Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 221: What enzyme is responsible for synthesizing cDNA from mRNA?

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

Explanation: **Explanation:** **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 mRNA template. This process reverses the "Central Dogma" of molecular biology (DNA → RNA). In the laboratory, this is a crucial step in **RT-PCR**, where mRNA is converted to cDNA to study gene expression, as cDNA lacks the non-coding introns present in genomic DNA. **2. Why Other Options are Incorrect:** * **DNA Polymerase:** This enzyme synthesizes DNA using a **DNA template** during replication. It requires a primer and works in the 5' to 3' direction but cannot utilize an RNA template. * **RNA Polymerase:** This enzyme performs **transcription**, synthesizing RNA from a DNA template. * **DNA Ligase:** Known as the "molecular glue," this enzyme joins DNA fragments (like Okazaki fragments) by forming phosphodiester bonds. It does not synthesize new DNA strands from templates. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Source:** Reverse transcriptase was first discovered in retroviruses (e.g., **HIV**). It is encoded by the *pol* gene. * **Telomerase:** A specialized reverse transcriptase (TERT) that maintains chromosomal ends (telomeres). Its activity is high in cancer cells and stem cells. * **Diagnostic Use:** RT-PCR is the gold standard for diagnosing RNA virus infections, such as **COVID-19 (SARS-CoV-2)** and monitoring **HIV viral load**. * **Inhibitors:** Nucleoside Reverse Transcriptase Inhibitors (NRTIs) like **Zidovudine (AZT)** are key drugs in HAART therapy for HIV.

Question 222: What is a frame shift mutation?

A. Substitution of an amino acid
B. Change in an mRNA base at the 3rd nucleotide of a codon
C. Resulting in a premature stop codon
D. Deletion or insertion of nucleotides not in multiples of three (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** The genetic code is read in non-overlapping triplets called **codons**. A **frameshift mutation** occurs when the number of nucleotides inserted or deleted is **not a multiple of three** (e.g., +1, +2, -1). This shifts the "reading frame" of the mRNA from the point of mutation onwards. Consequently, every subsequent codon is altered, leading to a completely different amino acid sequence and usually resulting in a non-functional protein or a premature stop codon (truncated protein). **2. Analysis of Incorrect Options:** * **Option A:** This describes a **Missense mutation**, where a single nucleotide substitution results in one different amino acid (e.g., Sickle Cell Anemia). * **Option B:** This often refers to a **Silent mutation**. Due to the "Wobble hypothesis," changes at the 3rd position frequently code for the same amino acid. * **Option C:** This is the definition of a **Nonsense mutation**. While a frameshift *can* lead to a premature stop codon, the primary mechanism of the mutation itself is the shift in the reading frame. **3. NEET-PG High-Yield Pearls:** * **Clinical Correlation:** **Duchenne Muscular Dystrophy (DMD)** is typically caused by frameshift mutations (severe), whereas **Becker Muscular Dystrophy** is caused by non-frameshift (in-frame) mutations (milder). * **Tay-Sachs Disease:** Often caused by a 4-base pair insertion in the HEXA gene (a classic frameshift example). * **Key Rule:** If an insertion/deletion is a **multiple of three**, it is an "in-frame" mutation; the reading frame remains intact, but specific amino acids are added or lost (e.g., Cystic Fibrosis $\Delta$F508).

Question 223: A change from UAC to UAG represents which type of mutation?

A. Nonsense mutation (Correct Answer)
B. Frameshift mutation
C. Deletion
D. Missense mutation

Explanation: **Explanation:** The correct answer is **Nonsense mutation**. **1. Why it is correct:** A nonsense mutation occurs when a single nucleotide substitution results in a **premature stop codon** (UAA, UAG, or UGA) within the mRNA sequence. In this case, the codon **UAC** (which codes for the amino acid Tyrosine) is changed to **UAG** (the "Amber" stop codon). This leads to the premature termination of protein synthesis, resulting in a truncated, usually non-functional protein. **2. Why the other options are incorrect:** * **Frameshift mutation:** This occurs due to the **insertion or deletion** of a number of nucleotides not divisible by three. This shifts the entire reading frame downstream, altering all subsequent amino acids. A single base substitution (like UAC to UAG) does not shift the reading frame. * **Deletion:** This involves the physical removal of one or more nucleotides from the DNA sequence. While a deletion can cause a frameshift, the question describes a specific base substitution (C to G), not a loss of genetic material. * **Missense mutation:** This is a point mutation where a single nucleotide change results in a codon that codes for a **different amino acid** (e.g., UAC to UAU is silent, but UAC to UCC would be missense). Since UAG is a stop codon and not an amino acid, it is specifically classified as nonsense. **High-Yield Clinical Pearls for NEET-PG:** * **Stop Codons:** Remember them as **U** **A**re **A**way (UAA), **U** **A**re **G**one (UAG), and **U** **G**o **A**way (UGA). * **Transition vs. Transversion:** UAC to UAG is a **transversion** (Pyrimidine 'C' replaced by Purine 'G'). * **Clinical Example:** Nonsense mutations are frequently seen in severe forms of **Beta-thalassemia** (β⁰) and **Duchenne Muscular Dystrophy (DMD)**. * **Nonsense-mediated decay:** The cell often recognizes and degrades mRNA containing premature stop codons to prevent the accumulation of toxic truncated proteins.

Question 224: Euchromatin is the region of DNA that is relatively

A. Uncondensed (Correct Answer)
B. Condensed
C. Overcondensed
D. Partially condensed

Explanation: **Explanation:** Chromatin exists in two functional states within the nucleus: **Euchromatin** and **Heterochromatin**. This distinction is based on the degree of DNA packaging and transcriptional activity. **Why Option A is Correct:** **Euchromatin** represents the "active" portion of the genome. It is **uncondensed** (loosely packed), which allows the transcriptional machinery, such as RNA polymerase and transcription factors, to access the DNA sequence. Under light microscopy, it appears light-stained because of its low density. Chemically, it is characterized by high levels of **histone acetylation**, which neutralizes the positive charge of histones, weakening their bond with DNA and promoting an "open" configuration. **Why Other Options are Incorrect:** * **Options B & C (Condensed/Overcondensed):** These describe **Heterochromatin**. Heterochromatin is tightly packed, appears dark-stained (basophilic), and is transcriptionally silent. It is characterized by **histone methylation** and deacetylation. * **Option D (Partially condensed):** While chromatin structure is dynamic, "uncondensed" is the specific technical term used to define the functional state of euchromatin in medical genetics. **High-Yield Clinical Pearls for NEET-PG:** * **Barr Body:** This is a classic example of **facultative heterochromatin** (an inactivated X chromosome in females). * **Histone Acetylation:** Increases transcription (associated with Euchromatin). * **DNA Methylation:** Usually "mutes" DNA (associated with Heterochromatin/Gene silencing). * **Constitutive Heterochromatin:** Regions that are always condensed (e.g., centromeres and telomeres).

Question 225: Xeroderma pigmentosum is caused due to:

A. DNA synthesis defect
B. Ectodermal cell migration defect
C. Abnormal nucleotide excision repair (Correct Answer)
D. Melanocyte proliferation

Explanation: **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by extreme sensitivity to ultraviolet (UV) radiation. 1. **Why Option C is Correct:** The core defect in XP is a mutation in the genes responsible for **Nucleotide Excision Repair (NER)**. Normally, UV light causes the formation of **pyrimidine dimers** (specifically thymine dimers) in DNA. The NER pathway is responsible for recognizing these bulky lesions, excising the damaged oligonucleotide strand (via endonucleases), and synthesizing new DNA. In XP patients, this repair mechanism fails, leading to the accumulation of mutations, skin carcinogenesis, and premature aging of the skin. 2. **Why Other Options are Incorrect:** * **Option A:** DNA synthesis (replication) is generally functional in XP; the issue is the inability to *repair* damage post-synthesis. * **Option B:** Ectodermal cell migration defects are associated with conditions like Waardenburg syndrome or certain craniofacial abnormalities, not DNA repair. * **Option C:** Melanocyte proliferation is a feature of nevi or melanoma, but it is a *consequence* of the unrepaired DNA damage in XP, not the primary cause. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Features:** Severe sunburn with minimal sun exposure, "parchment-like" skin (xeroderma), hyperpigmentation (pigmentosum), and a 1000-fold increased risk of skin cancers (BCC, SCC, and Melanoma). * **Enzyme Deficient:** Specifically, the **UV-specific endonuclease** (exinuclease) is often cited in exams. * **Associated Condition:** **Cockayne Syndrome** also involves NER defects but presents with "Mickey Mouse" facies and dwarfism without a significant increase in skin cancer. * **Diagnostic Test:** Chromosomal strand break analysis or unscheduled DNA synthesis (UDS) assays.

Question 226: Which of the following statements regarding telomerase is true?

A. It is a reverse transcriptase. (Correct Answer)
B. It is active in most somatic cells.
C. It is involved in protein translation.
D. It contributes to the shortening of DNA.

Explanation: **Explanation:** **1. Why Option A is Correct:** Telomerase is a specialized **ribonucleoprotein enzyme** that functions as a **RNA-dependent DNA polymerase**, also known as **reverse transcriptase**. It contains an internal RNA template (hTR) which it uses to synthesize repetitive DNA sequences (TTAGGG in humans) at the 3' ends of chromosomes (telomeres). This mechanism counteracts the "end-replication problem," where DNA polymerase cannot fully replicate the lagging strand, preventing the loss of vital genetic information. **2. Why Other Options are Incorrect:** * **Option B:** In humans, telomerase is highly active in **germ cells, stem cells, and cancer cells**. However, it is **inactive or has very low activity in most differentiated somatic cells**, leading to progressive telomere shortening and cellular senescence (the Hayflick limit). * **Option C:** Telomerase is involved in **DNA replication/maintenance**, not protein translation. Translation is the process of synthesizing proteins from mRNA via ribosomes. * **Option D:** Telomerase **prevents** the shortening of DNA. It is the *absence* of telomerase activity that leads to the progressive shortening of chromosomes during each cell division. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cancer Link:** Approximately 85–90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as double-stranded DNA breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by mutations in telomerase components, leading to premature aging and bone marrow failure. * **Sequence:** Remember the human telomeric repeat sequence: **5’-TTAGGG-3’**.

Question 227: During which phase of the cell cycle does DNA synthesis occur?

A. G1
B. S (Correct Answer)
C. G2
D. M

Explanation: **Explanation:** The cell cycle is a highly regulated sequence of events that leads to cell division. The correct answer is **S phase (Synthesis phase)**. **1. Why S phase is correct:** The S phase is the specific period during the interphase where **DNA replication** occurs. During this stage, the DNA content of the cell doubles (from 2n to 4n in diploid cells), ensuring that each daughter cell receives an identical set of chromosomes. Key enzymes involved here include DNA Polymerase, Helicase, and Topoisomerase. Additionally, **histone protein synthesis** occurs simultaneously to package the newly formed DNA. **2. Why other options are incorrect:** * **G1 (Gap 1):** This is the pre-synthetic phase. The cell grows in size and synthesizes RNA and proteins required for DNA replication, but no DNA synthesis occurs here. * **G2 (Gap 2):** This is the post-synthetic phase. The cell prepares for mitosis by synthesizing tubulin for spindle fibers and checking the replicated DNA for errors. * **M (Mitosis):** This is the actual phase of nuclear and cytoplasmic division (Prophase, Metaphase, Anaphase, Telophase). DNA is condensed and segregated, but not synthesized. **Clinical Pearls for NEET-PG:** * **G0 Phase:** Quiescent stage where cells (like neurons or muscle cells) exit the cycle and stop dividing. * **Checkpoints:** The **G1-S checkpoint** (regulated by Cyclin D/CDK4 and Rb protein) is the "Restriction Point"—once passed, the cell is committed to division. * **Pharmacology Link:** Many chemotherapy drugs are "S-phase specific," such as **Methotrexate, 5-Fluorouracil, and Cytarabine**, as they inhibit DNA synthesis.

Question 228: Which of the following statements about microRNAs (miRNAs) is false?

A. They can encode proteins. (Correct Answer)
B. They are typically 21-30 nucleotides in length.
C. They were discovered by Andrew Fire and Craig Mello.
D. They function by inhibiting gene expression.

Explanation: **Explanation:** **1. Why Option A is the Correct Answer (The False Statement):** MicroRNAs (miRNAs) are a class of **non-coding RNAs**. Their primary characteristic is that they do **not** serve as templates for translation into proteins. Instead, they function as regulatory molecules that modulate the expression of other protein-coding genes. In the central dogma, while mRNA carries the code for proteins, miRNA acts as a "silencer" of that code. **2. Analysis of Other Options:** * **Option B (Length):** This is true. Mature miRNAs are small, single-stranded RNA molecules, typically ranging from **21 to 25 nucleotides** (often generalized to 30 in broader contexts). * **Option C (Discovery):** This is true. **Andrew Fire and Craig Mello** were awarded the Nobel Prize in 2006 for their discovery of RNA interference (RNAi), specifically demonstrating how double-stranded RNA can silence gene expression. * **Option D (Function):** This is true. miRNAs inhibit gene expression post-transcriptionally. They bind to the 3' untranslated region (UTR) of target mRNAs, leading to either **mRNA degradation** or **inhibition of translation**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Biogenesis:** miRNAs are transcribed by **RNA Polymerase II**. Key enzymes involved in their processing include **Drosha** (in the nucleus) and **Dicer** (in the cytoplasm). * **RISC Complex:** miRNAs function by being loaded into the **RNA-induced silencing complex (RISC)**. * **OncomiRs:** miRNAs that are dysregulated in cancer. Some act as tumor suppressors, while others act as oncogenes. * **Therapeutic Potential:** Synthetic miRNA mimics or "antagomirs" (inhibitors) are being researched as targeted therapies for genetic disorders and malignancies.

Question 229: All of the following are associated with changes in genetic material, except?

A. Insertion
B. Deletion
C. Robertsonian translocation
D. Inversion (Correct Answer)

Explanation: ### Explanation The core concept of this question lies in distinguishing between **quantitative** changes (gain or loss of genetic material) and **structural** rearrangements. **1. Why Inversion is the Correct Answer:** An **Inversion** occurs when a segment of a chromosome breaks off, flips 180 degrees, and reattaches to the same chromosome. Crucially, this is a **balanced rearrangement**. There is no net gain or loss of genetic material; the genes are simply rearranged in a different linear order. Unless the break occurs exactly within a functional gene or a regulatory region, inversions are typically phenotypically silent. **2. Analysis of Incorrect Options:** * **Insertion (A):** This involves the addition of one or more nucleotide base pairs into a DNA sequence. This represents a net **increase** (gain) in genetic material. * **Deletion (B):** This involves the removal of a segment of DNA, ranging from a single base pair to a large chromosomal region. This represents a net **loss** of genetic material. * **Robertsonian Translocation (C):** This occurs between acrocentric chromosomes (13, 14, 15, 21, 22). The long arms fuse to form one large chromosome, while the short arms (p-arms) are lost. Because the p-arms of these chromosomes contain redundant rRNA genes, the loss is usually tolerated, but it is still a physical **loss** of genetic material. **3. Clinical Pearls for NEET-PG:** * **Paracentric Inversion:** Does not involve the centromere. * **Pericentric Inversion:** Involves the centromere (Hint: **Peri**centric = **P**resent centromere). * **Robertsonian Translocation:** Most common is between chromosomes **14 and 21**. This is a high-yield cause of Familial Down Syndrome. * **Balanced vs. Unbalanced:** Inversions and Translocations are "Balanced" (no change in amount), whereas Deletions and Insertions are "Unbalanced." The question asks for "changes in genetic material," implying a change in the *quantity* or *content* rather than just the *position*.

Question 230: Which one of the following is the complementary sequence of 5' TTAAGCTAC 3'?

A. 5' GTAGCTTAA 3' (Correct Answer)
B. 5' AATTCGCATG 3'
C. 5' CATGCGAATT 3'
D. 5' TTAAGCGTAC 3'

Explanation: **Explanation:** In molecular biology, DNA strands are **antiparallel** and follow **Chargaff’s rule** of base pairing. To find the complementary sequence, two rules must be applied: 1. **Base Pairing:** Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). 2. **Directionality:** The complementary strand runs in the opposite direction (3' to 5') relative to the template (5' to 3'). **Step-by-step derivation:** * **Template:** 5' T T A A G C T A C 3' * **Complement (3' to 5'):** 3' A A T T C G A T G 5' * **Reverse for standard notation (5' to 3'):** 5' G T A G C T T A A 3' **Analysis of Options:** * **Option A (Correct):** This is the 3' to 5' complement written in the standard 5' to 3' direction. * **Option B:** This is a "direct" complement (A for T, T for A) but fails to account for the antiparallel nature; it incorrectly keeps the 5' end matching the 5' end of the template. * **Option C:** This is a random sequence that does not follow base-pairing rules for the given template. * **Option D:** This is nearly identical to the original sequence with minor changes, representing a common distractor for students who do not reverse the direction. **Clinical Pearls for NEET-PG:** * **Chargaff’s Rule:** In double-stranded DNA, A+G (purines) = T+C (pyrimidines). This rule does not apply to single-stranded RNA or DNA. * **Bonding:** A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3 hydrogen bonds**. High G-C content increases the **Melting Temperature (Tm)** of DNA. * **Directionality:** DNA polymerase always synthesizes DNA in the **5' to 3' direction**, adding nucleotides to the 3' hydroxyl (-OH) group.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free