Molecular Biology and Genomics Practice Questions

Q: What are nucleosomes composed of?

DNA and Histones. ### Explanation **1. Why the Correct Answer is Right:** A **nucleosome** is the fundamental structural unit of chromatin, often described as "beads on a string." It consists of a segment of **DNA** (approximately 147 base pairs) wrapped 1.65 times around a protein core called the **histone octamer**. This octamer is composed of two copies each of four core histones: **H2A, H2B, H3, and H4**. The DNA is negatively charged (due to phosphate groups), while histones are positively charged (rich in Lysine and Arginine), allowing for a tight electrostatic bond that facilitates DNA packaging within the nucleus. **2. Why the Other Options are Wrong:** * **Option A & C:** RNA is not a structural component of the nucleosome core. While RNA is involved in transcription and regulation, the nucleosome is strictly a DNA-protein complex. * **Option D:** Although RNA may be present in the overall chromatin environment during transcription, it is not a constituent of the nucleosome unit itself. **3. NEET-PG High-Yield Facts & Clinical Pearls:** * **Linker Histone:** **Histone H1** is known as the "linker histone." It sits outside the nucleosome core and helps stabilize the 30-nm chromatin fiber. It is *not* part of the octamer. * **Epigenetics:** Acetylation of histones (by HATs) neutralizes their positive charge, relaxing chromatin (**Euchromatin**) and increasing transcription. Deacetylation (by HDACs) leads to condensed **Heterochromatin** and gene silencing. * **Drug Link:** Sodium valproate (anti-epileptic) acts as a Histone Deacetylase (HDAC) inhibitor. * **Amino Acid Richness:** Histones are exceptionally rich in **Lysine and Arginine**, which is a frequent MCQ point regarding their basic nature.

Q: Maternal disomy of chromosome 15 is seen in which genetic syndrome?

Prader-Willi syndrome. This question tests your knowledge of **Genomic Imprinting** and **Uniparental Disomy (UPD)**, high-yield concepts for NEET-PG. ### **Explanation of the Correct Answer** **Prader-Willi Syndrome (PWS)** occurs due to the loss of expression of genes on the **paternally derived** chromosome 15 (specifically the 15q11-q13 region). While the most common cause is a paternal deletion (70%), the second most common cause (25%) is **Maternal Uniparental Disomy**. In maternal UPD, the child inherits two copies of chromosome 15 from the mother and none from the father. Since the maternal copies are "silenced" (imprinted) in this specific region, the child lacks the necessary active paternal genes, leading to the PWS phenotype. ### **Analysis of Incorrect Options** * **Angelman Syndrome:** This is the "sister" syndrome to PWS. It results from the loss of the **maternally** derived 15q11-q13 region (specifically the *UBE3A* gene). It is caused by maternal deletion or **Paternal Disomy**. * **Klinefelter's Syndrome:** A chromosomal aneuploidy characterized by a **47, XXY** karyotype. It is not related to imprinting or UPD. * **Turner's Syndrome:** A chromosomal aneuploidy characterized by a **45, XO** karyotype (monosomy X). ### **NEET-PG High-Yield Clinical Pearls** * **Prader-Willi Presentation:** Infantile hypotonia, feeding difficulties initially followed by **hyperphagia and obesity**, hypogonadism, and small hands/feet. * **Angelman Presentation:** "Happy Puppet" syndrome—inappropriate laughter, ataxia, seizures, and severe intellectual disability. * **Mnemonic:** **P**rader-Willi = **P**aternal deletion / **M**aternal Disomy. **A**ngelman = **M**aternal deletion / **P**aternal Disomy. (Remember: **P**ader = **P**op/Paternal; **A**ngelman = **M**om/Maternal).

Q: In which phase of the cell cycle does proofreading occur?

S. **Explanation:** **Why S Phase is Correct:** DNA replication occurs exclusively during the **S (Synthesis) phase** of the cell cycle. Proofreading is an intrinsic function of **DNA Polymerases** (specifically Pol $\delta$ and Pol $\epsilon$ in eukaryotes), which possess **3' to 5' exonuclease activity**. This activity allows the enzyme to immediately identify, remove, and replace mismatched nucleotides as they are being incorporated. Since proofreading is a "real-time" correction mechanism coupled with DNA synthesis, it must occur during the S phase. **Why Other Options are Incorrect:** * **G1 Phase:** This is a pre-synthetic gap phase focused on cell growth and preparation for DNA replication. No new DNA is synthesized here, so proofreading does not occur. * **G2 Phase:** This is the post-synthetic gap phase. While **Mismatch Repair (MMR)** and other repair mechanisms (like homologous recombination) are active here to fix errors missed during replication, the specific process of "proofreading" by DNA polymerase has already concluded. * **M Phase:** This is the Mitosis phase where sister chromatids segregate. The DNA is highly condensed into chromosomes, making it inaccessible for replication-linked proofreading. **NEET-PG High-Yield Pearls:** * **Proofreading Direction:** Always **3' $\rightarrow$ 5' exonuclease activity**. * **Synthesis Direction:** Always **5' $\rightarrow$ 3' polymerase activity**. * **Clinical Correlation:** Defects in **Mismatch Repair (MMR)**—which acts as a "spell-check" after proofreading—lead to **Lynch Syndrome (HNPCC)**, characterized by microsatellite instability. * **Key Enzyme:** In eukaryotes, **DNA Polymerase $\gamma$** is responsible for proofreading mitochondrial DNA.

Q: A 7-year-old child of short stature presents with skin patches and learning difficulties. Diagnostic workup indicates a defect in DNA repair specifically related to double-strand breaks. What is the most likely diagnosis?

Bloom Syndrome. ***Bloom Syndrome***- This condition is caused by a mutation in the **BLM gene**, which encodes a **RecQ helicase** necessary for homologous recombination repair (a mechanism for **double-strand break** repair), consistent with the clinical presentation.- Key clinical features include **short stature**, **telangiectatic erythema** (skin patches, especially on sun exposure), and intellectual disability/learning difficulties. *Xeroderma Pigmentosum*- This disease results from a defect in **Nucleotide Excision Repair (NER)**, crucial for repairing DNA damage caused by UV light (e.g., pyrimidine dimers), not primarily double-strand breaks.- Patients exhibit extreme **photosensitivity** and have a drastically increased risk of developing multiple skin cancers early in life.*Cockayne Syndrome*- Cockayne syndrome results from defects in genes like *ERCC8* or *ERCC6*, impairing **Transcription-Coupled Repair (TCR)**, a specific type of NER.- While it presents with short stature (often described as **cachectic dwarfism**) and neurological defects, the primary DNA defect is not in the repair of double-strand breaks but rather UV adducts in transcribed DNA.*Lynch Syndrome*- Also known as Hereditary Non-Polyposis Colorectal Cancer (HNPCC), this syndrome is caused by defects in **Mismatch Repair (MMR)** genes (e.g., *MLH1*, *MSH2*).- It primarily predisposes individuals to colorectal and endometrial cancers and is not typically associated with the features of short stature and specific DNA double-strand break repair defects seen in this child.

Q: Which of the following enzymes is responsible for the immortality of cancer cells?

Telomerase. ***Telomerase*** **Telomerase** is a **ribonucleoprotein enzyme** that adds repetitive DNA sequences (TTAGGG) to the ends of chromosomes (**telomeres**), counteracting the natural shortening that occurs during cell division. The activation of **telomerase** is characteristic of most cancer cells, granting them the ability to divide indefinitely, fulfilling the hallmark of **unlimited replicative potential** (immortality). *Topoisomerase* This enzyme is crucial for relieving the **torsional strain** (supercoiling) in the DNA helix that arises ahead of the replication fork or during transcription. Its primary role is managing DNA structure and integrity, not determining cellular lifespan or **immortality** through **telomere** maintenance. *Helicase* **Helicases** are motor proteins that use energy from ATP hydrolysis to **unwind nucleic acid duplexes** (like the DNA double helix) in fundamental processes such as DNA replication, repair, and transcription. While essential for replication, it does not prevent the gradual loss of terminal DNA sequences (**telomeres**) required for cancer cell immortality. *RNA polymerase* This enzyme is responsible for **transcription**, the process of synthesizing an **RNA molecule** from a DNA template. Its function is focused on gene expression (protein synthesis) and is not directly involved in maintaining the length of **telomeres** or conferring replicative immortality upon cells.

Q: A patient presents with multiple colonic polyps and has been diagnosed with colorectal carcinoma. There is a strong family history of Hereditary Non-Polyposis Colorectal Cancer (HNPCC). Which DNA repair mechanism is most likely defective in this condition?

Mismatch repair. ***Mismatch repair***- **HNPCC (Lynch syndrome)** is caused by inherited germline mutations in genes (e.g., *MLH1*, *MSH2*) that are responsible for the **mismatch repair (MMR)** pathway. - The failure of MMR leads to the accumulation of errors, specifically in repetitive DNA sequences, resulting in **microsatellite instability** which drives carcinogenesis.*Nucleotide excision repair*- This mechanism repairs bulky helix-distorting lesions in DNA, most commonly **pyrimidine dimers** caused by UV radiation. - A classic disease associated with defective **NER** is **Xeroderma Pigmentosum**, which presents with extreme sun sensitivity and a high risk of skin cancers.*Base excision repair*- This pathway primarily corrects small, non-helix distorting damage, such as oxidized or alkylated bases, utilizing enzymes like **DNA glycosylases**.- While fundamental for DNA maintenance, primary defects in BER are not the underlying cause of pathogenesis in **Lynch syndrome**.*Double-strand break repair*- This mechanism repairs severe damage where the entire DNA helix is broken, typically via **Homologous Recombination (HR)** or **Non-Homologous End Joining (NHEJ)**. - Defects in HR are often linked to hereditary breast and ovarian cancers (e.g., *BRCA1*/ *BRCA2* mutations) and are distinct from the pathogenesis of **HNPCC**.

Q: In the liver, the Apo-B gene is completely translated to ApoB-100; in the intestine, it is translated to ApoB-48. Which of the following mechanism explains this?

RNA editing. ***RNA editing (Correct Answer)*** - This post-transcription modification involves a specific **cytidine deaminase** enzyme (APOBEC-1) found primarily in the intestine. - This enzyme converts a **CAA codon** (coding for Glutamine) into a **UAA stop codon** within the *ApoB* mRNA, truncating the protein from ApoB-100 to **ApoB-48**. *Gene splicing* - Gene splicing, including **alternative splicing**, involves differential removal of **introns** and joining of **exons** to create various mRNA transcripts from a single gene. - However, gene splicing does not involve the direct **nucleotide change** (C to U) necessary to create the premature stop codon responsible for shortening ApoB. *Alternative polyadenylation* - This process selects different cleavage sites towards the 3' end of the mRNA, influencing the length of the **3' untranslated region** and mRNA stability. - While it affects mRNA processing, it does not involve a **base conversion** that fundamentally alters the coding sequence by introducing a stop codon. *Gene rearrangements* - Gene rearrangements involve physical changes to the **genomic DNA** sequence itself (e.g., V(D)J recombination in immunoglobulins) and are typically irreversible. - The distinction between ApoB-100 and ApoB-48 is purely a **post-transcriptional** event changing the mRNA, not an alteration of the ApoB gene structure.

Q: How many PCR cycles are needed to produce half the amplified DNA copies produced by 27 cycles of PCR?

26. ***26 (Correct)*** - PCR amplification follows an **exponential growth** pattern, where the template DNA is theoretically doubled in each cycle. The product quantity is proportional to 2^n (where n is the cycle number). - If 27 cycles produce 2^27 copies, half of this amount is calculated as 2^27 / 2, which mathematically simplifies to 2^(27-1). Therefore, exactly **26 cycles** are needed. *25 (Incorrect)* - 25 cycles would produce 2^25 copies. Since 2^27 = 4 × 2^25, 25 cycles yield only **one-quarter** (25%) of the DNA produced by 27 cycles, not half. - In exponential amplification, a difference of two cycles corresponds to a **four-fold change** in product amount. *13 (Incorrect)* - 13 cycles would produce 2^13 copies, which represents an extremely small fraction of the total DNA generated after 27 cycles (2^27). - This amount belongs to the early phase of the reaction and is mathematically too low to be relevant to the calculation for half the **final product quantity**. *12 (Incorrect)* - 12 cycles produce 2^12 copies, which is 2^15 times less than 2^27 copies. - This number is irrelevant as it doesn't align with the required calculation based on the principles of **geometric progression** in PCR.

Question 1

What are nucleosomes composed of?

Accepted Answer

DNA and Histones

Answer

DNA and RNA

Answer

RNA and Histones

Answer

DNA, RNA, and Histones

Question 2

Maternal disomy of chromosome 15 is seen in which genetic syndrome?

Accepted Answer

Prader-Willi syndrome

Answer

Klinefelter's syndrome

Answer

Angelman syndrome

Answer

Turner's syndrome

Question 3

Which enzyme is associated with the aging process related to DNA?

Accepted Answer

Telomerase

Answer

Telosomerase

Answer

Topoisomerase

Answer

DNA polymerase

Question 4

What is true about crossing over?

Accepted Answer

Occurs between non-sister chromatids of homologous chromosomes

Answer

Occurs during the diplotene stage

Answer

Occurs between sister chromatids of homologous chromosomes

Answer

Occurs between sister chromatids of non-homologous chromosomes

Question 5

In which phase of the cell cycle does proofreading occur?

Accepted Answer

S

Answer

G1

Answer

G2

Answer

M

Question 6

A 7-year-old child of short stature presents with skin patches and learning difficulties. Diagnostic workup indicates a defect in DNA repair specifically related to double-strand breaks. What is the most likely diagnosis?

Accepted Answer

Bloom Syndrome

Answer

Lynch Syndrome

Answer

Xeroderma Pigmentosum

Answer

Cockayne Syndrome

Question 7

Which of the following enzymes is responsible for the immortality of cancer cells?

Accepted Answer

Telomerase

Answer

RNA polymerase

Answer

Helicase

Answer

Topoisomerase

Question 8

A patient presents with multiple colonic polyps and has been diagnosed with colorectal carcinoma. There is a strong family history of Hereditary Non-Polyposis Colorectal Cancer (HNPCC). Which DNA repair mechanism is most likely defective in this condition?

Accepted Answer

Mismatch repair

Answer

Double-strand break repair

Answer

Nucleotide excision repair

Answer

Base excision repair

Question 9

In the liver, the Apo-B gene is completely translated to ApoB-100; in the intestine, it is translated to ApoB-48. Which of the following mechanism explains this?

Accepted Answer

RNA editing

Answer

Gene splicing

Answer

Alternative polyadenylation

Answer

Gene rearrangements

Question 10

How many PCR cycles are needed to produce half the amplified DNA copies produced by 27 cycles of PCR?

Accepted Answer

26

Answer

12

Answer

25

Answer

13

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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