Molecular Biology and Genomics Practice Questions

Q: A 4-year-old female child complains of weakness and pain in joints. Her parents are known cases of sickle cell trait. Hemoglobin electrophoresis reveals an HbS pattern. This disease occurs because of which base alteration in DNA?

CTC → CAC. ### Explanation **Concept:** Sickle Cell Anemia is caused by a **point mutation (missense mutation)** in the $\beta$-globin gene located on chromosome 11. Specifically, there is a substitution of **Glutamic acid** (polar) by **Valine** (non-polar) at the **6th position** of the $\beta$-polypeptide chain. **Why Option A is Correct:** The question specifically asks for the **base alteration in DNA**. * The normal DNA sequence (template strand) for the 6th codon is **CTC**, which codes for the mRNA codon **GAG** (Glutamic acid). * In Sickle Cell Anemia, the Thymine (T) in the DNA template is replaced by Adenine (A). * Therefore, the DNA sequence changes from **CTC to CAC**. **Analysis of Incorrect Options:** * **Option B (GAG → GUG):** This represents the change in **mRNA**, not DNA. While the mRNA does change from GAG to GUG, the question specifically asks for the DNA alteration. * **Option C & D:** These are incorrect because they describe alterations in RNA. Furthermore, DNA contains Thymine (T), while RNA contains Uracil (U); "CAC" is a possible RNA sequence, but the primary mutation occurs at the genomic DNA level. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Molecular Mechanism:** Substitution of a hydrophilic amino acid (Glutamate) with a hydrophobic one (Valine) creates a "sticky patch," leading to HbS polymerization under deoxygenated conditions. * **Diagnosis:** * **Screening:** Solubility test (Sodium dithionite). * **Confirmatory:** Hb Electrophoresis (HbS moves slowest towards the Anode due to loss of negative charge). * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria.

Q: Which of the following is NOT a disease caused by trinucleotide repeats?

Hereditary Nonpolyposis Colon Cancer. ### Explanation The correct answer is **D. Hereditary Nonpolyposis Colon Cancer (HNPCC)**. **1. Why HNPCC is the correct answer:** Hereditary Nonpolyposis Colon Cancer (also known as Lynch Syndrome) is caused by mutations in **DNA Mismatch Repair (MMR) genes** (most commonly *MLH1, MSH2, MSH6,* and *PMS2*) [1]. This leads to **microsatellite instability (MSI)**, which involves changes in the length of short, repeated DNA sequences [1]. However, it is not classified as a "trinucleotide repeat expansion disorder," which involves a specific mechanism of dynamic mutations where the number of repeats increases across generations (anticipation). **2. Why the other options are incorrect:** * **Huntington’s Chorea:** Caused by a **CAG** repeat expansion in the *HTT* gene on chromosome 4. It is an autosomal dominant neurodegenerative disorder. * **Myotonic Dystrophy (Type 1):** Caused by a **CTG** repeat expansion in the *DMPK* gene on chromosome 19. It is the most common adult-onset muscular dystrophy. * **Friedreich’s Ataxia:** Caused by a **GAA** repeat expansion in the *FXN* gene (encoding Frataxin) on chromosome 9. Notably, it is the only common trinucleotide repeat disorder that is **autosomal recessive**. **3. NEET-PG High-Yield Clinical Pearls:** * **Anticipation:** This is the hallmark of trinucleotide repeat disorders, where the disease manifests at an earlier age and with increased severity in successive generations. * **Location of Repeats:** * **Exonic (Coding):** Huntington’s (CAG). * **Intronic (Non-coding):** Friedreich’s Ataxia (GAA). * **Untranslated Regions (UTR):** Fragile X (CGG in 5' UTR) and Myotonic Dystrophy (CTG in 3' UTR). * **Mnemonic for HNPCC:** Remember "Mismatch Repair" for Lynch Syndrome, not "Repeat Expansion."

Q: RNA polymerase recognizes which site on the DNA?

Promoter site. **Explanation:** **Why the Correct Answer is Right:** Transcription begins with the binding of **RNA polymerase** to a specific DNA sequence called the **Promoter site**. This site is located upstream (5') of the gene to be transcribed. The promoter serves as the "recognition signal" that tells RNA polymerase where to land, which strand to read, and in which direction to move. In prokaryotes, the **Sigma ($\sigma$) factor** is specifically responsible for recognizing the promoter (Pribnow box), while in eukaryotes, various transcription factors assist RNA polymerase in binding to sequences like the **TATA box**. **Analysis of Incorrect Options:** * **Initiation site:** This is the specific nucleotide (usually a purine) where the first RNA nucleotide is actually incorporated (the +1 site). While it is part of the promoter region, the enzyme *recognizes* the broader promoter sequence to position itself correctly at the initiation site. * **Regulator site:** These are DNA sequences (like enhancers or silencers) where regulatory proteins (activators or repressors) bind to modulate the frequency of transcription, rather than being the primary docking site for RNA polymerase. * **Stop site (Terminator):** This is the sequence that signals the RNA polymerase to detach from the DNA template and cease transcription. **High-Yield NEET-PG Pearls:** * **Pribnow Box (TATAAT):** The prokaryotic promoter located at -10 position. * **Hogness/TATA Box:** The eukaryotic promoter located at -25 position. * **Rifampicin:** A key clinical correlation; it inhibits the $\beta$-subunit of bacterial DNA-dependent RNA polymerase, preventing the initiation of transcription (used in Tuberculosis). * **$\alpha$-Amanitin:** Found in *Amanita phalloides* mushrooms; it specifically inhibits **RNA Polymerase II**, leading to severe liver failure.

Q: What is the nucleotide repeat associated with Spinocerebellar ataxia type 6?

CAG. **Explanation:** Spinocerebellar Ataxia Type 6 (SCA6) is an autosomal dominant neurodegenerative disorder characterized by progressive ataxia, dysarthria, and nystagmus. It is caused by a **CAG trinucleotide repeat expansion** in the *CACNA1A* gene on chromosome 19p13, which encodes the alpha-1A subunit of the P/Q-type voltage-gated calcium channel. Unlike other SCAs, SCA6 involves a very small expansion (usually 20–33 repeats) and is often considered a "channelopathy." **Analysis of Options:** * **A. CAG (Correct):** This is the most common repeat in the SCA family (SCA1, 2, 3, 6, 7, 17) and Huntington’s disease. These are "polyglutamine" diseases because CAG codes for the amino acid Glutamine [2]. * **B. CGG:** Associated with **Fragile X Syndrome**. It occurs in the *FMR1* gene and leads to intellectual disability and macroorchidism. * **C. GAA:** Associated with **Friedreich’s Ataxia**, the most common autosomal recessive ataxia [1]. It occurs in the *FXN* gene (Frataxin). * **D. CTG:** Associated with **Myotonic Dystrophy Type 1** (*DMPK* gene). **High-Yield Clinical Pearls for NEET-PG:** * **SCA3 (Machado-Joseph Disease):** The most common SCA worldwide (CAG repeat). * **Anticipation:** This phenomenon (earlier onset in successive generations) is common in triplet repeat disorders, though it is less pronounced in SCA6 compared to others. * **Pure Cerebellar Sign:** SCA6 is unique because it often presents as a "pure" cerebellar syndrome [1] without the significant extrapyramidal or cognitive features seen in other types.

Q: Oncogenes can be best studied by which of the following methods?

Transfection. ### Explanation **Why Transfection is the Correct Answer:** Transfection is the process of deliberately introducing naked or purified nucleic acids (DNA or RNA) into **eukaryotic cells**. In cancer research, this is the gold-standard method for studying oncogenes. By "transfecting" specific DNA sequences into normal mammalian cell lines (like NIH 3T3 cells), researchers can observe if the cells undergo malignant transformation (e.g., loss of contact inhibition, increased proliferation). This technique was instrumental in the discovery of the first human oncogene, *RAS*. **Analysis of Incorrect Options:** * **Transduction:** This involves the transfer of genetic material from one bacterium to another via a **bacteriophage** (virus). While viral vectors are used in gene therapy, "transduction" as a classical genetic term refers primarily to prokaryotic horizontal gene transfer. * **Transformation:** In a classical microbiology context, this is the uptake of naked DNA by **prokaryotic (bacterial) cells**. While the term is also used to describe a normal cell becoming cancerous, as a *method* of genetic study, it refers to bacteria. * **Conjugation:** This is "bacterial mating," where genetic material is transferred between two bacteria through direct cell-to-cell contact (via a sex pilus). It has no application in studying human oncogenes. **High-Yield Clinical Pearls for NEET-PG:** * **NIH 3T3 Assay:** The classic experiment where DNA from human bladder cancer was transfected into mouse fibroblasts, leading to the identification of the *H-RAS* oncogene. * **Transfection Methods:** Can be chemical (calcium phosphate, liposomes) or physical (electroporation). * **Key Distinction:** Remember: **Transformation** (Bacteria), **Transfection** (Eukaryotes), **Transduction** (Viruses).

Q: Analyze the following pedigree and give the mode of inheritance:

Mitochondrial inheritance. ***Mitochondrial inheritance*** - Shows **maternal transmission** to all offspring regardless of sex, as mitochondrial DNA is inherited exclusively from the mother through the **cytoplasm of the ovum**. - Affects **both males and females equally** with no paternal transmission, which is characteristic of **extranuclear inheritance** patterns. *Autosomal recessive* - Would show **horizontal pattern** with affected siblings having unaffected parents, and **consanguinity** increasing risk. - Requires **two mutated copies** of the gene, showing **25% recurrence risk** in each pregnancy from carrier parents. *Autosomal dominant* - Would demonstrate **vertical transmission** through generations with **affected parent** typically having affected offspring. - Shows **50% recurrence risk** and affects **both sexes equally**, but with paternal transmission possible. *X-linked dominant* - Would show **affected males** passing the trait to **all daughters** but **no sons**, creating a distinctive pattern. - **Affected females** would have a **50% chance** of passing the trait to children of both sexes, with more severe manifestation in males.

Q: Which of the following enzymes is part of DNA dependent RNA polymerase?

Primase. **Explanation:** The correct answer is **Primase**. **Why Primase is the correct answer:** DNA replication cannot be initiated *de novo* because DNA polymerases require a free 3'-OH group to add nucleotides. To overcome this, the enzyme **Primase** (specifically DnaG in prokaryotes) synthesizes a short stretch of RNA (approximately 10 nucleotides) called a **primer**. Since Primase uses a DNA template to synthesize an RNA strand, it is biochemically classified as a **DNA-dependent RNA polymerase**. Once the primer is in place, DNA polymerase III can begin elongation. **Analysis of Incorrect Options:** * **DNA ligase:** This enzyme joins DNA fragments (like Okazaki fragments) by catalyzing the formation of phosphodiester bonds. It does not synthesize RNA. * **DNA polymerase III:** This is a DNA-dependent **DNA** polymerase. It synthesizes DNA by adding deoxyribonucleotides, but it cannot initiate synthesis without a primer. * **RNA transcriptase:** This term is often confused with RNA polymerase used in transcription. While RNA polymerase is also a DNA-dependent RNA polymerase, "Primase" is the specific enzyme integrated into the replisome during DNA replication. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Prokaryotes vs. Eukaryotes:** In eukaryotes, the primase activity is associated with **DNA Polymerase α** (alpha). * **Directionality:** Primase synthesizes the primer in the **5' to 3'** direction. * **Requirement:** The leading strand requires only one primer, whereas the lagging strand requires multiple primers (one for each Okazaki fragment). * **Removal:** In prokaryotes, RNA primers are removed by **DNA Polymerase I** (via its 5' to 3' exonuclease activity), not by Primase.

Question 1

A 4-year-old female child complains of weakness and pain in joints. Her parents are known cases of sickle cell trait. Hemoglobin electrophoresis reveals an HbS pattern. This disease occurs because of which base alteration in DNA?

Accepted Answer

CTC → CAC

Answer

GAG → GUG

Answer

CTC → CAC in RNA

Answer

GAG → CAC in RNA

Question 2

Which of the following is NOT a disease caused by trinucleotide repeats?

Accepted Answer

Hereditary Nonpolyposis Colon Cancer

Answer

Huntington's chorea

Answer

Myotonic dystrophy

Answer

Friedreich's ataxia

Question 3

RNA polymerase recognizes which site on the DNA?

Accepted Answer

Promoter site

Answer

Initiation site

Answer

Regulator site

Answer

Stop site

Question 4

What is true about linkage analysis in familial gene disorders?

Accepted Answer

A characteristic DNA polymorphism in a family is associated with the disorder.

Answer

Useful to create a pedigree chart to show affected and non-affected family members.

Answer

Used to create a pedigree chart to show non-paternity.

Answer

Pedigree charts are used to identify disease frequency.

Question 5

The short and long arms of chromosomes are designated respectively as:

Accepted Answer

q and p arms

Answer

p and q arms

Answer

m and q arms

Answer

l and s arms

Question 6

Which statement regarding gene transcription in human cells is true?

Accepted Answer

Transcription reads the DNA template strand in the 3' to 5' direction.

Answer

Transcription always begins at an AUG codon, which codes for methionine.

Answer

Transcription does not require local unwinding of the two DNA strands.

Answer

Transcription occurs on only one of the two copies of each chromosome found in each cell.

Question 7

What is the nucleotide repeat associated with Spinocerebellar ataxia type 6?

Accepted Answer

CAG

Answer

CGG

Answer

GAA

Answer

CTG

Question 8

Oncogenes can be best studied by which of the following methods?

Accepted Answer

Transfection

Answer

Transduction

Answer

Transformation

Answer

Conjugation

Question 9

Analyze the following pedigree and give the mode of inheritance:

Accepted Answer

Mitochondrial inheritance

Answer

Autosomal recessive

Answer

Autosomal dominant

Answer

X-linked dominant

Question 10

Which of the following enzymes is part of DNA dependent RNA polymerase?

Accepted Answer

Primase

Answer

DNA ligase

Answer

DNA polymerase III

Answer

RNA transcriptase

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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