Molecular Biology and Genomics Practice Questions

Q: What is the rapid method for chromosome identification in intersex individuals?

FISH. **Explanation:** The correct answer is **FISH (Fluorescent In Situ Hybridization)**. In the management of intersex disorders (Disorders of Sex Development - DSD), rapid identification of chromosomal sex is critical for parental counseling and clinical decision-making. 1. **Why FISH is correct:** FISH uses fluorescently labeled DNA probes that bind to specific sequences on the X and Y chromosomes. It can be performed on **interphase nuclei** (non-dividing cells), meaning it does not require cell culture. This allows for a rapid turnaround time, typically **24 to 48 hours**, making it the fastest reliable method for identifying sex chromosomes in a clinical setting. 2. **Why other options are incorrect:** * **Karyotyping:** While it is the "Gold Standard" for definitive diagnosis, it requires cells to be in metaphase. This necessitates cell culture, which takes **7 to 14 days**, making it too slow for immediate rapid identification. * **PCR (Polymerase Chain Reaction):** While very fast at amplifying DNA sequences (like the SRY gene), it is generally used for targeted gene mutation analysis rather than comprehensive chromosome identification or detecting mosaicism. * **SSCP (Single Strand Conformation Polymorphism):** This is a technique used to detect small mutations or polymorphisms in DNA sequences; it is not used for chromosome identification. **Clinical Pearls for NEET-PG:** * **Gold Standard for Chromosomal Analysis:** Conventional G-banded Karyotyping. * **Rapid Sex Determination:** FISH (specifically looking for X and Y centromeric probes). * **Barr Body:** A quick but outdated screening method; it represents the inactive X chromosome. * **SRY Gene:** Located on the short arm of the Y chromosome; its presence is the primary determinant of male sexual differentiation.

Q: A 10-year-old girl presents with numerous freckles on her face, neck, arms, and hands, along with unusual sensitivity to sunlight. Two basal cell carcinomas are identified on her face. Which of the following DNA repair processes is most likely to be defective in this patient?

Removal of pyrimidine dimers from DNA. **Explanation:** The clinical presentation of extreme photosensitivity, extensive freckling, and early-onset skin cancers (basal cell carcinomas) in a young child is classic for **Xeroderma Pigmentosum (XP)**. **1. Why the Correct Answer is Right:** XP is an autosomal recessive disorder caused by a deficiency in **Nucleotide Excision Repair (NER)**. Ultraviolet (UV) radiation from sunlight causes the formation of **pyrimidine dimers** (specifically thymine dimers), which create bulky distortions in the DNA helix. In healthy individuals, the NER pathway identifies these lesions, excises the damaged oligonucleotide strand, and replaces it. In XP patients, this repair mechanism fails, leading to the accumulation of mutations and a 1000-fold increased risk of skin malignancies. **2. Analysis of Incorrect Options:** * **Option A:** Repair of double-stranded breaks (via homologous recombination) is defective in conditions like **Ataxia-Telangiectasia** and **BRCA1/2** mutations. * **Option B:** Removal of mismatched bases involves **Mismatch Repair (MMR)**. Defects here lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). * **Option D:** Removal of uracil is the first step of **Base Excision Repair (BER)**, initiated by uracil DNA glycosylase. This pathway handles small, non-bulky lesions (like deamination), not UV-induced dimers. **3. NEET-PG High-Yield Pearls:** * **Enzyme Defect:** Most commonly a deficiency in **UV-specific endonuclease**. * **Clinical Triad:** Photosensitivity, Pigmentary changes (freckling), and early Skin Cancer. * **Associated Findings:** Some variants (XP-De Sanctis-Cacchione) present with neurological abnormalities and hypogonadism. * **Key Concept:** NER acts during the **G1 phase** of the cell cycle to repair "bulky" lesions.

Q: A patient with HNPCC (hereditary nonpolyposis colon cancer) exhibits genes with microsatellite instability. This instability is a consequence of the failure of which DNA repair mechanism?

Mismatch repair. ### Explanation **1. Why Mismatch Repair (MMR) is Correct:** Hereditary Nonpolyposis Colon Cancer (HNPCC), also known as **Lynch Syndrome**, is caused by germline mutations in genes responsible for **Mismatch Repair (MMR)**, most commonly **MLH1, MSH2**, MSH6, and PMS2. The MMR system identifies and fixes errors that occur during DNA replication, such as mispaired bases (e.g., G-T instead of G-C) or small insertions/deletions. When MMR is defective, repetitive DNA sequences known as **microsatellites** (short tandem repeats) vary in length across different cells. This phenomenon is called **Microsatellite Instability (MSI)**, which serves as a hallmark diagnostic marker for Lynch Syndrome. **2. Why Other Options are Incorrect:** * **Base Excision Repair (BER):** Repairs "small" damage to single bases (e.g., deamination of cytosine to uracil or damage from oxidation). It uses glycosylases and is not associated with MSI. * **Homologous Recombination (HR):** A high-fidelity mechanism for repairing **double-strand breaks (DSBs)** using a sister chromatid as a template. Defects in HR (e.g., BRCA1/2 mutations) lead to breast and ovarian cancers, not HNPCC. * **Non-homologous End Joining (NHEJ):** An error-prone mechanism for repairing DSBs without a template. Defects in NHEJ are associated with **SCID** (Severe Combined Immunodeficiency). **3. Clinical Pearls for NEET-PG:** * **Lynch Syndrome Inheritance:** Autosomal Dominant. * **Cancer Spectrum:** Primarily proximal colon cancer (right-sided), but also increases risk for endometrial, ovarian, and gastric cancers. * **Amsterdam Criteria:** Used clinically to identify families at risk (3-2-1 rule: 3 relatives, 2 generations, 1 diagnosed before age 50). * **Nucleotide Excision Repair (NER):** (Not listed but high-yield) Repairs bulky lesions like pyrimidine dimers caused by UV light; deficiency leads to **Xeroderma Pigmentosum**.

Q: Which of the following statements about microsatellites is false?

Repeat size is typically more than 10 to 15 nucleotides. **Explanation:** Microsatellites, also known as **Short Tandem Repeats (STRs)**, are small sequences of non-coding DNA (usually 1–6 base pairs long) that are repeated multiple times in tandem. **1. Why Option A is the Correct Answer (False Statement):** The repeat size of a microsatellite is typically **1 to 6 nucleotides**. If the repeat unit is larger (usually 10 to 100 nucleotides), it is classified as a **Minisatellite** (Variable Number Tandem Repeats or VNTRs). Therefore, the statement that the repeat size is more than 10 to 15 nucleotides is incorrect. **2. Analysis of Other Options:** * **Option B (More prone to variation):** Microsatellites are highly polymorphic because they are prone to "replication slippage," leading to frequent gains or losses of repeat units. This makes them excellent markers for DNA fingerprinting and linkage analysis. * **Option C (Found in colonic carcinoma):** Defects in Mismatch Repair (MMR) genes (like *MLH1, MSH2*) lead to **Microsatellite Instability (MSI)**. This is a hallmark of **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer - HNPCC). * **Option D (DNA repeats present):** By definition, microsatellites consist of tandemly repeated DNA sequences (e.g., CACACACA). **High-Yield Clinical Pearls for NEET-PG:** * **Microsatellites:** 1–6 bp repeats; used in DNA profiling (CODIS). * **Minisatellites:** 10–100 bp repeats; used in original DNA fingerprinting (Jeffreys' probes). * **Microsatellite Instability (MSI):** Tested via PCR to screen for Lynch Syndrome. * **Trinucleotide Repeat Disorders:** A subset of microsatellite expansions causing diseases like Huntington’s (CAG) and Fragile X (CGG).

Q: An 8-year-old girl in a developing country presents with significant corneal scarring and multiple cutaneous skin lesions in sun-exposed areas. Neuro-developmental delay has been present since 3 months of age. What is the probable diagnosis that is due to defective nucleotide excision repair?

Xeroderma pigmentosum. **Explanation** The clinical presentation of severe photosensitivity, cutaneous lesions (such as hyperpigmentation and telangiectasia) in sun-exposed areas, and corneal scarring, combined with neurodevelopmental delay, is classic for **Xeroderma Pigmentosum (XP)**. **1. Why Xeroderma Pigmentosum is Correct:** XP is an autosomal recessive disorder caused by a defect in **Nucleotide Excision Repair (NER)**. Normally, NER identifies and removes bulky DNA adducts, specifically **pyrimidine dimers** (thymine dimers) formed by Ultraviolet (UV) radiation. In XP, these mutations go unrepaired, leading to extreme UV sensitivity, a 1000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, Melanoma), and in some subtypes (like De Sanctis-Cacchione syndrome), progressive neurological impairment. **2. Why Other Options are Incorrect:** * **Werner Syndrome:** Known as "adult progeria," it involves premature aging due to a mutation in the *WRN* gene (DNA helicase). It typically manifests in the second decade of life, not infancy. * **Rothmund-Thomson Syndrome:** Characterized by poikiloderma, sparse hair, and skeletal abnormalities. While it involves DNA repair defects (*RECQL4* helicase), it is not primarily a defect of the NER pathway. * **Bloom Syndrome:** Caused by a mutation in the *BLM* gene (RecQ helicase), leading to chromosomal instability. It presents with short stature and a "butterfly" rash, but the hallmark is **sister chromatid exchange**, not NER failure. **High-Yield Clinical Pearls for NEET-PG:** * **NER Mechanism:** Uses **Endonucleases** to nick the damaged strand, **DNA Polymerase δ/ε** to fill the gap, and **Ligase** to seal it. * **Key Association:** XP patients must avoid all sunlight ("Children of the Night"). * **Differential:** Cockayne Syndrome also involves NER defects but is characterized by "mickey mouse" facies and dwarfism without an increased risk of skin cancer.

Q: Which of the following is a nonsense codon?

UAG. ### Explanation **Correct Option: A (UAG)** In molecular biology, **nonsense codons** (also known as stop codons or termination codons) are sequences of three nucleotides in mRNA that do not code for any amino acid. Instead, they signal the termination of protein synthesis by causing the ribosome to detach from the mRNA strand. There are three nonsense codons: 1. **UAG** (Amber) 2. **UAA** (Ochre) 3. **UGA** (Opal) **Analysis of Incorrect Options:** * **B. AUG:** This is the **initiation (start) codon**. It signals the beginning of translation and codes for the amino acid **Methionine** in eukaryotes (and N-formylmethionine in prokaryotes). * **C. AGG:** This is a sense codon that codes for the amino acid **Arginine**. * **D. UUA:** This is a sense codon that codes for the amino acid **Leucine**. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Nonsense Mutation:** A point mutation that changes a sense codon into a nonsense codon, resulting in a **prematurely truncated, usually non-functional protein**. This is a common cause of genetic diseases like β-thalassemia and Duchenne Muscular Dystrophy. * **Read-through Therapy:** Certain drugs, like **Aminoglycosides** (e.g., Gentamicin) or **Ataluren**, can sometimes induce the ribosome to "skip" a premature stop codon, potentially treating diseases caused by nonsense mutations. * **Mnemonic:** To remember the stop codons: **U** **A**re **G**one (UAG), **U** **A**re **A**way (UAA), **U** **G**o **A**way (UGA). * **Exceptions:** In human **mitochondria**, UGA codes for Tryptophan rather than acting as a stop codon.

Q: The gene for the Rh antigen is located on which chromosome?

1. **Explanation:** The Rh blood group system is the second most important system in transfusion medicine after ABO. It is encoded by two highly homologous genes: **RHD** (which determines Rh positivity) and **RHCE** (which determines C, c, E, and e antigens). Both of these genes are located on the **short arm of Chromosome 1 (1p36.11)**. **Analysis of Options:** * **A. Chromosome 1 (Correct):** This is the location of the *RHD* and *RHCE* genes. These genes are inherited as a linkage complex. * **B. Chromosome 19:** This chromosome carries the genes for the **Lewis (Le)** blood group and the **H substance** (FUT1 gene) and **Secretor** (FUT2 gene) status. * **C. Chromosome 4:** This is the location of the **MNS blood group** system (GYPA and GYPB genes). * **D. Chromosome 9:** This chromosome carries the **ABO gene** (at 9q34.2), which encodes the glycosyltransferases responsible for the A and B antigens. **High-Yield Clinical Pearls for NEET-PG:** 1. **Nature of Antigens:** Unlike ABO antigens (which are carbohydrates), Rh antigens are **transmembrane proteins**. 2. **Rh Null Phenotype:** A rare condition where individuals lack all Rh antigens; their RBCs show structural abnormalities (stomatocytosis) and mild hemolytic anemia. 3. **HDN:** Rh incompatibility is the most common cause of severe **Hemolytic Disease of the Newborn (HDN)**, typically occurring in the second pregnancy of an Rh-negative mother with an Rh-positive fetus. 4. **Inheritance:** Rh antigens are inherited in an autosomal dominant fashion. If a person has at least one *RHD* gene, they are Rh-positive.

Question 1

What is the rapid method for chromosome identification in intersex individuals?

Accepted Answer

FISH

Answer

PCR

Answer

SSCP

Answer

Karyotyping

Question 2

A 10-year-old girl presents with numerous freckles on her face, neck, arms, and hands, along with unusual sensitivity to sunlight. Two basal cell carcinomas are identified on her face. Which of the following DNA repair processes is most likely to be defective in this patient?

Accepted Answer

Removal of pyrimidine dimers from DNA

Answer

Repair of double-stranded breaks

Answer

Removal of mismatched bases from the 3' end of Okazaki fragments

Answer

Removal of uracil from DNA

Question 3

A patient with HNPCC (hereditary nonpolyposis colon cancer) exhibits genes with microsatellite instability. This instability is a consequence of the failure of which DNA repair mechanism?

Accepted Answer

Mismatch repair

Answer

Base excision repair

Answer

Homologous recombination repair

Answer

Non-homologous end joining repair

Question 4

Which of the following statements about microsatellites is false?

Accepted Answer

Repeat size is typically more than 10 to 15 nucleotides

Answer

More prone to variation

Answer

Found in colonic carcinoma

Answer

DNA repeats present

Question 5

An 8-year-old girl in a developing country presents with significant corneal scarring and multiple cutaneous skin lesions in sun-exposed areas. Neuro-developmental delay has been present since 3 months of age. What is the probable diagnosis that is due to defective nucleotide excision repair?

Accepted Answer

Xeroderma pigmentosum

Answer

Werner syndrome

Answer

Rothmund-Thomson syndrome

Answer

Bloom syndrome

Question 6

Which of the following are considered examples of genetic polymorphism?

Accepted Answer

SNPs and Microsatellites

Answer

Microsatellites and Mutations

Answer

Mutations and Translocations

Answer

SNPs and Translocations

Question 7

Which of the following is a nonsense codon?

Accepted Answer

UAG

Answer

AUG

Answer

AGG

Answer

UUA

Question 8

Same amino acid is coded by multiple codons due to:

Accepted Answer

Degeneracy

Answer

Frame-shift mutation

Answer

Transcription

Answer

Mutation

Question 9

All of the following events occur during mitosis except:

Accepted Answer

DNA replication

Answer

Cytokinesis

Answer

Chromatids separate

Answer

The kinetochore becomes evident

Question 10

The gene for the Rh antigen is located on which chromosome?

Accepted Answer

1

Answer

19

Answer

4

Answer

9

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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