Molecular Biology and Genomics Practice Questions

Q: The CAP protein in the lac operon serves as an example of which of the following?

Positive regulator. ### Explanation **1. Why Option A is Correct:** The **Catabolite Activator Protein (CAP)**, also known as cAMP Receptor Protein (CRP), is a classic example of a **positive regulator**. In the absence of glucose, levels of cyclic AMP (cAMP) rise. cAMP binds to CAP, forming a cAMP-CAP complex. This complex binds to a specific site near the *lac* promoter, enhancing the affinity of RNA polymerase for the promoter. This "recruitment" significantly increases the rate of transcription. Therefore, CAP acts as an activator that "turns on" or upregulates gene expression. **2. Why Other Options are Incorrect:** * **B. Negative Regulator:** This refers to the **Lac Repressor protein** (encoded by the *lacI* gene). The repressor binds to the operator to block transcription; CAP does the opposite. * **C. Attenuation:** This is a regulatory mechanism seen in the **Tryptophan (*trp*) operon**, involving premature termination of transcription based on the speed of translation. It is not a feature of the *lac* operon. * **D. Constitutive Expression:** This refers to genes that are expressed at a constant rate regardless of environmental conditions (e.g., "housekeeping genes"). The *lac* operon is **inducible**, not constitutive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Glucose Effect:** The *lac* operon is subject to "Catabolite Repression." Even if lactose is present, the operon is not fully active if glucose is available, because glucose inhibits Adenylate Cyclase, lowering cAMP levels and preventing CAP binding. * **Dual Control:** Remember that for maximal *lac* operon expression, two conditions must be met: **High Lactose** (to remove the repressor) and **Low Glucose** (to allow CAP binding). * **DNA Binding Motif:** CAP utilizes a **Helix-turn-helix** motif to bind to DNA, a common feature in prokaryotic regulatory proteins.

Q: Which of the following is NOT a disease of defective DNA repair?

Adenosis polyposis coli. **Explanation:** The correct answer is **Adenosis polyposis coli (APC)**. **1. Why APC is the correct answer:** Familial Adenomatous Polyposis (FAP) is caused by a mutation in the **APC gene**, which is a **tumor suppressor gene** involved in the **Wnt signaling pathway**. The APC protein normally promotes the degradation of β-catenin. When mutated, β-catenin accumulates, translocates to the nucleus, and triggers the transcription of genes (like *c-myc*) that drive cell proliferation. It is a defect in **cell signaling and growth regulation**, not a direct defect in DNA repair mechanisms. **2. Analysis of incorrect options (DNA Repair Defects):** * **Severe Combined Immunodeficiency (SCID):** Specifically, the **RS-SCID** variant is caused by mutations in genes like *DCLRE1C* (Artemis), which are essential for **Non-Homologous End Joining (NHEJ)** during V(D)J recombination. * **Bloom Syndrome:** Caused by a mutation in the *BLM* gene, which encodes a **DNA Helicase**. This leads to genomic instability due to defective repair during DNA replication (homologous recombination). * **BRCA 1:** This gene is critical for the repair of double-strand breaks via **Homologous Recombination**. Mutations lead to an inability to repair DNA damage, significantly increasing the risk of breast and ovarian cancers. **Clinical Pearls for NEET-PG:** * **Hereditary Non-Polyposis Colorectal Cancer (HNPCC/Lynch Syndrome):** Often confused with APC, this *is* a DNA repair defect (specifically **Mismatch Repair/MMR**). * **Xeroderma Pigmentosum:** Defect in **Nucleotide Excision Repair (NER)**; inability to repair UV-induced thymine dimers. * **Ataxia Telangiectasia:** Defect in the *ATM* gene, which senses double-strand breaks. * **Fanconi Anemia:** Defect in the repair of DNA inter-strand cross-links.

Q: What is the function of single-strand binding proteins (SSBs) during DNA replication?

Prevents premature reannealing of strands. **Explanation:** **1. Why Option C is Correct:** During DNA replication, the enzyme **Helicase** unwinds the double helix, creating two single strands. These strands are naturally complementary and have a high affinity for one another. **Single-strand binding proteins (SSBs)** bind to these exposed strands to stabilize them and **prevent premature reannealing** (re-forming the double helix) or the formation of secondary structures like hairpins. This ensures the strands remain accessible as templates for DNA Polymerase. **2. Analysis of Incorrect Options:** * **Option A (Relieves torsional strain):** This is the function of **DNA Topoisomerases** (e.g., DNA Gyrase in prokaryotes). They create nicks in the DNA to resolve supercoiling caused by unwinding. * **Option B (Deoxynucleotide polymerization):** This is the primary catalytic function of **DNA Polymerase III** (in prokaryotes) or **DNA Polymerase $\delta$ and $\epsilon$** (in eukaryotes). * **Option D (Seals the nick):** This is the function of **DNA Ligase**, which catalyzes the formation of phosphodiester bonds between Okazaki fragments on the lagging strand. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cooperativity:** SSB binding is "cooperative"—once one SSB molecule binds, it makes it easier for subsequent SSB molecules to bind to the adjacent DNA. * **Eukaryotic Equivalent:** In eukaryotes, the functional equivalent of the prokaryotic SSB is **Replication Protein A (RPA)**. * **Protection:** SSBs also protect the single-stranded DNA from being degraded by **nucleases** present in the cell. * **Drug Target:** Topoisomerase inhibitors (Option A) are clinically significant; for example, **Fluoroquinolones** inhibit bacterial DNA Gyrase, while **Etoposide/Teniposide** inhibit human Topoisomerase II.

Q: What is considered the most powerful cloning vector?

Cosmid. ### Explanation **Correct Option: C (Cosmid)** In the context of standard cloning vectors, "power" is often defined by the **insert capacity**—the size of the foreign DNA fragment a vector can carry. A **Cosmid** is a hybrid vector containing the *cos* sequences (cohesive ends) of the Lambda phage and the essential elements of a plasmid. * **Mechanism:** Because it utilizes the packaging machinery of a bacteriophage but replicates like a plasmid, it can accommodate large DNA fragments ranging from **30 to 45 kb**. This makes it significantly more "powerful" for genomic library construction compared to standard plasmids or phages. **Analysis of Incorrect Options:** * **A. Plasmid:** These are small, circular extrachromosomal DNA. While easy to manipulate, they have a limited insert capacity, typically **<10 kb**. * **B. Virus:** While viral vectors (like Retrovirus or Adenovirus) are essential for gene therapy, as cloning tools, their capacity is generally restricted by the viral capsid size, usually smaller than cosmids. * **D. Phage:** Bacteriophages (like Lambda phage) typically carry inserts between **8 to 25 kb**. While more efficient than plasmids, they fall short of the cosmid’s capacity. **High-Yield NEET-PG Pearls:** * **Hierarchy of Insert Capacity (Smallest to Largest):** Plasmid (<10 kb) < Phage (up to 25 kb) < Cosmid (up to 45 kb) < BAC (Bacterial Artificial Chromosome: 100–300 kb) < YAC (Yeast Artificial Chromosome: 200–2000 kb). * **YAC** is the most powerful vector overall, but among the standard options provided in traditional exams, **Cosmid** is the preferred answer for high-capacity cloning. * **Cosmid components:** Plasmid (Origin of replication + Antibiotic resistance) + Phage (*cos* sites for packaging).

Q: What does palindromic DNA imply?

All of the above. **Explanation:** **Palindromic DNA** refers to sequences of double-stranded DNA where the reading direction (5' to 3') is the same on both complementary strands. For example: 5'-GAATTC-3' 3'-CTTAAG-5' **Why Option D is Correct:** 1. **Short stretches of DNA:** Palindromic sequences are typically short, usually ranging from 4 to 8 base pairs in length. 2. **Recognized by specific restriction endonucleases:** This is the most significant functional feature. Restriction enzymes (molecular scissors) specifically bind to and cut DNA at these symmetrical palindromic sites. 3. **Codes for bacterial resistance:** In a clinical context, many antibiotic resistance genes (found on plasmids) are flanked by or contain palindromic sequences (transposons or "jumping genes"). These sequences allow the resistance genes to be excised and integrated into different parts of the bacterial genome or shared via horizontal gene transfer. **Analysis of Options:** * **Option A & B:** These are structural and functional definitions of palindromes in molecular biology. * **Option C:** While not all palindromes code for resistance, the mechanism of moving resistance genes (via transposons and integrons) relies heavily on these inverted repeat/palindromic sequences. Since A and B are definitively true, and C is true in the context of microbial genetics, "All of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Type II Restriction Endonucleases:** These are the most commonly used in recombinant DNA technology because they cut within the palindromic recognition site. * **Sticky vs. Blunt Ends:** Palindromic cuts can result in "sticky ends" (staggered cuts, e.g., *EcoRI*) or "blunt ends" (straight cuts, e.g., *SmaI*). * **Zinc Finger Motifs:** These are common DNA-binding proteins that often recognize palindromic sequences in the major groove of DNA.

Q: Heritable changes in gene expression not caused by alterations in the DNA sequence refers to which of the following?

Epigenetics. **Explanation:** **1. Why Epigenetics is Correct:** Epigenetics refers to the study of heritable changes in gene function that occur **without changing the underlying DNA sequence** (the "A-T-C-G" code). Instead, these changes involve chemical modifications to the DNA or associated proteins that dictate whether a gene is "turned on" or "off." The primary mechanisms include: * **DNA Methylation:** Usually occurs at CpG islands; typically leads to gene silencing. * **Histone Modification:** Acetylation (increases transcription) or Methylation (can increase or decrease transcription). * **Non-coding RNAs:** Such as miRNA and siRNA that regulate translation. **2. Why the Other Options are Incorrect:** * **Genetics:** This is the study of heredity and variation involving the actual DNA sequence and its transmission from parents to offspring. * **Mutations:** These are permanent, structural alterations in the DNA sequence (e.g., point mutations, deletions, insertions). Unlike epigenetic changes, mutations change the "blueprint" itself. * **Transposons:** Also known as "jumping genes," these are mobile DNA sequences that can move from one location to another within the genome, potentially causing mutations or altering genome size. **3. NEET-PG High-Yield Clinical Pearls:** * **Genomic Imprinting:** A classic epigenetic phenomenon where only one allele (either maternal or paternal) is expressed. Examples: **Prader-Willi Syndrome** (paternal deletion/maternal imprinting) and **Angelman Syndrome** (maternal deletion/paternal imprinting) on Chromosome 15. * **Cancer:** Hypermethylation of tumor suppressor genes (like *p53* or *RB*) can lead to silencing and subsequent oncogenesis. * **Drug Target:** **5-Azacytidine** is a DNA methyltransferase inhibitor used in treating myelodysplastic syndromes by reversing gene silencing.

Question 1

The CAP protein in the lac operon serves as an example of which of the following?

Accepted Answer

Positive regulator

Answer

Negative regulator

Answer

Attenuation

Answer

Constitutive expression

Question 2

What is the common feature of Nuclear Localization Signals?

Accepted Answer

Rich in basic amino acid residues and may be located anywhere in the protein sequence

Answer

Rich in acidic amino acid residues and located at the C-terminus of the protein

Answer

Rich in basic amino acid residues and located at the C-terminus of the protein

Answer

Rich in acidic amino acid residues and may be located anywhere in the protein sequence

Question 3

Which of the following is NOT a disease of defective DNA repair?

Accepted Answer

Adenosis polyposis coli

Answer

Severe combined immunodeficiency disease

Answer

Bloom syndrome

Answer

Breast cancer susceptibility 1 (BRCA 1)

Question 4

What is the function of single-strand binding proteins (SSBs) during DNA replication?

Accepted Answer

Prevents premature reannealing of strands

Answer

Relieves torsional strain

Answer

Deoxynucleotide polymerization

Answer

Seals the nick between Okazaki fragments

Question 5

What is considered the most powerful cloning vector?

Accepted Answer

Cosmid

Answer

Plasmid

Answer

Virus

Answer

Phage

Question 6

Which of the following statements is true about protein translation?

Accepted Answer

Protein translation involves three main steps: initiation, elongation, and termination.

Answer

IF-2 (Initiation Factor 2) prevents the reassociation of ribosomal subunits.

Answer

IF-3 and 1A are involved in the binding of the initiating codon to the ribosome.

Answer

None of the above statements are true.

Question 7

Which of the following methods cannot be used to detect the point mutation in the beta (β)-globin gene that causes sickle cell anemia?

Accepted Answer

Northern blot analysis

Answer

Polymerase chain reaction with allele-specific oligonucleotide hybridization

Answer

Southern blot analysis

Answer

DNA sequencing

Question 8

What is microRNA?

Accepted Answer

Gene Silencing RNA

Answer

Splicing RNA

Answer

Snurps

Answer

Ribonuclease

Question 9

What does palindromic DNA imply?

Accepted Answer

All of the above

Answer

Short stretches of DNA

Answer

Recognised by specific restriction endonuclease

Answer

Codes for bacterial resistance

Question 10

Heritable changes in gene expression not caused by alterations in the DNA sequence refers to which of the following?

Accepted Answer

Epigenetics

Answer

Genetics

Answer

Mutations

Answer

Transposons

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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