Molecular Biology and Genomics Practice Questions

Q: A woman who is trying to conceive has a sister whose child has an autosomal recessive disease characterized by the dysfunction of mucus-secreting cells, leading to abnormally thick mucus that obstructs the pancreatic ducts, bronchi, bronchioles, and bile ducts. Which of the following tests can be performed to determine if this woman and her husband are carriers of this disease?

PCR and sequencing. ### Explanation **1. Why PCR and Sequencing is Correct:** The clinical presentation (thick mucus obstructing pancreatic and bile ducts, bronchi, and bronchioles) is diagnostic of **Cystic Fibrosis (CF)**, an autosomal recessive disorder caused by mutations in the **CFTR gene** on chromosome 7. Since the woman’s niece/nephew has the disease, she has a 50% chance of being a carrier. To determine carrier status, we must identify specific DNA mutations. **PCR (Polymerase Chain Reaction)** is used to amplify the CFTR gene segments, followed by **DNA Sequencing** (or Allele-Specific Oligonucleotide probes) to detect point mutations or small deletions (like the common **ΔF508**). This is the gold standard for identifying specific genetic variations in prospective parents. **2. Why Incorrect Options are Wrong:** * **Northern Blot:** Used to detect and quantify **RNA**. While it measures gene expression, it is not used for routine carrier screening or identifying specific genomic DNA mutations. * **Southern Blot:** Used to detect specific **DNA** sequences, particularly large insertions, deletions, or restriction fragment length polymorphisms (RFLPs). While it can detect DNA changes, it is labor-intensive and less precise than PCR-sequencing for the variety of point mutations found in CF. * **Western Blot:** Used to detect specific **proteins** (e.g., confirming HIV or Lyme disease). It would analyze the CFTR protein itself, which is not the standard method for carrier screening. **3. Clinical Pearls for NEET-PG:** * **Cystic Fibrosis:** Most common lethal genetic disease in Caucasians; defect in **Chloride channel** (CFTR). * **Diagnosis:** Sweat Chloride Test (>60 mEq/L) is the initial test of choice for symptomatic patients. * **High-Yield Blotting Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** (nothing) * **W**estern = **P**rotein

Q: What enzyme is used for mapping hypersensitive sites in recombinant DNA research?

DNase I. **Explanation:** **DNase I (Deoxyribonuclease I)** is the correct answer because it is the specific endonuclease used to identify **DNase-hypersensitive sites (DHSs)** in chromatin. 1. **Mechanism:** In eukaryotic cells, DNA is tightly wrapped around histones. However, active regulatory regions (like promoters and enhancers) are "open" or nucleosome-free to allow transcription factor binding. These open regions are highly sensitive to cleavage by low concentrations of DNase I. By digesting chromatin with DNase I and performing Southern blotting or sequencing, researchers can map these hypersensitive sites to identify transcriptionally active areas of the genome. **Analysis of Incorrect Options:** * **DNA Ligase:** This enzyme functions as "molecular glue" that joins two DNA fragments by forming phosphodiester bonds. It is used in DNA replication and repair, not for mapping structural sensitivity. * **DNA Polymerase I:** Primarily involved in prokaryotic DNA replication and repair (filling gaps). In research, its "Klenow fragment" is used for DNA labeling or sequencing, but it cannot identify open chromatin sites. * **Polynucleotide Kinase (PNK):** This enzyme transfers a phosphate group from ATP to the 5' end of a DNA or RNA strand. It is used for end-labeling nucleic acids, not for structural mapping. **High-Yield Clinical Pearls for NEET-PG:** * **Hypersensitivity = Activity:** DNase I hypersensitive sites are hallmarks of **transcriptionally active DNA**. * **Heterochromatin vs. Euchromatin:** Heterochromatin (tightly packed) is DNase-resistant, while Euchromatin (loosely packed) contains DNase-hypersensitive sites. * **Diagnostic Use:** DNase I is also used clinically as a nebulized drug (**Dornase alfa**) in **Cystic Fibrosis** to digest the thick DNA-rich mucus in the lungs, reducing viscosity.

Q: Which of the following conditions is NOT caused by the overexpression of a trinucleotide repeat?

Alzheimer's disease. **Explanation:** The correct answer is **Alzheimer’s disease**. This condition is a neurodegenerative disorder primarily associated with the accumulation of amyloid-beta plaques and tau tangles [1]. Its genetic basis involves mutations in the **APP, PSEN1, and PSEN2** genes (early-onset) or the **APOE-ε4** allele (late-onset), rather than trinucleotide repeat expansions. **Analysis of Options:** * **Fragile X Syndrome:** Caused by a **CGG** repeat expansion in the *FMR1* gene. It is the most common inherited cause of intellectual disability. * **Huntington Disease:** An autosomal dominant disorder caused by **CAG** repeats in the *HTT* gene on chromosome 4 [2]. It exhibits "anticipation," where the disease manifests earlier in successive generations. * **Spinocerebellar Ataxia (SCA) Type 2:** Part of a group of autosomal dominant ataxias caused by **CAG** repeats (polyglutamine diseases) affecting the *ATXN2* gene [2]. **NEET-PG High-Yield Pearls:** 1. **Anticipation:** This phenomenon (earlier onset/increased severity in offspring) is a hallmark of trinucleotide repeat disorders due to further expansion during gametogenesis. 2. **Repeat Locations:** * **CAG (Polyglutamine):** Huntington’s, SCA, Spinal Bulbar Muscular Ataxia (Kennedy disease). * **CGG:** Fragile X (Non-coding region). * **GAA:** Friedreich’s Ataxia (Intronic). * **CTG:** Myotonic Dystrophy. 3. **Alzheimer’s Association:** Down Syndrome patients have an increased risk of Alzheimer’s because the *APP* gene is located on **Chromosome 21**.

Q: Which of the following processes does NOT occur during RNA processing?

Chemical hydrolysis. **Explanation:** RNA processing (post-transcriptional modification) is a highly regulated enzymatic process that converts a primary transcript (hnRNA) into a functional mature RNA molecule. **Why Chemical Hydrolysis is the Correct Answer:** Chemical hydrolysis refers to the non-specific breakdown of phosphodiester bonds, typically caused by alkaline conditions or heat, leading to RNA degradation. Unlike the other options, this is a **spontaneous or destructive chemical reaction**, not a programmed biological step in the maturation of RNA. In the cell, RNA cleavage is always **enzymatic** (mediated by RNases/ribozymes), not chemical. **Analysis of Incorrect Options:** * **Terminal Addition:** This occurs during the addition of the **5' 7-methylguanosine cap** and the **3' Poly-A tail** in mRNA, as well as the addition of the **CCA sequence** to the 3' end of tRNA by nucleotidyltransferase. * **Nucleoside Modification:** This is a hallmark of tRNA and rRNA processing. Examples include the conversion of uridine to **pseudouridine** or **dihydrouridine**, and the methylation of bases or ribose sugars. * **Nucleoside Cleavage:** This involves the precise enzymatic cutting of the RNA backbone. It is essential for **splicing** (removal of introns) and the liberation of individual rRNA and tRNA species from large polycistronic primary transcripts. **High-Yield Clinical Pearls for NEET-PG:** * **Splicing Defect:** Mutations at splice sites are a common cause of **β-Thalassemia**. * **Autoimmunity:** Antibodies against **snRNPs** (Small Nuclear Ribonucleoproteins), specifically **Anti-Smith (Anti-Sm) antibodies**, are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **RNA Stability:** The 5' cap and 3' poly-A tail are crucial for protecting mRNA from exonuclease degradation and facilitating translation initiation.

Q: Synthesis of protein occurs on:

Poly ribosomes. **Explanation:** **Why Polyribosomes is the correct answer:** Protein synthesis (translation) occurs on **ribosomes**. When multiple ribosomes attach to a single strand of messenger RNA (mRNA) to translate it simultaneously, the complex is called a **polyribosome** (or polysome). This arrangement allows the cell to produce multiple copies of the same polypeptide efficiently from a single mRNA molecule. Ribosomes can either be free in the cytosol (synthesizing proteins for internal use) or attached to the Rough Endoplasmic Reticulum (synthesizing proteins for secretion or membranes). **Why other options are incorrect:** * **Mitochondria:** While mitochondria contain their own DNA and some ribosomes (mitoribosomes) to synthesize a few organelle-specific proteins, they are primarily the site of ATP production (oxidative phosphorylation), not the primary site for general cellular protein synthesis. * **Nucleus:** This is the site of **transcription** (DNA to mRNA) and DNA replication. Protein synthesis does not occur here because ribosomes are located in the cytoplasm. * **Golgi bodies:** These are responsible for the **post-translational modification**, sorting, and packaging of proteins received from the Endoplasmic Reticulum. They do not synthesize proteins themselves. **High-Yield Facts for NEET-PG:** * **Ribosome Subunits:** Eukaryotes have 80S ribosomes (40S + 60S); Prokaryotes have 70S (30S + 50S). * **Clinical Correlation:** Many antibiotics target protein synthesis by binding to specific ribosomal subunits (e.g., **A**minoglycosides/Tetracyclines act on **30S**; **C**hloramphenicol/Erythromycin act on **50S**—Mnemonic: **"Buy AT 30, CELL at 50"**). * **Signal Sequence:** Proteins destined for secretion have a "signal peptide" that directs the ribosome to the Rough ER.

Q: Karyotyping is performed during which phase of the cell cycle?

Metaphase. **Explanation:** **Why Metaphase is the Correct Answer:** Karyotyping is the process of pairing and ordering all the chromosomes of an organism to detect numerical or structural abnormalities. **Metaphase** is the ideal stage for this procedure because chromosomes reach their **maximum state of condensation** during this phase. At this point, the sister chromatids are clearly visible and aligned at the equatorial plate, making them easiest to stain, identify, and count under a light microscope. **Analysis of Incorrect Options:** * **Anaphase:** During this phase, sister chromatids separate and move toward opposite poles. Because the chromosomes are moving and fragmented into individual chromatids, they cannot be accurately aligned for a karyotype. * **Telophase:** Chromosomes begin to de-condense (uncoil) back into chromatin to reform the nucleus. Their lack of defined structure makes visualization impossible. * **S phase:** This is part of interphase where DNA replication occurs. Chromosomes exist as a loose mass of chromatin fibers and are not visible as distinct rod-like structures. **High-Yield Clinical Pearls for NEET-PG:** * **Colchicine/Colcemid:** This alkaloid is used in the laboratory to arrest cells in metaphase by inhibiting microtubule formation (spindle poisons). * **Common Staining:** **G-banding (Giemsa stain)** is the most common technique used in karyotyping to produce characteristic light and dark bands. * **Samples used:** Peripheral blood **T-lymphocytes** (stimulated by Phytohemagglutinin) are the most common source for postnatal karyotyping. * **Indications:** Used to diagnose conditions like Down Syndrome (Trisomy 21), Turner Syndrome (45, XO), and Chronic Myeloid Leukemia (Philadelphia chromosome).

Q: A highly specific inhibitor that targets the phosphorylation activity of TFIIH is added to an in vitro transcription reaction. Which one of the following steps is most likely to be affected?

Promoter clearance. ### Explanation **Why "Promoter Clearance" is the correct answer:** Transcription initiation by RNA Polymerase II (Pol II) requires a group of General Transcription Factors (GTFs). **TFIIH** is a multi-subunit complex with two critical enzymatic activities: 1. **Helicase activity:** It unwinds the DNA to form the "open complex." 2. **Kinase activity:** It phosphorylates the **Carboxy-Terminal Domain (CTD)** of RNA Polymerase II (specifically at Serine 5). This phosphorylation of the CTD acts as a "molecular switch." It triggers a conformational change that releases the polymerase from the promoter and the transcription initiation complex, allowing it to move forward along the DNA template. This transition from initiation to elongation is known as **Promoter Clearance**. If TFIIH phosphorylation is inhibited, the polymerase remains "stuck" at the promoter. **Analysis of Incorrect Options:** * **A & C (Binding of Pol II and Recruitment of TFIID):** These are early steps in the assembly of the Pre-Initiation Complex (PIC). TFIID is the first factor to bind (via TBP), and Pol II recruitment occurs before TFIIH exerts its kinase activity. * **D (Open promoter complex formation):** While TFIIH is involved in this step, it uses its **helicase** (ATP-dependent unwinding) activity, not its **phosphorylation** activity, to create the transcription bubble. **NEET-PG High-Yield Pearls:** * **TFIIH & Disease:** Mutations in TFIIH subunits (XPB/XPD helicases) are associated with **Xeroderma Pigmentosum**, **Cockayne Syndrome**, and **Trichothiodystrophy** because TFIIH also functions in Nucleotide Excision Repair (NER). * **CTD Phosphorylation:** Phosphorylation at Serine 5 (by TFIIH) is for promoter clearance/capping; phosphorylation at Serine 2 (by P-TEFb) is for productive elongation and splicing. * **Alpha-Amanitin:** Remember that this toxin from *Amanita phalloides* specifically inhibits RNA Polymerase II.

Q: A frameshift mutation does not occur in multiples of which number?

3. **Explanation:** The genetic code is **triplet-based**, meaning a sequence of three nucleotides (a codon) codes for a single amino acid. The "reading frame" is established at the start codon and continues sequentially. **Why 3 is the correct answer:** A **frameshift mutation** occurs when the number of nucleotides inserted or deleted is **not a multiple of three**. If exactly three nucleotides (or any multiple of 3, such as 6 or 9) are added or removed, it results in the gain or loss of entire amino acids without shifting the downstream reading frame. This is known as an **in-frame mutation**. For a mutation to be classified as a "frameshift," it must disrupt the triplet grouping, altering every subsequent codon and usually leading to a premature stop codon (nonsense mutation). **Analysis of Incorrect Options:** * **Options A, C, and D (2, 4, 5):** These numbers are not multiples of three. If 2, 4, or 5 nucleotides are inserted or deleted, the triplet alignment is broken. For example, deleting 2 bases shifts the frame by +1, while deleting 4 bases shifts it by +2. Therefore, these numbers **do** cause frameshift mutations. **Clinical Pearls & High-Yield Facts:** * **Cystic Fibrosis:** The most common mutation ($\Delta$F508) is an **in-frame deletion** of 3 nucleotides (phenylalanine), not a frameshift. * **Duchenne vs. Becker Muscular Dystrophy:** Duchenne is typically caused by **frameshift mutations** (severe phenotype), whereas Becker is often caused by **in-frame mutations** (milder phenotype). * **Consequence:** Frameshift mutations are generally more deleterious than point mutations because they alter the entire primary structure of the protein C-terminal to the mutation.

Q: All of the following are true regarding satellite DNA EXCEPT?

It is transcriptionally active.. ### Explanation **Satellite DNA** refers to highly repetitive DNA sequences that form a significant portion of the eukaryotic genome. Understanding its characteristics is crucial for molecular biology and forensic genetics. **Why Option D is the Correct Answer (The Exception):** Satellite DNA is primarily **transcriptionally inactive**. It is composed of non-coding sequences that do not translate into proteins. Structurally, it is categorized as **constitutive heterochromatin**, which is highly condensed and chemically modified (e.g., methylated) to prevent gene expression. Therefore, the statement that it is transcriptionally active is incorrect. **Analysis of Incorrect Options:** * **Option A:** Satellite DNA consists of short sequences (ranging from a few to hundreds of base pairs) that are **repeated in tandem** (head-to-tail). This distinguishes it from interspersed repeats like SINEs and LINEs. * **Option B:** Large blocks of satellite DNA (Macro-satellites) are characteristically found at the **centromeres**, where they play a structural role in spindle fiber attachment during mitosis. * **Option C:** Satellite DNA is also frequently clustered at the **telomeres** (the ends of chromosomes) and the secondary constrictions of acrocentric chromosomes, providing genomic stability. **High-Yield Facts for NEET-PG:** 1. **Buoyant Density:** The name "satellite" comes from the fact that these sequences have a different G-C content than the rest of the genome, causing them to form a separate "satellite" band during **cesium chloride (CsCl) density gradient centrifugation**. 2. **VNTRs and STRs:** Minisatellites (VNTRs) and Microsatellites (STRs) are types of satellite DNA used in **DNA Fingerprinting** due to their high degree of polymorphism. 3. **Trinucleotide Repeats:** Microsatellite expansions are responsible for diseases like **Huntington’s chorea** and **Fragile X syndrome**.

Question 1

A woman who is trying to conceive has a sister whose child has an autosomal recessive disease characterized by the dysfunction of mucus-secreting cells, leading to abnormally thick mucus that obstructs the pancreatic ducts, bronchi, bronchioles, and bile ducts. Which of the following tests can be performed to determine if this woman and her husband are carriers of this disease?

Accepted Answer

PCR and sequencing

Answer

Northern blot

Answer

Southern blot

Answer

Western blot

Question 2

What enzyme is used for mapping hypersensitive sites in recombinant DNA research?

Accepted Answer

DNase I

Answer

DNA ligase

Answer

DNA polymerase I

Answer

Polynucleotide kinase

Question 3

Which of the following conditions is NOT caused by the overexpression of a trinucleotide repeat?

Accepted Answer

Alzheimer's disease

Answer

Fragile X syndrome

Answer

Huntington disease

Answer

Spinocerebellar ataxia Type 2

Question 4

Base substitution of GAC (Asp) to GAG (Glu) is an example of which type of mutation?

Accepted Answer

Non-sense mutation

Answer

Point mutation

Answer

Silent mutation

Answer

Conserved mutation

Question 5

Which of the following processes does NOT occur during RNA processing?

Accepted Answer

Chemical hydrolysis

Answer

Terminal addition

Answer

Nucleoside modification

Answer

Nucleoside cleavage

Question 6

Synthesis of protein occurs on:

Accepted Answer

Poly ribosomes

Answer

Mitochondria

Answer

Nucleus

Answer

Golgi bodies

Question 7

Karyotyping is performed during which phase of the cell cycle?

Accepted Answer

Metaphase

Answer

Anaphase

Answer

Telophase

Answer

S phase

Question 8

A highly specific inhibitor that targets the phosphorylation activity of TFIIH is added to an in vitro transcription reaction. Which one of the following steps is most likely to be affected?

Accepted Answer

Promoter clearance

Answer

Binding of RNA polymerase to promoter sequence

Answer

Recruitment of TFIID

Answer

Open promoter complex formation

Question 9

A frameshift mutation does not occur in multiples of which number?

Accepted Answer

3

Answer

2

Answer

4

Answer

5

Question 10

All of the following are true regarding satellite DNA EXCEPT?

Accepted Answer

It is transcriptionally active.

Answer

It consists of repeated DNA sequences arranged in tandem.

Answer

It is typically clustered around the centromeres.

Answer

It is often found clustered around the telomeres.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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