Molecular Biology and Genomics Practice Questions

Q: Apolipoprotein B48 and Apolipoprotein B100 are synthesized from the same mRNA due to a difference in what process?

Differential RNA processing. ### Explanation The synthesis of Apolipoprotein B48 (Apo B48) and Apolipoprotein B100 (Apo B100) from a single gene is a classic example of **RNA Editing**, which falls under the broader category of **Differential RNA processing**. **Why the correct answer is right:** Both proteins are encoded by the *APOB* gene. In the liver, the mRNA is translated directly to produce **Apo B100** (the full-length protein). However, in the small intestine, a specific enzyme called **Cytidine Deaminase** acts on the mRNA. It converts a Cytosine (C) to Uracil (U) at codon 2153. This change transforms the original glutamine codon (**CAA**) into a premature stop codon (**UAA**). Consequently, translation terminates early, producing **Apo B48**, which represents the N-terminal 48% of the full protein. This tissue-specific modification of the mRNA sequence after transcription is a form of differential RNA processing. **Why incorrect options are wrong:** * **Splicing (Options A & B):** Splicing involves the removal of introns and joining of exons. While "Alternative Splicing" can create different proteins from one gene (e.g., Calcitonin/CGRP), the Apo B diversity is specifically due to **base substitution (editing)**, not the rearrangement of exons. * **Chemical modification (Option A):** While RNA editing is a chemical change (deamination), "chemical modification" usually refers to 5' capping or 3' polyadenylation, which do not change the coding sequence to create different protein isoforms in this context. **NEET-PG High-Yield Pearls:** * **Apo B100:** Found in VLDL, IDL, and LDL; synthesized in the **Liver**. * **Apo B48:** Found in Chylomicrons; synthesized in the **Small Intestine**. * **Enzyme:** Cytidine Deaminase (C $\rightarrow$ U editing). * **Memory Aid:** **L**iver = **L**ong (B100); **S**mall Intestine = **S**hort (B48).

Q: Which of the following eukaryotic transcription factors recognizes the promoter?

TFIID. ### Explanation In eukaryotes, RNA Polymerase II cannot initiate transcription alone; it requires a group of proteins called **General Transcription Factors (GTFs)**. **1. Why TFIID is correct:** **TFIID** is the first component of the basal transcription apparatus to bind to the promoter. It is a multi-protein complex consisting of the **TATA-binding protein (TBP)** and TBP-associated factors (TAFs). TBP specifically recognizes and binds to the **TATA box** (located approximately 25-30 base pairs upstream of the transcription start site), effectively "marking" the promoter for the recruitment of RNA Polymerase II and other GTFs (TFIIA, TFIIB, TFIIE, TFIIF, and TFIIH). **2. Why the other options are incorrect:** * **Rho (A):** This is a protein involved in **prokaryotic** transcription termination (Rho-dependent termination). It is not involved in promoter recognition. * **Pribnow box (B):** This is a **DNA sequence** (TATAAT) found in **prokaryotic** promoters (the -10 element). It is a site, not a transcription factor. * **Sigma (C):** The **Sigma factor ($\sigma$)** is a subunit of the **prokaryotic** RNA polymerase holoenzyme. It is responsible for promoter recognition in bacteria, not eukaryotes. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **TFIIH** is unique because it has both **helicase** activity (to unwind DNA) and **kinase** activity (to phosphorylate the CTD of RNA Polymerase II). * Defects in TFIIH subunits are associated with clinical conditions like **Xeroderma Pigmentosum** and **Cockayne Syndrome** due to its role in Nucleotide Excision Repair (NER). * **Alpha-amanitin** (from the *Amanita phalloides* mushroom) specifically inhibits **RNA Polymerase II**, leading to severe hepatotoxicity.

Q: Which enzyme is responsible for DNA supercoiling?

DNA gyrase. **Explanation:** The correct answer is **DNA gyrase**. DNA supercoiling is a critical process that manages the topological stress induced during the unwinding of the DNA double helix. **1. Why DNA Gyrase is Correct:** DNA gyrase is a specialized **Type II Topoisomerase** found in prokaryotes. It is unique because it is the only enzyme capable of actively introducing **negative supercoils** into DNA using energy derived from ATP hydrolysis. This process neutralizes the positive supercoiling (overwinding) that occurs ahead of the replication fork, allowing DNA replication and transcription to proceed smoothly. **2. Why the Other Options are Incorrect:** * **DNA Polymerase I:** Primarily involved in DNA repair and the removal of RNA primers (via its 5' to 3' exonuclease activity) during lagging strand synthesis. * **DNA Polymerase II:** Mainly functions in DNA repair mechanisms when the replication fork stalls. * **DNA Topoisomerase:** While DNA gyrase is a *type* of topoisomerase, "DNA Topoisomerase" (specifically Type I) generally relaxes supercoils by cutting a single strand without requiring ATP. In the context of NEET-PG, if "DNA gyrase" is an option, it is the most specific and correct answer for the active introduction of supercoils. **3. Clinical Pearls & High-Yield Facts:** * **Pharmacology Link:** DNA gyrase is the primary target of **Quinolones and Fluoroquinolones** (e.g., Ciprofloxacin). These drugs inhibit the enzyme, leading to double-strand breaks and bacterial cell death. * **Eukaryotic Equivalent:** Eukaryotes do not have DNA gyrase; they use Topoisomerase II to relax supercoils, which is targeted by anticancer drugs like **Etoposide**. * **Energy Requirement:** Remember that Type I Topoisomerases are ATP-independent, whereas Type II (including Gyrase) are **ATP-dependent**.

Q: A 10-year-old child presents with short stature, external ophthalmoplegia, gait abnormality, and sensorineural hearing loss. Lab findings reveal elevated fasting blood glucose levels. What is true about the DNA involved in this condition?

Circular. **Explanation:** The clinical presentation of short stature, external ophthalmoplegia, gait abnormality (ataxia), sensorineural hearing loss, and diabetes mellitus (elevated glucose) is characteristic of **Kearns-Sayre Syndrome (KSS)**. KSS is a mitochondrial myopathy caused by large-scale deletions in **Mitochondrial DNA (mtDNA)**. **1. Why the Correct Answer is Right:** Mitochondrial DNA differs significantly from nuclear DNA. It is **circular**, double-stranded, and lacks introns. Unlike nuclear DNA, it is not enclosed within a nucleus but resides in the mitochondrial matrix. Because KSS is a mitochondrial disorder, the DNA involved is circular. **2. Why Incorrect Options are Wrong:** * **Double-helix (Option B):** While mtDNA is double-stranded, "Circular" is the more specific defining characteristic used to differentiate it from the linear structure of nuclear DNA in medical exams. * **Bounded by nuclear envelope (Option C):** This describes nuclear DNA. mtDNA is located in the mitochondria, reflecting its endosymbiotic evolutionary origin. * **Packed into chromatin (Option D):** Nuclear DNA is wrapped around histone proteins to form chromatin. mtDNA is "naked" (lacks histones) and is instead organized into structures called nucleoids. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Inheritance:** Transmitted only through the mother (maternal inheritance). All children of an affected mother are at risk, but an affected father cannot pass it on. * **Heteroplasmy:** The coexistence of mutated and wild-type mtDNA within a single cell, explaining the clinical variability in mitochondrial diseases. * **Mitochondrial DNA Facts:** Encodes 13 polypeptides (oxidative phosphorylation subunits), 22 tRNAs, and 2 rRNAs. It has a higher mutation rate than nuclear DNA due to a lack of robust repair mechanisms and proximity to free radicals.

Q: The CG region is involved in which of the following processes?

Methylation. **Explanation:** The correct answer is **Methylation**. In eukaryotic genomes, DNA methylation occurs predominantly at **CpG islands**—regions where a Cytosine nucleotide is followed by a Guanine nucleotide (connected by a phosphate group). **1. Why Methylation is Correct:** DNA methylation is a key epigenetic mechanism catalyzed by **DNA Methyltransferases (DNMTs)**. The enzyme adds a methyl group to the 5th carbon of the cytosine ring, forming **5-methylcytosine**. CpG islands are typically located in or near promoter regions of genes. When these regions are methylated, it generally leads to **gene silencing** (transcriptional repression) by preventing the binding of transcription factors or recruiting chromatin-remodeling proteins. **2. Why Other Options are Incorrect:** * **Acetylation:** This primarily occurs on **lysine residues of histone proteins**, not directly on the DNA sequence. Histone acetylation (by HATs) usually relaxes chromatin (euchromatin) and activates transcription. * **Phosphorylation:** This is a post-translational modification of proteins (e.g., enzymes or histones) or a step in nucleotide metabolism, but it is not the primary regulatory modification of CG regions in DNA. * **DNA Replication:** While CG-rich regions can influence the stability of the DNA helix due to triple hydrogen bonding, "CG regions" as a regulatory motif are specifically associated with the biochemical process of methylation. **Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** Methylation of CpG islands is the basis for imprinting (e.g., **Prader-Willi** and **Angelman syndromes**). * **Fragile X Syndrome:** Characterized by CGG triplet repeat expansion leading to hypermethylation of the FMR1 gene and subsequent silencing. * **Cancer:** Hypermethylation of tumor suppressor genes (like *p16*) is a common finding in various malignancies. * **SAM (S-adenosylmethionine):** Acts as the universal methyl donor for this reaction.

Q: Real-time PCR is primarily used for what purpose?

Quantifying the extent of DNA amplification. **Explanation:** Real-time PCR, also known as **Quantitative PCR (qPCR)**, is a modification of the standard PCR technique. While traditional PCR only allows for "end-point" analysis (checking results after all cycles are finished), qPCR monitors the amplification process **as it occurs** (in real-time). **Why Option D is Correct:** The core principle of qPCR is the use of fluorescent dyes (like SYBR Green) or sequence-specific probes (like TaqMan). These markers emit fluorescence proportional to the amount of DNA synthesized. By measuring this fluorescence at each cycle, clinicians can determine the initial concentration of the target DNA template. This makes it the gold standard for **quantification**. **Analysis of Incorrect Options:** * **Option A:** Amplification of RNA requires **Reverse Transcriptase PCR (RT-PCR)**. While qPCR is often combined with RT-PCR to measure RNA levels (RT-qPCR), the "Real-time" component specifically refers to the quantification, not the RNA-to-DNA conversion. * **Option B:** This describes **Standard PCR**. While qPCR does amplify DNA, its *primary* and distinguishing purpose is quantification. * **Option C:** PCR techniques are exclusive to nucleic acids. Protein amplification is not a function of PCR (Proteins are analyzed via Western Blot or ELISA). **High-Yield Clinical Pearls for NEET-PG:** * **Cycle Threshold (Ct) Value:** The number of cycles required for the fluorescent signal to cross the background threshold. **Lower Ct = Higher initial viral/DNA load.** * **Clinical Use:** It is the diagnostic mainstay for measuring **Viral Load** (e.g., HIV, Hepatitis B/C) and monitoring response to antiviral therapy. * **Distinction:** Do not confuse **RT-PCR** (Reverse Transcriptase) with **Real-time PCR**. RT-PCR handles RNA; Real-time PCR handles quantification. (Note: COVID-19 testing uses both: RT-qPCR).

Q: Which of the following methods cannot be used to detect subtelomeric fractions in children with idiopathic MR?

Laser diode index. **Explanation:** Subtelomeric rearrangements (deletions or duplications) are a significant cause of idiopathic mental retardation (MR), accounting for approximately 5–10% of cases. Detecting these micro-aberrations requires high-resolution molecular cytogenetic techniques. **Why "Laser Diode Index" is the correct answer:** A **Laser Diode** is a semiconductor device used in optical fiber communications and barcode readers; it is **not a diagnostic medical technique** for genetic analysis. There is no established "Laser Diode Index" used in cytogenetics or the detection of chromosomal fractions. **Analysis of Incorrect Options:** * **FISH (Fluorescence In Situ Hybridization):** Historically the "gold standard" for detecting subtelomeric deletions. It uses fluorescent probes specific to the telomeric regions to visualize missing or extra segments under a microscope. * **MALDI (Matrix-Assisted Laser Desorption/Ionization):** This is a type of Mass Spectrometry (MALDI-TOF). It is used for high-throughput analysis of DNA fragments and can detect single nucleotide polymorphisms (SNPs) and small deletions/insertions in subtelomeric regions. * **Array CGH (Comparative Genomic Hybridization):** Currently the first-line investigation for children with unexplained developmental delay or MR. It provides a much higher resolution than standard karyotyping, allowing for the detection of submicroscopic copy number variants (CNVs) across the entire genome, including subtelomeres. **Clinical Pearls for NEET-PG:** * **First-line investigation** for idiopathic MR/Developmental Delay: **Array CGH** (Microarray). * **Standard Karyotyping** resolution is ~5 Mb; subtelomeric rearrangements are often <5 Mb, necessitating molecular techniques. * Common subtelomeric syndrome: **22q13.3 deletion syndrome** (Phelan-McDermid syndrome).

Q: What is telomerase?

RNA dependent DNA polymerase. **Explanation:** **Telomerase** is a specialized enzyme responsible for maintaining the length of telomeres (the repetitive TTAGGG sequences at the ends of eukaryotic chromosomes). 1. **Why Option B is Correct:** Telomerase is a **ribonucleoprotein** complex. It contains an internal RNA template (known as TERC) that it uses to synthesize complementary DNA sequences. Because it uses an **RNA template** to synthesize **DNA**, it is functionally an **RNA-dependent DNA polymerase** (a type of reverse transcriptase). This activity prevents the "end-replication problem," where chromosomes shorten with each cell division. 2. **Why Other Options are Incorrect:** * **Option A (DNA-dependent RNA polymerase):** These enzymes (like RNA Polymerase I, II, and III) use DNA as a template to synthesize RNA (transcription). * **Option C (RNA-dependent RNA polymerase):** These are primarily found in RNA viruses (e.g., Poliovirus, SARS-CoV-2) to replicate their RNA genomes; they are not naturally occurring in human telomere maintenance. * **Option D (DNA-dependent DNA polymerase):** These are standard DNA polymerases (e.g., Pol $\alpha, \delta, \epsilon$) used in semi-conservative DNA replication. **High-Yield Clinical Pearls for NEET-PG:** * **Cellular Aging:** Telomerase is active in germ cells, stem cells, and cancer cells, but is **absent or low in most somatic cells**, leading to cellular senescence (the Hayflick limit). * **Cancer:** Approximately 85-90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Components:** It consists of **TERT** (Telomerase Reverse Transcriptase - the catalytic protein) and **TERC** (Telomerase RNA component - the template). * **Clinical Correlation:** Mutations in telomerase components can lead to **Dyskeratosis Congenita**, characterized by bone marrow failure and premature aging.

Question 1

Apolipoprotein B48 and Apolipoprotein B100 are synthesized from the same mRNA due to a difference in what process?

Accepted Answer

Differential RNA processing

Answer

Splicing and chemical modification

Answer

Splicing

Answer

All of the above

Question 2

What is the rapid method for chromosome identification in interphase?

Accepted Answer

FISH

Answer

PCR

Answer

SSCP

Answer

Karyotyping

Question 3

Which of the following eukaryotic transcription factors recognizes the promoter?

Accepted Answer

TFIID

Answer

Rho

Answer

Pribnow box

Answer

Sigma

Question 4

Which enzyme is responsible for DNA supercoiling?

Accepted Answer

DNA gyrase

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

DNA topoisomerase

Question 5

A four-year-old child is diagnosed with Duchenne muscular dystrophy, an X-linked recessive disorder. Genetic analysis shows that the patient's gene for the muscle protein dystrophin contains a mutation in its promoter region. What would be the most likely effect of this mutation?

Accepted Answer

Initiation of dystrophin transcription will be deficient.

Answer

Tailing of dystrophin mRNA will be defective.

Answer

Capping of dystrophin mRNA will be defective.

Answer

Termination of dystrophin transcription will be deficient.

Question 6

A 10-year-old child presents with short stature, external ophthalmoplegia, gait abnormality, and sensorineural hearing loss. Lab findings reveal elevated fasting blood glucose levels. What is true about the DNA involved in this condition?

Accepted Answer

Circular

Answer

Double-helix

Answer

Bounded by nuclear envelope

Answer

Packed into chromatin

Question 7

The CG region is involved in which of the following processes?

Accepted Answer

Methylation

Answer

Acetylation

Answer

Phosphorylation

Answer

DNA Replication

Question 8

Real-time PCR is primarily used for what purpose?

Accepted Answer

Quantifying the extent of DNA amplification

Answer

The amplification of RNA molecules

Answer

The amplification of specific DNA segments

Answer

The amplification of proteins

Question 9

Which of the following methods cannot be used to detect subtelomeric fractions in children with idiopathic MR?

Accepted Answer

Laser diode index

Answer

FISH

Answer

MALDI

Answer

Array comparative genomic hybridization

Question 10

What is telomerase?

Accepted Answer

RNA dependent DNA polymerase

Answer

DNA dependent RNA polymerase

Answer

RNA dependent RNA polymerase

Answer

DNA dependent DNA polymerase

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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