High-Yield Clinical Pearls for NEET-PG:** * **Phenotypic Heterogeneity:** Different mutations in the same gene can result in different clinical phenotypes (e.g., different mutations in the *FGFR* gene causing different skeletal dysplasias). * **Pleiotropy:** A single gene mutation resulting in multiple, seemingly unrelated phenotypic effects (e.g., Marfan Syndrome affecting the eyes, heart, and skeleton). * **Penetrance:** The percentage of individuals with a specific genotype who actually exhibit the associated phenotype. If a person has the gene but no symptoms, it is called "reduced penetrance."

Question 207: Which of the following is not required for PCR, a cell-free test tube method for amplifying a target sequence of DNA?

A. Taq polymerase
B. Deoxynucleotide triphosphates (dNTPs)
C. Primer
D. Radiolabeled DNA probe (Correct Answer)

Explanation: **Explanation** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. To understand why a **radiolabeled DNA probe** is not required, one must look at the fundamental components of the PCR "cocktail." 1. **Why the Correct Answer is Right:** A **radiolabeled DNA probe** is used in hybridization techniques like Southern Blotting to *detect* a specific DNA sequence after it has been separated by electrophoresis. In PCR, the goal is *amplification* (synthesis). While probes are used in Real-Time PCR (qPCR) for quantification, they are not a fundamental requirement for the basic PCR process itself. Furthermore, modern labs use fluorescent dyes rather than hazardous radioactive labels. 2. **Why the Other Options are Wrong:** * **Taq Polymerase (A):** A heat-stable DNA polymerase (derived from *Thermus aquaticus*) is essential to synthesize new DNA strands at high temperatures without denaturing. * **dNTPs (B):** Deoxynucleotide triphosphates (dATP, dCTP, dGTP, dTTP) are the "building blocks" required to construct the new DNA polymer. * **Primers (C):** PCR requires two synthetic oligonucleotide primers that are complementary to the sequences flanking the target DNA. They provide the 3'-OH group necessary for DNA polymerase to initiate synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (94-96°C) → Annealing (50-65°C) → Extension (72°C). * **RT-PCR vs. qPCR:** Reverse Transcriptase PCR (RT-PCR) is used to amplify RNA (e.g., for COVID-19 testing), while Quantitative PCR (qPCR) measures DNA concentration in real-time. * **Magnesium ions (MgCl₂):** A critical cofactor for Taq polymerase activity; its concentration can affect the specificity of the reaction.

Question 208: Which of the following statements is true about Polymerase Chain Reaction (PCR)?

A. It is carried out by thermostable DNA polymerase.
B. The amplification process is exponential.
C. The amplification is sequence-specific.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. It relies on thermal cycling, consisting of denaturation, annealing, and extension. * **Option A (Thermostable DNA polymerase):** Because the denaturation step requires high temperatures (approx. 95°C) to separate DNA strands, a heat-stable enzyme is essential. The most common is **Taq Polymerase**, isolated from the bacterium *Thermus aquaticus*, which remains functional at high temperatures. * **Option B (Exponential amplification):** PCR follows the formula $2^n$, where '$n$' is the number of cycles. Since the product of one cycle serves as the template for the next, the target DNA amount doubles every cycle, leading to exponential growth. * **Option C (Sequence-specific):** The specificity of PCR is determined by **synthetic oligonucleotide primers**. These primers are designed to be complementary to the sequences flanking the target region, ensuring only the desired segment is amplified. Since all three statements accurately describe the fundamental principles of PCR, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **RT-PCR (Reverse Transcriptase PCR):** Used for RNA viruses (e.g., SARS-CoV-2, HIV) to convert RNA to cDNA before amplification. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green). * **Components required:** Template DNA, Primers, dNTPs (Deoxynucleotide triphosphates), Mg²⁺ (cofactor), and Taq Polymerase. * **Applications:** Diagnosis of genetic mutations, forensic analysis (DNA fingerprinting), and detection of infectious agents.

Question 209: Which of the following is true regarding aminoacyl-tRNA synthetase?

A. Isoaccepting tRNA
B. Implements the genetic code
C. Attachment of amino acid to the 3' end of tRNA (Correct Answer)
D. Editing function

Explanation: ### Explanation **Aminoacyl-tRNA synthetase (aaRS)** is the enzyme responsible for the "charging" of tRNA, a critical step in translation. **1. Why Option C is Correct:** The primary function of aaRS is to catalyze a two-step reaction: first, activating an amino acid with ATP to form aminoacyl-AMP, and second, transferring that amino acid to the **3' hydroxyl group (CCA tail)** of its cognate tRNA. This ester bond formation provides the energy required for peptide bond synthesis during translation. **2. Analysis of Other Options:** * **Option A (Isoaccepting tRNA):** This term refers to different tRNA molecules that carry the same amino acid but have different anticodons. While aaRS must recognize all isoaccepting tRNAs for a specific amino acid, the enzyme itself is not the "isoaccepting tRNA." * **Option B (Implements the genetic code):** While aaRS is often called the "translator" of the genetic code because it matches an amino acid to an anticodon, the **genetic code itself** is implemented by the ribosome-mRNA-tRNA complex during codon-anticodon pairing. (Note: Some texts consider aaRS as the "second genetic code," but Option C is the definitive biochemical function). * **Option D (Editing function):** While many (but not all) aaRS enzymes possess a **proofreading/editing site** to hydrolyze incorrectly attached amino acids, this is a secondary quality-control mechanism, not the primary defining function of the enzyme class. **High-Yield Clinical Pearls for NEET-PG:** * **Energy Requirement:** The charging process consumes **two high-energy phosphate bonds** (ATP → AMP + PPi). * **Specificity:** There are generally **20 types** of aaRS, one for each amino acid. * **Clinical Correlation:** **Mupirocin** (topical antibiotic) acts by inhibiting bacterial isoleucyl-tRNA synthetase, preventing protein synthesis. * **Autoimmunity:** Antibodies against histidyl-tRNA synthetase (**Anti-Jo-1 antibodies**) are diagnostic markers for Dermatomyositis and Polymyositis.

Question 210: The process by which DNA is converted to mRNA is called as?

A. Transcription (Correct Answer)
B. Translation
C. DNA replication
D. None

Explanation: ### Explanation The correct answer is **Transcription**. This process is the first step of the **Central Dogma of Molecular Biology**, where the genetic information stored in DNA is copied into a complementary strand of messenger RNA (mRNA). #### Why Transcription is Correct: In eukaryotes, transcription occurs in the nucleus. The enzyme **RNA Polymerase II** reads the template strand of DNA (3' to 5') to synthesize mRNA in a 5' to 3' direction. This mRNA then carries the genetic "blueprint" from the nucleus to the cytoplasm for protein synthesis. #### Why Other Options are Incorrect: * **Translation:** This is the process where the genetic code carried by mRNA is decoded by ribosomes to synthesize a specific **polypeptide/protein**. It occurs in the cytoplasm. * **DNA Replication:** This is the process of producing two identical replicas of DNA from one original DNA molecule. It occurs during the **S-phase** of the cell cycle to ensure genetic continuity during cell division. #### NEET-PG High-Yield Pearls: * **Reverse Transcription:** The process of converting RNA back into DNA (seen in Retroviruses like HIV), catalyzed by the enzyme **Reverse Transcriptase**. * **Post-Transcriptional Modifications:** In eukaryotes, the initial transcript (hnRNA) undergoes 5' capping, 3' polyadenylation, and splicing (removal of introns) to become mature mRNA. * **Inhibitors:** **Rifampicin** inhibits bacterial RNA polymerase (used in TB treatment), while **Alpha-amanitin** (from *Amanita phalloides* mushrooms) inhibits eukaryotic RNA Polymerase II. * **Promoter Region:** The **TATA box** (Goldberg-Hogness box) is a key promoter element in eukaryotes that helps position RNA Polymerase II for transcription initiation.

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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