Molecular Biology and Genomics Practice Questions

Q: What is the function of mitochondrial DNA?

Encodes proteins of the respiratory chain. ### Explanation **Correct Answer: B. Encodes proteins of the respiratory chain** Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule (approx. 16.5 kb) that follows maternal inheritance. Its primary function is to encode essential components of the **Oxidative Phosphorylation (OXPHOS)** system. Specifically, human mtDNA encodes **13 polypeptides**, all of which are subunits of the respiratory chain complexes (Complex I, III, IV, and V). It also encodes 2 rRNAs and 22 tRNAs required for their translation within the mitochondria. **Why other options are incorrect:** * **Option A:** Proteins of the cell membrane are encoded by **nuclear DNA** and synthesized by ribosomes in the cytosol or on the rough endoplasmic reticulum. * **Option C:** While mitochondria replicate independently of the cell cycle, the overall process of cell replication (mitosis/meiosis) is governed by nuclear DNA and regulatory proteins like cyclins and CDKs. * **Option D:** While mtDNA does form its own specific mitochondrial rRNAs (12S and 16S), the vast majority of cellular rRNA (which forms the cytoplasmic ribosomes) is synthesized in the **nucleolus** by RNA Polymerase I. **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother; thus, mitochondrial diseases (e.g., **MELAS, LHON, MERRF**) affect both sons and daughters, but only daughters can pass the trait to the next generation. * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (normal and mutated mtDNA) within a cell. This explains the variable clinical severity of mitochondrial diseases. * **High Mutation Rate:** mtDNA lacks histones and has less efficient repair mechanisms compared to nuclear DNA, making it 10 times more prone to mutations.

Q: Which of the following statements describing restriction endonucleases is true?

They cleave both strands in duplex DNA. **Explanation:** **Restriction Endonucleases (REs)**, often called "molecular scissors," are bacterial enzymes used in recombinant DNA technology. The correct answer is **D** because these enzymes function by recognizing specific palindromic sequences and catalyzing the hydrolysis of phosphodiester bonds on **both strands** of the DNA duplex. **Why the other options are incorrect:** * **Option A:** While many REs (like *EcoRI*) create "sticky" or overhanging ends, others (like *SmaI*) cut straight across the duplex to produce **blunt ends**. Therefore, they do not *always* yield overhanging ends. * **Option B:** REs generally **do not** recognize methylated DNA. In fact, bacteria use **DNA methylation** (via methyltransferases) to protect their own genome from being cleaved by their own restriction enzymes. This is known as the Restriction-Modification System. * **Option C:** REs recognize specific **palindromic sequences** (typically 4–8 base pairs long that read the same 5'→3' on both strands), not triplet repeats. Triplet repeats are associated with genetic disorders like Huntington’s disease. **High-Yield Clinical Pearls for NEET-PG:** * **Type II REs** are the most commonly used in labs because they cut DNA at specific, predictable sites and do not require ATP. * **Restriction Fragment Length Polymorphism (RFLP):** A technique using REs to detect genetic variations (e.g., in sickle cell anemia, where a mutation abolishes a specific *MstII* recognition site). * **Palindromes:** Remember that REs look for symmetry. Example: 5'-GAATTC-3' (the complementary strand is also 5'-GAATTC-3').

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).

Question 1

What is the function of mitochondrial DNA?

Accepted Answer

Encodes proteins of the respiratory chain

Answer

Encodes proteins of the cell membrane

Answer

Helps in cell replication

Answer

Formation of rRNA

Question 2

Which of the following statements describing restriction endonucleases is true?

Accepted Answer

They cleave both strands in duplex DNA

Answer

They always yield overhanging single-stranded ends

Answer

They recognize methylated DNA sequences

Answer

They recognize triplet repeats

Question 3

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 4

What is the rapid method for chromosome identification in interphase?

Accepted Answer

FISH

Answer

PCR

Answer

SSCP

Answer

Karyotyping

Question 5

Which of the following eukaryotic transcription factors recognizes the promoter?

Accepted Answer

TFIID

Answer

Rho

Answer

Pribnow box

Answer

Sigma

Question 6

Which enzyme is responsible for DNA supercoiling?

Accepted Answer

DNA gyrase

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

DNA topoisomerase

Question 7

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 8

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 9

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

Accepted Answer

Methylation

Answer

Acetylation

Answer

Phosphorylation

Answer

DNA Replication

Question 10

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

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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