Molecular Biology and Genomics Practice Questions

Q: Transcription is the synthesis of:

A single-stranded complementary copy of DNA. **Explanation:** Transcription is the fundamental process of gene expression where the genetic information stored in **DNA** is copied into **RNA**. **Why Option A is Correct:** During transcription, the enzyme **RNA polymerase** reads the template strand of DNA (3' to 5') and synthesizes a **single-stranded** RNA molecule in the 5' to 3' direction. This RNA transcript is **complementary** to the DNA template strand (with Uracil replacing Thymine) and identical in sequence to the coding (non-template) strand. **Why Other Options are Incorrect:** * **Option B:** DNA is double-stranded, but the resulting RNA transcript is always single-stranded. Double-stranded DNA synthesis occurs during **Replication**, not transcription. * **Option C:** Synthesis of a complementary copy of RNA is characteristic of **RNA-dependent RNA replication** (seen in certain RNA viruses) or **Reverse Transcription** (where DNA is made from RNA). * **Option D:** While rRNA is indeed produced via transcription, it is only one specific type of RNA. Transcription encompasses the synthesis of **all** RNA types, including mRNA and tRNA; therefore, "a complementary copy of DNA" is the more accurate general definition. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** Transcription always proceeds in the **5' → 3' direction**. * **Enzymes (Eukaryotes):** * **RNA Pol I:** Nucleolus; synthesizes 28S, 18S, and 5.8S **rRNA**. * **RNA Pol II:** Nucleoplasm; synthesizes **mRNA** and snRNA (Inhibited by **α-amanitin** from *Amanita phalloides* mushrooms). * **RNA Pol III:** Nucleoplasm; synthesizes **tRNA** and 5S rRNA. * **Prokaryotes:** A single RNA polymerase (multimeric enzyme) synthesizes all types of RNA. **Rifampicin** inhibits the β-subunit of this bacterial RNA polymerase.

Q: What hematological condition is associated with 'hn RNA'?

Beta-thalassemia. **Explanation:** The correct answer is **Beta-thalassemia**. **Underlying Concept:** Heterogeneous nuclear RNA (hnRNA) is the primary transcript produced by RNA polymerase II, containing both exons (coding) and introns (non-coding). Before it becomes functional mRNA, it must undergo **post-transcriptional processing**, which includes 5' capping, 3' polyadenylation, and **splicing** (removal of introns). In certain forms of **$\beta$-thalassemia**, mutations occur at the splice donor or acceptor sites. These mutations interfere with the normal splicing of hnRNA into mRNA. Consequently, the defective hnRNA cannot be processed correctly, leading to a deficiency of functional $\beta$-globin chains. This specific molecular pathology—**defective splicing of hnRNA**—is a classic high-yield association for $\beta$-thalassemia. **Analysis of Incorrect Options:** * **Sickle cell anemia:** This is caused by a **point mutation** (missense mutation) in the DNA where Glutamic acid is replaced by Valine at the 6th position of the $\beta$-globin chain. It does not involve hnRNA processing defects. * **Thalassemia (General):** While $\alpha$-thalassemia also involves globin chain deficiency, it is most commonly caused by **gene deletions** rather than the specific hnRNA splicing defects typically highlighted in $\beta$-thalassemia questions. **NEET-PG High-Yield Pearls:** * **hnRNA vs. mRNA:** hnRNA is the "pre-mRNA" found only in the nucleus; mRNA is the processed version found in the cytoplasm. * **Splicing Marker:** Small nuclear ribonucleoproteins (snRNPs or "snurps") are responsible for splicing hnRNA. * **$\beta$-Thalassemia Mutations:** Most $\beta$-thalassemias are due to point mutations (splicing, promoter, or chain termination), whereas $\alpha$-thalassemias are usually due to deletions.

Q: In a family, the mother is phenotypically normal, but the father has a genetic disease. All their daughters are carriers, and all their sons are normal. What is the pattern of inheritance of this disease?

X-linked recessive. ### Explanation The inheritance pattern described is characteristic of **X-linked recessive (XLR)** inheritance. **1. Why X-linked Recessive is Correct:** In XLR disorders, the father carries the mutated gene on his single X chromosome ($X^d Y$). Since he must pass his Y chromosome to his sons, **none of his sons will inherit the disease** or the gene from him. However, he must pass his X chromosome to all his daughters. Because the mother is phenotypically normal (assumed homozygous $XX$), the daughters receive one normal X from the mother and one mutated X from the father ($X^d X$), making them **obligate carriers**. **2. Why the Other Options are Incorrect:** * **X-linked Dominant:** If the father had an X-linked dominant condition, all daughters would inherit the mutated X and would be **clinically affected**, not just carriers. * **Autosomal Dominant:** A father with an autosomal dominant disease has a 50% chance of passing the trait to **both** sons and daughters. The gender-specific distribution (only daughters affected as carriers) rules this out. * **Autosomal Recessive:** For all daughters to be carriers and all sons to be normal, the mother would have to be homozygous normal and the father affected. While possible, autosomal inheritance does not typically show a 100% gender-segregated outcome in a standard pedigree analysis unless by chance; the "all daughters/all sons" pattern is the classic "textbook" description of X-linked transmission. **3. High-Yield NEET-PG Pearls:** * **Criss-cross inheritance:** XLR traits are transmitted from an affected father to his grandsons through his carrier daughters. * **Key Examples:** Hemophilia A and B, Duchenne Muscular Dystrophy (DMD), Red-Green Color Blindness, and G6PD Deficiency. * **Rule of Thumb:** If a father has an X-linked disease, his sons are safe (no male-to-male transmission), but all his daughters are carriers.

Q: What is the function of TTAGGG repeat-binding proteins?

Telomere maintenance. **Explanation:** The correct answer is **Telomere maintenance**. **1. Why Telomere Maintenance is Correct:** Telomeres are repetitive DNA sequences (TTAGGG in humans) located at the ends of linear chromosomes. **TTAGGG repeat-binding proteins**, primarily known as the **Shelterin complex** (including TRF1 and TRF2), bind specifically to these repeats. Their primary function is **telomere maintenance**, which involves protecting the chromosome ends from being recognized as double-stranded DNA breaks, preventing end-to-end fusion, and regulating the access of the enzyme telomerase to the DNA strand. **2. Why Other Options are Incorrect:** * **Telomere Elongation & Synthesis (Options A & C):** These processes are specifically carried out by **Telomerase**, a ribonucleoprotein (reverse transcriptase). While binding proteins regulate telomerase activity, they do not synthesize or elongate the DNA themselves. * **Telomere Capping (Option D):** While "capping" is a specific *mechanism* used by these proteins to protect the ends (forming the T-loop), "maintenance" is the broader, more accurate functional term used in standard biochemistry to describe the overall stability and regulation of telomeric length and integrity. **3. Clinical Pearls for NEET-PG:** * **The End Replication Problem:** DNA polymerase cannot replicate the 3' end of linear chromosomes, leading to progressive shortening. * **Hayflick Limit:** The finite number of times a normal somatic cell population will divide before cell division stops (senescence), governed by telomere length. * **Cancer Link:** Approximately 85–90% of cancer cells upregulate **telomerase**, allowing them to bypass senescence and achieve "immortality." * **Shelterin Complex:** Key proteins include TRF1, TRF2, RAP1, TIN2, TPP1, and POT1. Mutations in these can lead to **Dyskeratosis Congenita**.

Q: A potent inhibitor of protein synthesis that acts as an analogue of aminoacyl t-RNA is:

Puromycin. **Explanation:** **Puromycin** is the correct answer because it is a structural analogue of the 3' end of **aminoacyl-tRNA** (specifically tyrosinyl-tRNA). Due to this structural similarity, it enters the 'A' site of the ribosome during translation in both prokaryotes and eukaryotes. It incorporates itself into the growing polypeptide chain, leading to **premature chain termination** and the release of incomplete peptides. **Analysis of Incorrect Options:** * **Mitomycin C:** This is an alkylating agent that causes **DNA cross-linking**, primarily inhibiting DNA synthesis rather than acting as a tRNA analogue. * **Streptomycin:** An aminoglycoside that binds to the **30S ribosomal subunit**. It interferes with the initiation of protein synthesis and causes misreading of mRNA, but it does not mimic tRNA. * **Nalidixic Acid:** A quinolone that inhibits **DNA Gyrase** (Topoisomerase II) in bacteria, thereby preventing DNA replication. **High-Yield Clinical Pearls for NEET-PG:** * **Puromycin Unique Feature:** It is non-selective and inhibits protein synthesis in **both prokaryotes and eukaryotes**, making it unsuitable for clinical use as an antibiotic but useful in research. * **Inhibitors of the 50S Subunit:** Remember the mnemonic **"CLEAN"** (Chloramphenicol, Clindamycin, Erythromycin/Macrolides, Azithromycin, Linezolid). * **Inhibitors of the 30S Subunit:** Remember **"AT"** (Aminoglycosides, Tetracyclines). * **Diphtheria Toxin:** Inhibits eukaryotic translation by ADP-ribosylation of **Elongation Factor-2 (EF-2)**.

Q: Xeroderma pigmentosum is caused due to a defect in which of the following processes?

Nucleotide excision repair. **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by extreme sensitivity to ultraviolet (UV) radiation. **Why Nucleotide Excision Repair (NER) is correct:** UV light causes the formation of **pyrimidine dimers** (specifically thymine dimers) in DNA, which distort the double helix. In healthy individuals, these bulky lesions are removed via the **Nucleotide Excision Repair (NER)** pathway. This process involves specific endonucleases (XP proteins A through G) that "cut" the damaged strand, followed by DNA polymerase and ligase to fill the gap. In XP patients, a deficiency in these **UV-specific endonucleases** prevents the repair of these dimers, leading to accumulated mutations and carcinogenesis. **Why other options are incorrect:** * **Base pairing:** Refers to the hydrogen bonding between A-T and G-C; while dimers disrupt this, the *defect* is in the repair mechanism, not the chemical property of base pairing itself. * **Mismatch repair (MMR):** This system corrects errors (like mispaired bases) that occur during DNA replication. Defects in MMR lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). * **Translocation:** This is a chromosomal abnormality where a segment of one chromosome breaks off and attaches to another (e.g., t(9;22) in CML). It is not a DNA repair pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Severe sunburn on minimal sun exposure, "parchment-like" skin, excessive freckling, and telangiectasia. * **Complications:** 2000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma) before age 10. * **Key Enzyme:** The most common defect is in **UV-specific endonuclease**. * **Contrast:** While XP involves NER, **Ataxia-telangiectasia** involves defects in repairing double-strand breaks.

Q: RNA polymerase does not require:

Primer. **Explanation:** The fundamental difference between DNA replication and transcription lies in the requirement for a primer. **RNA Polymerase** is capable of initiating the synthesis of an RNA chain *de novo* by joining two nucleotides together. In contrast, DNA Polymerase requires a pre-existing 3'-OH group (provided by an RNA primer) to begin polymerization. * **Why Option D is Correct:** RNA polymerase can initiate transcription by simply binding to a specific promoter sequence on the DNA. It does not require a primer to start the synthesis of the RNA strand. * **Why Option A is Incorrect:** RNA polymerase requires a **double-stranded DNA (dsDNA)** template. It reads the template strand in the 3' → 5' direction to synthesize RNA in the 5' → 3' direction. * **Why Option B is Incorrect:** Transcription requires **activated precursors** in the form of ribonucleoside triphosphates (ATP, GTP, UTP, and CTP). The energy for phosphodiester bond formation is derived from the cleavage of high-energy phosphate bonds in these nucleotides. * **Why Option C is Incorrect:** RNA polymerase is a metalloenzyme. It requires **divalent metal ions** (usually $Mg^{2+}$ or $Mn^{2+}$) as cofactors to stabilize the transition state and facilitate the catalytic process. **High-Yield Facts for NEET-PG:** * **Rifampicin:** Inhibits the $\beta$-subunit of bacterial RNA polymerase, preventing the initiation of transcription (used in Tuberculosis). * **$\alpha$-Amanitin:** A toxin from the *Amanita phalloides* mushroom that specifically inhibits RNA Polymerase II, leading to liver failure. * **Sigma ($\sigma$) Factor:** A subunit of prokaryotic RNA polymerase required for the **recognition of the promoter** site; it dissociates after the initiation of transcription.

Q: Which is the longest chromosome?

Chromosome 1. **Explanation:** In the human genome, chromosomes are numbered roughly in descending order of their physical size (length of DNA and number of base pairs), with the exception of the sex chromosomes. **1. Why Chromosome 1 is correct:** Chromosome 1 is the **largest (longest) human autosome**. It contains approximately **249 million base pairs**, representing about 8% of the total DNA in a human cell. It also boasts the highest gene density, housing over 2,000 identified genes. In a standard karyotype, it is the first chromosome displayed due to its superior length. **2. Why the other options are incorrect:** * **Chromosome 21:** This is the **smallest human autosome** (by base pair count, though Chromosome 22 was historically thought to be smaller). It contains roughly 48 million base pairs. Clinical significance: Trisomy 21 causes Down Syndrome. * **Chromosome 14:** This is a medium-sized acrocentric chromosome. It is significant in immunology as it contains the Gene locus for the Immunoglobulin Heavy chain (IgH). * **Chromosome X:** While large, the X chromosome ranks 8th in terms of size (approx. 155 million base pairs), making it significantly shorter than Chromosome 1. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Longest Chromosome:** Chromosome 1. * **Smallest Chromosome:** Chromosome Y (in terms of total base pairs), followed by Chromosome 21 (smallest autosome). * **Gene Richest Chromosome:** Chromosome 1. * **Gene Poorest Chromosome:** Chromosome Y. * **Acrocentric Chromosomes:** 13, 14, 15, 21, and 22 (Important for Robertsonian translocations). * **Denver Classification:** Chromosomes are classified into 7 groups (A-G) based on size and centromere position; Chromosome 1 belongs to **Group A** (large metacentric).

Question 1

Transcription is the synthesis of:

Accepted Answer

A single-stranded complementary copy of DNA

Answer

A double-stranded complementary copy of DNA

Answer

A complementary copy of RNA

Answer

A complementary copy of rRNA

Question 2

What hematological condition is associated with 'hn RNA'?

Accepted Answer

Beta-thalassemia

Answer

Thalassemia

Answer

Sickle cell anemia

Answer

None of the above

Question 3

In a family, the mother is phenotypically normal, but the father has a genetic disease. All their daughters are carriers, and all their sons are normal. What is the pattern of inheritance of this disease?

Accepted Answer

X-linked recessive

Answer

X-linked dominant

Answer

Autosomal dominant

Answer

Autosomal recessive

Question 4

What is the function of TTAGGG repeat-binding proteins?

Accepted Answer

Telomere maintenance

Answer

Telomere elongation

Answer

Telomere synthesis

Answer

Telomere capping

Question 5

A potent inhibitor of protein synthesis that acts as an analogue of aminoacyl t-RNA is:

Accepted Answer

Puromycin

Answer

Mitomycin C

Answer

Streptomycin

Answer

Nalidixic acid

Question 6

Which type of RNA molecule undergoes the least post-translational modification?

Accepted Answer

Prokaryotic messenger RNA (mRNA)

Answer

Transfer RNA (t-RNA)

Answer

Prokaryotic ribosomal RNA (r-RNA)

Answer

Eukaryotic ribosomal RNA (r-RNA)

Question 7

Xeroderma pigmentosum is caused due to a defect in which of the following processes?

Accepted Answer

Nucleotide excision repair

Answer

Base pairing

Answer

Mismatch repair

Answer

Translocation

Question 8

RNA polymerase does not require:

Accepted Answer

Primer

Answer

Template (ds DNA)

Answer

Activated precursors (ATP, GTP, UTP, CTP)

Answer

Divalent metal ions (Mn2+, Mg2+)

Question 9

A pregnant woman with a family history of fragile X syndrome undergoes prenatal testing of her fetus. PCR analysis to amplify the appropriate region of the FMR1 gene is attempted using DNA from amniotic fluid cells, but no amplified products are obtained. Which of the following is the most appropriate next step?

Accepted Answer

Southern blot analysis of DNA from the amniotic fluid cells

Answer

Routine karyotyping of the amniotic fluid cells

Answer

Routine karyotyping of the unaffected father

Answer

PCR analysis of the mother's FMR1 gene

Question 10

Which is the longest chromosome?

Accepted Answer

Chromosome 1

Answer

Chromosome 21

Answer

Chromosome 14

Answer

Chromosome X

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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