Molecular Biology and Genomics Practice Questions

Q: Primers are removed by which of the following enzymes, except?

None of the above. In eukaryotic DNA replication, the removal of RNA primers is a coordinated process involving multiple enzymes. The question asks which enzyme does **not** participate in this process; since all listed enzymes are involved, the correct answer is **None of the above**. ### **Mechanism of Primer Removal (Eukaryotes)** Unlike prokaryotes (where DNA Polymerase I removes primers via 5’→3’ exonuclease activity), eukaryotes utilize a "strand displacement" mechanism: 1. **DNA Polymerase δ (Delta):** As it synthesizes the Okazaki fragment, it encounters the RNA primer of the preceding fragment. It displaces the 5’ end of the primer, creating a "flap." 2. **RNase H1:** This enzyme recognizes and degrades the RNA portion of the RNA-DNA hybrid primer, leaving a single ribonucleotide attached to the DNA. 3. **FEN1 (Flap Endonuclease 1):** This enzyme acts as an endonuclease to clip off the RNA "flaps" displaced by Polymerase δ. It is essential for removing the final ribonucleotide and ensuring the gap is ready for ligation. ### **Analysis of Options** * **Option A (Delta Polymerase):** Correctly involved; it provides the motor force to displace the primer. * **Option B (RNase H1):** Correctly involved; it degrades the bulk of the RNA primer. * **Option C (FEN1):** Correctly involved; it cleaves the displaced flap to complete primer removal. ### **High-Yield Clinical Pearls for NEET-PG** * **Prokaryotic Equivalent:** In *E. coli*, **DNA Polymerase I** is the sole enzyme responsible for primer removal due to its unique **5’ to 3’ exonuclease activity**. * **DNA Ligase:** Once primers are removed and the gap is filled by Pol δ, DNA Ligase I seals the phosphodiester bond. * **PCNA (Proliferating Cell Nuclear Antigen):** Acts as a sliding clamp that increases the processivity of Pol δ and recruits FEN1 to the replication fork.

Q: Which of the following types of mutations generally leads to a truncated protein?

Frameshift mutation. ### Explanation **Why Frameshift Mutation is Correct:** A **frameshift mutation** occurs when a number of nucleotides (not divisible by three) are inserted into or deleted from a DNA sequence. Since the genetic code is read in non-overlapping triplets (codons), this shifts the "reading frame" of the mRNA. This shift almost inevitably leads to the premature encounter of a **stop codon** (UAA, UAG, or UGA) further downstream. The premature termination of translation results in a **truncated (shortened) protein**, which is usually non-functional and often degraded via nonsense-mediated decay. **Analysis of Incorrect Options:** * **Deletion/Insertion (Options A & C):** While these *can* cause a frameshift, they only do so if the number of bases involved is not a multiple of three. If 3, 6, or 9 bases are deleted/inserted (in-frame mutations), the reading frame remains intact, resulting in the loss or gain of specific amino acids without truncating the entire protein. * **Missense Mutation (Option D):** This is a point mutation where a single nucleotide change results in a codon that codes for a *different* amino acid. The protein length remains the same, though its function may be altered (e.g., Hemoglobin S in Sickle Cell Anemia). **High-Yield Clinical Pearls for NEET-PG:** * **Duchenne Muscular Dystrophy (DMD):** Caused by **frameshift mutations** leading to a complete absence of functional dystrophin (severe). * **Becker Muscular Dystrophy (BMD):** Caused by **in-frame mutations**, resulting in a truncated but partially functional dystrophin (milder). * **Nonsense Mutation:** Another key cause of protein truncation where a point mutation directly changes an amino acid codon into a stop codon. * **Tay-Sachs Disease:** Often caused by a 4-base pair insertion (frameshift) in the *HEXA* gene.

Q: In which of the following conditions does a trinucleotide repeat mutation occur in the coding regions of DNA?

Huntington's disease. ### Explanation Trinucleotide repeat expansion disorders are characterized by an increase in the number of specific three-nucleotide sequences. The clinical manifestation depends on whether the expansion occurs in a **coding region** (exons) or a **non-coding region** (introns or UTRs). #### Why Huntington’s Disease is Correct: **Huntington’s disease** is caused by a **CAG** repeat expansion within the **coding region** (exon 1) of the *HTT* gene on chromosome 4. Since CAG codes for the amino acid **Glutamine**, these are known as **Polyglutamine (PolyQ) diseases**. The expansion leads to a "gain-of-function" mutation, resulting in an abnormally long polyglutamine tract that causes the huntingtin protein to misfold and become neurotoxic. #### Why Other Options are Incorrect: * **Myotonic Dystrophy (Type 1):** The **CTG** repeat occurs in the **3' Untranslated Region (UTR)** of the *DMPK* gene. It is not translated into protein but causes RNA toxicity. * **Fragile X Syndrome:** The **CGG** repeat occurs in the **5' Untranslated Region (UTR)** of the *FMR1* gene. This leads to hypermethylation of the promoter, gene silencing, and a loss of protein function. * **Friedreich’s Ataxia:** The **GAA** repeat occurs within the **first intron** (non-coding) of the *FXN* gene, leading to impaired transcription and decreased levels of the protein frataxin. #### High-Yield Clinical Pearls for NEET-PG: * **Anticipation:** The phenomenon where the disease becomes more severe or has an earlier onset in successive generations (most prominent in Huntington’s when inherited from the father). * **Mnemonic for Huntington's:** "**C**AG: **C**audate atrophy, **A**cetylcholine decrease, **G**ABA decrease." * **Location Summary:** * **Fragile X:** 5' UTR (CGG) * **Friedreich’s Ataxia:** Intron (GAA) * **Huntington’s:** Exon (CAG) * **Myotonic Dystrophy:** 3' UTR (CTG)

Q: Which enzyme catalyzes the hydrolytic step leading to the release of the polypeptide chain from ribosomes?

Peptidyl transferase. **Explanation:** The process of translation termination occurs when a ribosome encounters a **stop codon** (UAA, UAG, or UGA) in the A-site. Since there are no aminoacyl-tRNAs that recognize these codons, **Release Factors (RFs)** bind to the ribosome instead. **1. Why Peptidyl Transferase is correct:** The enzyme **peptidyl transferase** (a ribozyme activity of the 28S rRNA in eukaryotes and 23S rRNA in prokaryotes) normally catalyzes peptide bond formation. However, during termination, the binding of Release Factors alters its specificity. Instead of transferring the growing polypeptide chain to an amino acid, it catalyzes the **hydrolysis** of the ester bond between the tRNA and the polypeptide chain using a water molecule. This hydrolytic step releases the completed protein into the cytosol. **2. Why other options are incorrect:** * **Stop codons (A):** These are the *signals* for termination, not the enzymes that catalyze the chemical reaction of release. * **AUG codon (C):** This is the initiation codon that codes for Methionine; it plays no role in the termination or release of the polypeptide. **High-Yield Clinical Pearls for NEET-PG:** * **Ribozyme:** Peptidyl transferase is a classic example of a ribozyme (an RNA molecule with catalytic activity). * **Energy Requirement:** Termination and ribosome dissociation require energy in the form of **GTP hydrolysis**. * **Antibiotic Link:** Macrolides (e.g., Erythromycin) and Chloramphenicol act by inhibiting the peptidyl transferase center, thereby halting bacterial protein synthesis. * **Release Factors:** In eukaryotes, a single factor (**eRF1**) recognizes all three stop codons.

Q: Which of the following statements regarding complementary DNA (cDNA) is NOT true?

It contains both noncoding and coding sequences.. ### Explanation **Why Option D is the Correct Answer (The "NOT True" Statement):** The fundamental characteristic of **cDNA (complementary DNA)** is that it is synthesized from a mature mRNA template. In eukaryotes, mature mRNA has already undergone **post-transcriptional modification (splicing)**, where introns (non-coding sequences) are removed and exons (coding sequences) are joined together. Therefore, cDNA contains **only coding sequences (exons)** and lacks the introns found in genomic DNA. This makes it essential for expressing eukaryotic genes in prokaryotic systems (like *E. coli*), which cannot perform splicing. **Analysis of Incorrect Options:** * **Option A:** This is a definition. cDNA is synthesized by using mRNA as a template, making it complementary to the RNA sequence. * **Option B:** The enzyme **Reverse Transcriptase** (an RNA-dependent DNA polymerase) is strictly required to transcribe the single-stranded RNA into a DNA strand. * **Option C:** To produce human insulin commercially, the human insulin cDNA is inserted into bacterial plasmids. Since bacteria cannot splice human pre-mRNA, using cDNA (which lacks introns) is the only way to ensure the correct protein is synthesized. **NEET-PG High-Yield Pearls:** * **cDNA Library vs. Genomic Library:** A cDNA library represents only the genes being actively expressed (the transcriptome) in a specific tissue, whereas a genomic library contains the entire genome (introns, exons, promoters). * **Reverse Transcriptase:** Originally derived from retroviruses (like HIV). It requires an **oligo-dT primer** that binds to the poly-A tail of mRNA to initiate synthesis. * **Clinical Use:** cDNA is used in **RT-PCR** (Reverse Transcription Polymerase Chain Reaction) to detect RNA viruses like SARS-CoV-2 or to measure gene expression levels in oncology.

Q: A patient presents with skin bullae upon sun exposure. A defect in which of the following is responsible for this condition?

Thymidine dimers. ### Explanation The clinical presentation of **skin bullae (blisters) upon sun exposure** is characteristic of **Xeroderma Pigmentosum (XP)**, an autosomal recessive genetic disorder. **1. Why Thymidine Dimers is Correct:** Ultraviolet (UV) radiation from sunlight causes adjacent pyrimidine bases (usually two thymines) in DNA to form abnormal covalent bonds, known as **Thymidine Dimers**. In healthy individuals, these are repaired by the **Nucleotide Excision Repair (NER)** pathway. Patients with XP have a genetic defect in the NER enzymes (specifically UV-specific endonucleases). This leads to an accumulation of unrepaired thymidine dimers, causing DNA mutations, cell death (blistering), and a significantly high risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **2. Why the Other Options are Incorrect:** * **Trinucleotide repeats:** These are associated with "Dynamic Mutations" seen in disorders like Huntington’s disease, Fragile X syndrome, and Friedreich’s ataxia, which do not involve photosensitivity. * **Sugar changes:** Alterations in the deoxyribose sugar backbone are not the primary mechanism of UV-induced damage or XP. * **DNA methylation:** This is an epigenetic modification involved in gene silencing (e.g., Genomic Imprinting disorders like Prader-Willi or Angelman syndromes) and is not triggered by UV light. **Clinical Pearls for NEET-PG:** * **Key Enzyme Defect:** UV-specific endonuclease (excision nuclease). * **Early Signs:** Severe sunburn after minimal exposure, "parchment-like" dry skin, and excessive freckling (lentigines) before age 2. * **Associated Condition:** **Cockayne Syndrome** also involves NER defects but presents with "Mickey Mouse" facies and dwarfism without a high risk of skin cancer. * **Diagnostic Test:** Chromosomal breakage study or measuring DNA repair rates in cultured fibroblasts.

Q: During unwinding of DNA, relief of supercoil is made possible by?

Topoisomerase. ### Explanation **Correct Answer: A. Topoisomerase** **Mechanism:** As DNA helicase unwinds the double helix, the DNA ahead of the replication fork becomes overwound, leading to **positive supercoiling** (torsional strain). If not relieved, this strain would halt replication. **Topoisomerases** are enzymes that relieve this tension by creating transient breaks in the DNA backbone. * **Topoisomerase I** cuts a single strand (no ATP required). * **Topoisomerase II** cuts both strands (requires ATP). Once the supercoil is relaxed, the enzyme ligates the phosphodiester bonds back together. **Why other options are incorrect:** * **B. Gyrase:** While DNA Gyrase is a specific type of Topoisomerase II found in **prokaryotes**, "Topoisomerase" is the more general and accurate term for the process of relieving supercoils across all organisms. In many exam contexts, if both are present, Topoisomerase is the broader physiological answer. * **C. Helicase:** This enzyme is responsible for **unwinding** the DNA double helix by breaking hydrogen bonds between nitrogenous bases. It *causes* the supercoiling ahead of the fork rather than relieving it. * **D. Polymerase:** DNA Polymerases are responsible for **synthesizing** new DNA strands by adding nucleotides; they do not possess the nuclease activity required to relax supercoils. **High-Yield Clinical Pearls for NEET-PG:** 1. **Fluoroquinolones** (e.g., Ciprofloxacin) inhibit bacterial **DNA Gyrase** and Topoisomerase IV. 2. **Anticancer Drugs:** * **Etoposide/Teniposide** inhibit Topoisomerase II. * **Irinotecan/Topotecan** inhibit Topoisomerase I. 3. **DNA Gyrase** is unique because it can introduce *negative* supercoils, a feature specific to bacteria.

Q: Maternal chromosome 15 disomy results in which of the following conditions?

Prader Willi Syndrome. This question tests your knowledge of **Genomic Imprinting**, a phenomenon where certain genes are expressed in a parent-of-origin-specific manner. ### Explanation of the Correct Answer **Prader-Willi Syndrome (PWS)** occurs due to the **loss of the paternal contribution** of the 15q11-q13 region. This can happen via three mechanisms: 1. **Paternal Deletion (70%):** Deletion of the long arm of paternal chromosome 15. 2. **Maternal Uniparental Disomy (UPD) (25-30%):** The individual inherits two copies of chromosome 15 from the mother and **none from the father**. Since the maternal genes in this region are normally "silenced" (imprinted), the absence of paternal genes leads to PWS. 3. **Imprinting defects.** ### Why the Other Options are Incorrect * **D. Angelman Syndrome:** This is the "sister" condition caused by the **loss of the maternal contribution** (specifically the *UBE3A* gene) on chromosome 15. It is caused by maternal deletion or **Paternal UPD**. * **B. Fragile X Syndrome:** A trinucleotide repeat disorder (CGG) on the X chromosome, characterized by macro-orchidism and intellectual disability. * **C. Marfan’s Syndrome:** An autosomal dominant connective tissue disorder caused by a mutation in the *FBN1* gene on chromosome 15, but it does not involve imprinting or disomy. ### NEET-PG High-Yield Pearls * **Mnemonic:** **P**aternal deletion = **P**rader Willi; **M**aternal deletion = **A**ngelman (**M**AMA - **M**aternal **A**ngelman). * **Clinical Features of PWS:** Hyperphagia (leading to obesity), hypogonadism, small hands/feet, and almond-shaped eyes. * **Clinical Features of Angelman:** "Happy Puppet" syndrome—inappropriate laughter, jerky movements (ataxia), and seizures. * **Diagnosis:** DNA methylation analysis is the gold standard screening test to detect imprinting defects.

Question 1

Primers are removed by which of the following enzymes, except?

Accepted Answer

None of the above

Answer

Delta Polymerase

Answer

RNase H1

Answer

FEN1

Question 2

Which of the following types of mutations generally leads to a truncated protein?

Accepted Answer

Frameshift mutation

Answer

Deletion

Answer

Insertion

Answer

Missense mutation

Question 3

In which of the following conditions does a trinucleotide repeat mutation occur in the coding regions of DNA?

Accepted Answer

Huntington's disease

Answer

Myotonic dystrophy

Answer

Fragile X syndrome

Answer

Friedreich's ataxia

Question 4

Which enzyme catalyzes the hydrolytic step leading to the release of the polypeptide chain from ribosomes?

Accepted Answer

Peptidyl transferase

Answer

Stop codons

Answer

AUG codon

Answer

All of the above

Question 5

Which of the following statements regarding complementary DNA (cDNA) is NOT true?

Accepted Answer

It contains both noncoding and coding sequences.

Answer

It is DNA complementary to messenger RNA (mRNA).

Answer

Reverse transcriptase is required for its synthesis.

Answer

Synthesis of human insulin involves cDNA.

Question 6

A 3-year-old boy presents with progressive developmental delay, ataxia, seizures, and inappropriate laughter since infancy. DNA analysis shows he has inherited both number 15 chromosomes from his father. These findings are most likely indicative of which of the following genetic mechanisms?

Accepted Answer

Genomic imprinting

Answer

Maternal inheritance pattern

Answer

Mutation of mitochondrial DNA

Answer

Trinucleotide repeat expansion

Question 7

A patient presents with skin bullae upon sun exposure. A defect in which of the following is responsible for this condition?

Accepted Answer

Thymidine dimers

Answer

Trinucleotide repeats

Answer

Sugar changes

Answer

DNA methylation

Question 8

During unwinding of DNA, relief of supercoil is made possible by?

Accepted Answer

Topoisomerase

Answer

Gyrase

Answer

Helicase

Answer

Polymerase

Question 9

Which of the following statements regarding gene regulation is false?

Accepted Answer

The regulator gene is typically inducible.

Answer

The repressor protein binds to the operator gene.

Answer

Regulator genes produce repressor subunits.

Answer

Isopropyl thiogalactoside (IPTG) acts as an inducer but not a substrate.

Question 10

Maternal chromosome 15 disomy results in which of the following conditions?

Accepted Answer

Prader Willi Syndrome

Answer

Fragile X syndrome

Answer

Marfan's syndrome

Answer

Angelman Syndrome

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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