Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 621: What is the function of the MYC gene?

A. Protein kinase inhibitor
B. Growth factor inhibitor
C. GTPase
D. Transcription activator (Correct Answer)

Explanation: The **MYC gene** (specifically c-MYC) is a proto-oncogene that encodes a nuclear protein functioning as a **transcription activator**. It plays a pivotal role in cell cycle progression, apoptosis, and cellular transformation. ### Why the Correct Answer is Right The MYC protein contains a **basic Helix-Loop-Helix Leucine Zipper (bHLH-Zip)** domain. It forms a heterodimer with another protein called **MAX**. This MYC-MAX complex binds to specific DNA sequences known as **E-boxes** (CACGTG), leading to the recruitment of histone acetyltransferases (HATs). This remodels chromatin and activates the transcription of genes required for the G1 to S phase transition (e.g., Cyclin D, CDK4, and E2F). ### Why Other Options are Wrong * **A. Protein kinase inhibitor:** These are molecules like p21, p27, or p16 (CDK inhibitors) that halt the cell cycle. MYC actually promotes the cell cycle. * **B. Growth factor inhibitor:** MYC is a downstream effector of growth factor signaling (like the MAPK pathway); it does not inhibit them. * **C. GTPase:** This describes the **RAS** family of oncogenes. RAS proteins cycle between active GTP-bound and inactive GDP-bound states to relay signals. ### High-Yield Clinical Pearls for NEET-PG * **Burkitt Lymphoma:** Characterized by the **t(8;14)** translocation, which moves the c-MYC gene (chromosome 8) next to the Immunoglobulin Heavy Chain (IgH) promoter (chromosome 14), leading to constitutive MYC expression. * **Amplification:** **N-myc** is associated with Neuroblastoma (poor prognosis), while **L-myc** is associated with Small Cell Lung Cancer. * **Double Minutes:** Overexpression of MYC can sometimes be seen cytogenetically as "double minute" chromosomes or HSRs (Homogeneously Staining Regions).

Question 622: The base sequence of the strand of DNA used as the template for transcription has the base sequence GATCTAC. What is the base sequence of the RNA product?

A. CTAGATG
B. GTAGATC
C. GAUCUAC
D. GUAGAUC (Correct Answer)

Explanation: ### Explanation **1. Understanding the Core Concept** Transcription is the process where a DNA template is used to synthesize a complementary RNA strand. To determine the sequence of the RNA product, you must apply two fundamental rules: * **Complementary Base Pairing:** DNA bases pair with RNA bases as follows: * DNA **G** pairs with RNA **C** * DNA **C** pairs with RNA **G** * DNA **T** pairs with RNA **A** * DNA **A** pairs with RNA **U** (Uracil replaces Thymine in RNA). * **Antiparallel Orientation:** DNA is read in the **3' → 5'** direction by RNA polymerase to synthesize RNA in the **5' → 3'** direction. Unless specified otherwise, sequences are written in the 5' → 3' direction. **Applying the rules to GATCTAC (Template):** * G → C * A → U * T → A * C → G * T → A * A → U * C → G The complementary sequence is **CUAGAUG**. However, standard notation requires reading the product in the reverse (5' → 3') direction if the template was 3' → 5', or simply matching the polarity. Looking at the options, **GUAGAUC** represents the complementary sequence written in the standard orientation relative to the template. **2. Analysis of Incorrect Options** * **Option A (CTAGATG):** This uses Thymine (T) instead of Uracil (U). RNA never contains Thymine. * **Option B (GTAGATC):** This is a DNA sequence and does not follow proper base-pairing rules for RNA. * **Option C (GAUCUAC):** This is the "Coding Strand" equivalent (replacing T with U) but not the complement of the template strand. **3. NEET-PG High-Yield Pearls** * **Template Strand:** Also called the **Antisense** or Non-coding strand. * **Coding Strand:** Also called the **Sense** strand. Its sequence is identical to the mRNA (except T is replaced by U). * **RNA Polymerase:** Does not require a primer (unlike DNA Polymerase). * **Directionality:** RNA synthesis always occurs in the **5' to 3'** direction.

Question 623: 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?

A. Repair of double-stranded breaks
B. Removal of mismatched bases from the 3' end of Okazaki fragments
C. Removal of pyrimidine dimers from DNA (Correct Answer)
D. Removal of uracil from DNA

Explanation: **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.

Question 624: 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?

A. Mismatch repair (Correct Answer)
B. Base excision repair
C. Homologous recombination repair
D. Non-homologous end joining repair

Explanation: ### 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**.

Question 625: Which of the following statements about microsatellites is false?

A. Repeat size is typically more than 10 to 15 nucleotides (Correct Answer)
B. More prone to variation
C. Found in colonic carcinoma
D. DNA repeats present

Explanation: **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).

Question 626: 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?

A. Werner syndrome
B. Xeroderma pigmentosum (Correct Answer)
C. Rothmund-Thomson syndrome
D. Bloom syndrome

Explanation: **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.

Question 627: Which of the following are considered examples of genetic polymorphism?

A. SNPs and Microsatellites (Correct Answer)
B. Microsatellites and Mutations
C. Mutations and Translocations
D. SNPs and Translocations

Explanation: **Explanation:** **Genetic polymorphism** is defined as the occurrence of two or more clearly different phenotypes (alleles) in the same population, where the frequency of the rarest allele is at least **1%**. If the frequency is less than 1%, it is considered a rare mutation rather than a polymorphism. 1. **Why Option A is Correct:** * **SNPs (Single Nucleotide Polymorphisms):** These are the most common type of genetic variation, involving a change in a single nucleotide base. They occur approximately every 300–1000 base pairs and serve as vital markers for mapping diseases. * **Microsatellites (Short Tandem Repeats - STRs):** These consist of repeating sequences of 2–6 base pairs. Due to their high degree of variability (polymorphism) between individuals, they are the "gold standard" for **DNA fingerprinting** and linkage analysis. 2. **Why Other Options are Incorrect:** * **Mutations (Options B & C):** While polymorphisms originate from mutations, the term "mutation" in a clinical context usually refers to a rare (<1%), often pathological change that disrupts normal function. Polymorphisms are generally considered "normal" variations. * **Translocations (Options C & D):** These are structural chromosomal abnormalities where a segment of one chromosome breaks off and attaches to another. These are typically sporadic or associated with specific malignancies (e.g., Philadelphia chromosome) rather than stable, common variations in the general population. **High-Yield Facts for NEET-PG:** * **RFLP (Restriction Fragment Length Polymorphism):** The first DNA marker used for genomic mapping; it relies on variations in DNA sequences recognized by restriction enzymes. * **Minisatellites (VNTRs):** Repeats of 10–100 base pairs; used in earlier DNA profiling. * **Clinical Utility:** SNPs are used in **Genome-Wide Association Studies (GWAS)** to identify susceptibility to complex diseases like Diabetes and Hypertension.

Question 628: Which of the following conditions is characterized by a CGG repeat sequence?

A. Fragile X syndrome (Correct Answer)
B. Huntington's chorea
C. Dentatorubral-pallidoluysian atrophy
D. Machado-Joseph disease

Explanation: **Explanation:** This question tests knowledge of **Trinucleotide Repeat Expansion disorders**, a high-yield topic in medical genetics. **1. Why Fragile X Syndrome is Correct:** Fragile X syndrome is caused by the expansion of a **CGG repeat** in the 5' untranslated region of the ***FMR1*** gene on the X chromosome. * **Normal:** < 55 repeats. * **Full Mutation:** > 200 repeats. This expansion leads to hypermethylation of the promoter, silencing the gene and resulting in a deficiency of the Fragile X Mental Retardation Protein (FMRP), which is essential for neural development. **2. Analysis of Incorrect Options:** Options B, C, and D are all **Polyglutamine (polyQ) diseases**, which are characterized by **CAG repeats** (coding for Glutamine) within the coding region of the gene: * **Huntington’s chorea:** CAG repeat in the *HTT* gene (Chromosome 4). * **Dentatorubral-pallidoluysian atrophy (DRPLA):** CAG repeat in the *ATN1* gene. * **Machado-Joseph disease (SCA3):** CAG repeat in the *ATXN3* gene. **3. Clinical Pearls for NEET-PG:** * **Anticipation:** These diseases often show increased severity or earlier onset in successive generations due to further expansion of repeats during gametogenesis. * **Fragile X Clinical Triad:** Intellectual disability, macroorchidism (post-pubertal), and long face with large everted ears. * **Friedreich’s Ataxia:** Characterized by **GAA** repeats (Intron 1 of Frataxin gene). * **Myotonic Dystrophy (Type 1):** Characterized by **CTG** repeats (*DMPK* gene). **Mnemonic for Fragile X:** **C**hin (protruding), **G**iant **G**onads (**CGG**).

Question 629: Degeneracy of codon is related to which process?

A. Transcription
B. Translation (Correct Answer)
C. Post-translation modification
D. None of the above

Explanation: ### Explanation **Why Translation is the Correct Answer:** Degeneracy (or redundancy) of the genetic code refers to the fact that a single amino acid can be coded by more than one codon. This phenomenon is fundamentally linked to **Translation**, the process where mRNA sequences are decoded into polypeptide chains. The molecular basis of degeneracy is explained by the **Wobble Hypothesis** (proposed by Francis Crick). During translation, the pairing between the 3' base of the mRNA codon and the 5' base of the tRNA anticodon is non-standard. This allows a single tRNA to recognize multiple codons, ensuring that even if there are minor mutations or errors in the third nucleotide of a codon, the correct amino acid is often still incorporated into the protein. **Why Other Options are Incorrect:** * **Transcription:** This is the synthesis of RNA from a DNA template. While the genetic code is "written" here, degeneracy specifically describes the relationship between codons and amino acids, which only manifests during protein synthesis (translation). * **Post-translational modification:** This occurs *after* the polypeptide chain has been synthesized (e.g., phosphorylation, glycosylation). It involves chemical changes to the protein structure, not the decoding of the triplet codons. **NEET-PG High-Yield Pearls:** * **Wobble Position:** Degeneracy usually involves the **3rd nucleotide** of the codon. * **Exceptions to Degeneracy:** Only two amino acids are coded by a single codon (**Non-degenerate**): **Methionine (AUG)** and **Tryptophan (UGG)**. * **Universal Code:** The genetic code is nearly universal, but exceptions exist in **Mitochondrial DNA** (e.g., UGA codes for Tryptophan instead of a Stop codon). * **Clinical Significance:** Degeneracy provides a "buffer" against mutations; a change in the third base often results in a **Silent Mutation**, preserving protein function.

Question 630: Which of the following is a nonsense codon?

A. UAG (Correct Answer)
B. AUG
C. AGG
D. UUA

Explanation: ### 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.

Practice by Chapter

DNA Replication and Repair Mechanisms

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Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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