Molecular Biology and Genomics Practice Questions

Q: Which of the following diseases is NOT characterized by a defect in DNA repair mechanisms?

Huntington's disease. **Explanation:** The correct answer is **Huntington’s disease**. Unlike the other options, Huntington’s disease is a **Trinucleotide Repeat Expansion disorder** (specifically CAG repeats on chromosome 4). It is characterized by protein misfolding and aggregation (Huntingtin protein) rather than a primary defect in DNA repair. **Analysis of Options:** * **Xeroderma Pigmentosa (XP):** A classic DNA repair disorder caused by a defect in **Nucleotide Excision Repair (NER)**. Patients cannot repair pyrimidine dimers formed by UV light, leading to extreme photosensitivity and early skin cancers. * **Fanconi Anemia (Note: Option says Fanconi syndrome, but in the context of DNA repair, Fanconi Anemia is implied):** This is caused by defects in the **interstrand cross-link repair** mechanism. It presents with bone marrow failure, physical abnormalities, and high cancer risk. (Note: *Fanconi Syndrome* is a renal proximal tubule defect; *Fanconi Anemia* is the DNA repair defect). * **Hereditary Nonpolyposis Colon Cancer (HNPCC/Lynch Syndrome):** Caused by mutations in **Mismatch Repair (MMR)** genes (e.g., MSH2, MLH1). This leads to microsatellite instability and increased risk of colorectal and endometrial cancers. **High-Yield Clinical Pearls for NEET-PG:** * **Ataxia Telangiectasia:** Defect in **Non-homologous end joining (NHEJ)** or ATM gene (double-strand break repair). * **Bloom Syndrome / Cockayne Syndrome:** Also categorized as DNA repair/stability defects. * **Huntington’s Disease Hallmark:** Shows **"Anticipation"** (earlier onset in successive generations) and is inherited in an Autosomal Dominant pattern.

Q: What are the components of the 50S ribosomal subunit?

23S RNA & 5S RNA. ### Explanation The ribosome is the cellular machinery responsible for protein synthesis (translation). In prokaryotes (like bacteria), the ribosome is **70S**, consisting of a small **30S** subunit and a large **50S** subunit. **1. Why Option D is Correct:** The **50S (large) subunit** of the prokaryotic ribosome is composed of two types of ribosomal RNA (rRNA) and approximately 34 proteins (L1–L34). The specific rRNA components are: * **23S rRNA:** Acts as a ribozyme (peptidyl transferase) to catalyze peptide bond formation. * **5S rRNA:** Provides structural stability. **2. Analysis of Incorrect Options:** * **Option A (5S, 5.8S, & 28S):** These are the components of the **60S (large) subunit of Eukaryotic ribosomes**. Eukaryotes have an 80S ribosome (40S + 60S). * **Option B (16S & 23S):** 16S rRNA is the signature component of the **30S (small) subunit** in prokaryotes, not the 50S subunit. * **Option C (23S & 5.8S):** This is a mismatch. 5.8S is exclusively eukaryotic, while 23S is exclusively prokaryotic. **3. High-Yield Clinical Pearls for NEET-PG:** * **Antibiotic Targets:** Many antibiotics selectively target the 50S subunit (e.g., **Macrolides, Chloramphenicol, Clindamycin, and Linezolid**) or the 30S subunit (e.g., **Aminoglycosides, Tetracyclines**). This selectivity is based on the structural differences between prokaryotic (70S) and eukaryotic (80S) ribosomes. * **Shine-Dalgarno Sequence:** The **16S rRNA** (of the 30S subunit) contains a sequence complementary to the mRNA's Shine-Dalgarno sequence, which is essential for initiation of translation in bacteria. * **Svedberg Unit (S):** It measures the sedimentation rate, which depends on both mass and shape (which is why 50S + 30S = 70S, not 80S).

Q: Which gene product is transcribed but not translated?

t-RNA. ### Explanation The central dogma of molecular biology involves two primary steps: **Transcription** (DNA to RNA) and **Translation** (RNA to Protein). While most genes code for proteins, a significant portion of the genome codes for **Non-coding RNAs (ncRNAs)**. These molecules are transcribed but function directly as RNA without ever being translated into proteins. **Why t-RNA is the Correct Answer:** Transfer RNA (t-RNA) is a classic example of a non-coding RNA. It is transcribed by **RNA Polymerase III** in eukaryotes. Its primary function is to act as an "adapter" molecule during protein synthesis, carrying specific amino acids to the ribosome. Because the t-RNA molecule itself is the final functional product, it does not undergo translation. Other examples include r-RNA, snRNA, and miRNA. **Analysis of Incorrect Options:** * **A. Glycosyl transferase:** This is an enzyme (protein) responsible for attaching carbohydrate chains to proteins or lipids. Being a protein, it must be translated from mRNA. * **C. Keratin:** This is a structural intermediate filament protein found in hair, skin, and nails. It is the product of mRNA translation. * **D. Histone:** Histones are highly alkaline proteins that package DNA into nucleosomes. They are translated from specific histone mRNAs (which are unique because they lack a poly-A tail). **NEET-PG High-Yield Pearls:** * **RNA Polymerase Roles:** Remember **1, 2, 3 → R, M, T** (Pol I for r-RNA, Pol II for m-RNA, Pol III for t-RNA). * **t-RNA Structure:** Contains unusual bases like **Pseudouridine** and **Dihydrouridine**. The 3' end always carries the sequence **CCA**, which is the amino acid attachment site. * **Small Nuclear RNA (snRNA):** Another transcribed-only molecule; it is essential for **splicing** (removal of introns).

Q: Multiple proteins from a single gene may be generated by:

Alternative splicing. ### Explanation **Correct Option: A. Alternative Splicing** The central dogma states that one gene typically codes for one protein; however, **alternative splicing** is the primary mechanism that allows for proteomic diversity. During post-transcriptional modification, different combinations of **exons** (coding regions) from a single pre-mRNA transcript are joined together, while introns are removed. This results in multiple unique mRNA isoforms, which are then translated into different proteins with distinct functions or tissue specificities. * *Example:* The **Calcitonin gene** produces Calcitonin in the thyroid but undergoes alternative splicing to produce Calcitonin Gene-Related Peptide (CGRP) in neural tissue. **Why Incorrect Options are Wrong:** * **B. Mutations in the promoter:** The promoter region controls the *initiation and rate* of transcription. Mutations here typically lead to increased, decreased, or abolished protein synthesis, but they do not change the protein's primary structure or create multiple variants. * **C. RNA interference (RNAi):** This is a regulatory mechanism (involving siRNA or miRNA) that leads to **gene silencing** by degrading mRNA or inhibiting translation. It reduces protein expression rather than generating new protein variants. * **D. mRNA capping:** This involves adding a 7-methylguanosine cap to the 5' end. It is essential for mRNA stability, nuclear export, and translation initiation, but it does not alter the coding sequence. **High-Yield Clinical Pearls for NEET-PG:** * **Spliceosomes:** These are the molecular machines (composed of snRNPs) that perform splicing. * **Clinical Correlation:** Mutations in splice sites are responsible for approximately 15% of genetic diseases, including **Beta-thalassemia** and **Spinal Muscular Atrophy (SMA)**. * **Isoforms:** Different proteins produced from the same gene via alternative splicing are called isoforms.

Q: Friedreich's ataxia is caused due to triplet repeats of which nucleotide sequence?

GAA. **Explanation:** **Friedreich's Ataxia (FRDA)** is an autosomal recessive neurodegenerative disorder characterized by progressive ataxia, loss of deep tendon reflexes, and hypertrophic cardiomyopathy. **Why GAA is correct:** The molecular basis of Friedreich's Ataxia is the **expansion of a GAA trinucleotide repeat** located in the first intron of the **FXN gene** on chromosome 9. This gene encodes **Frataxin**, a mitochondrial protein involved in iron-sulfur cluster biogenesis. The expansion leads to transcriptional silencing (via heterochromatin formation), resulting in a deficiency of Frataxin. This causes mitochondrial iron overload, increased oxidative stress, and subsequent damage to the spinal cord (spinocerebellar tracts) and heart. **Analysis of Incorrect Options:** * **AGG:** This sequence is not typically associated with a major trinucleotide repeat disorder. However, **CGG** repeats are seen in Fragile X Syndrome. * **UAA:** This is a **stop codon** (Ochre). While mutations can create premature stop codons (nonsense mutations), it is not the basis for triplet repeat expansion diseases. * **AUG:** This is the **start codon** (codes for Methionine). It initiates translation and is not associated with expansion disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Unlike most triplet repeat disorders (which are Autosomal Dominant), Friedreich's Ataxia is **Autosomal Recessive**. * **Anticipation:** It does *not* typically show significant clinical anticipation, unlike Huntington’s or Fragile X. * **Clinical Triad:** Progressive limb ataxia, absent knee/ankle jerks, and **Hypertrophic Cardiomyopathy** (the most common cause of death). * **Skeletal Deformities:** Kyphoscoliosis and **Pes Cavus** (high arched feet) are classic findings. * **Diabetes:** About 10-20% of patients develop Diabetes Mellitus due to pancreatic beta-cell dysfunction.

Q: Which of the following is a classic example of a missense mutation?

Sickle cell disease. **Explanation:** **Sickle Cell Disease (SCD)** is the classic prototype of a **missense mutation**. In SCD, a single base substitution occurs in the 6th codon of the $\beta$-globin gene (GAG $\rightarrow$ GTG). This point mutation results in the replacement of **Glutamic acid** (a polar, negatively charged amino acid) with **Valine** (a non-polar, hydrophobic amino acid). This single amino acid change causes the hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Analysis of Incorrect Options:** * **Thalassemia:** Most commonly results from **splice-site mutations** or **nonsense mutations** (leading to premature stop codons) in $\beta$-thalassemia, or large **gene deletions** in $\alpha$-thalassemia. It is a quantitative defect in globin chain synthesis, rather than a qualitative structural change. * **Sideroblastic Anemia:** This is primarily a defect in heme synthesis (often due to ALAS2 enzyme deficiency) rather than a specific point mutation in the globin gene. * **Hemochromatosis:** While the most common cause (HFE gene mutation, C282Y) is technically a missense mutation, it is not considered the "classic" teaching example for this genetic concept in medical biochemistry compared to Sickle Cell Disease. **High-Yield Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** The SCD mutation (A $\rightarrow$ T) is a **transversion** (purine to pyrimidine). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because it loses the negative charge of glutamic acid. * **Sticky Patches:** The substitution of Valine creates "sticky" hydrophobic patches on the surface of the hemoglobin molecule.

Q: Which of the following is NOT a function of restriction endonucleases?

They are non-specific to base pairs.. **Explanation:** Restriction Endonucleases (REs), often called "molecular scissors," are enzymes primarily derived from bacteria where they serve as a defense mechanism against viral DNA. **Why Option D is the Correct Answer:** The defining characteristic of restriction endonucleases is their **high specificity**. They recognize and bind to specific DNA sequences, typically 4 to 8 base pairs long, known as **palindromic sequences** (reading the same 5'→3' on both strands). Because they only cut at these precise recognition sites, saying they are "non-specific" is factually incorrect, making it the right choice for this "NOT" question. **Analysis of Incorrect Options:** * **Option A:** REs function by hydrolyzing the phosphodiester bonds on **both strands** of the DNA double helix at specific points within or near the recognition site. * **Option B:** Many REs (like *EcoRI*) make staggered cuts, leaving short, single-stranded overhangs known as **sticky (cohesive) ends**. These are highly useful in recombinant DNA technology as they can easily hybridize with complementary sequences. * **Option C:** Some REs (like *SmaI*) cut straight across the DNA at the same position on both strands, resulting in **blunt ends**. These lack overhangs and are generally more difficult to ligate. **High-Yield Clinical Pearls for NEET-PG:** * **Type II REs** are the most commonly used in genetic engineering because they cut exactly at the recognition site and do not require ATP. * **Naming Convention:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., *EcoRI*: *Escherichia coli*, strain RY13, 1st enzyme). * **Restriction Fragment Length Polymorphism (RFLP):** A technique using REs to detect genetic variations/mutations, crucial in forensic medicine and prenatal diagnosis of diseases like Sickle Cell Anemia.

Q: Genomic imprinting is observed in which of the following conditions?

Angelman syndrome. **Explanation:** **Genomic Imprinting** is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. This means one allele is "silenced" (via DNA methylation) depending on whether it was inherited from the mother or the father. **Why Angelman Syndrome is Correct:** Angelman syndrome and Prader-Willi syndrome are the classic examples of imprinting defects involving **Chromosome 15 (q11-q13)**. * **Angelman Syndrome ("Happy Puppet"):** Occurs when the **maternal** allele (UBE3A gene) is deleted or mutated, while the paternal allele is normally imprinted (silenced). * **Prader-Willi Syndrome:** Occurs when the **paternal** allele is deleted, while the maternal allele is silenced. **Analysis of Incorrect Options:** * **A & B (Klinefelter’s and Down’s Syndrome):** These are **Aneuploidies** (numerical chromosomal aberrations) caused by nondisjunction during meiosis. They do not involve gene silencing via imprinting. * **D (Hydatidiform Mole):** While a complete mole involves only paternal chromosomes (and is an example of genomic imbalance), it is a gestational trophoblastic disease rather than a classic "imprinting disorder" syndrome like Angelman. **High-Yield Clinical Pearls for NEET-PG:** * **Uniparental Disomy (UPD):** A common cause of imprinting disorders where an individual inherits two copies of a chromosome from one parent and none from the other. * **Angelman Presentation:** Inappropriate laughter, seizures, ataxia, and severe intellectual disability. * **Prader-Willi Presentation:** Hyperphagia (obesity), hypogonadism, and hypotonia. * **Other Imprinting Disorders:** Beckwith-Wiedemann Syndrome (Chromosome 11p15).

Question 1

Which of the following diseases is NOT characterized by a defect in DNA repair mechanisms?

Accepted Answer

Huntington's disease

Answer

Xeroderma Pigmentosa

Answer

Fanconi syndrome

Answer

Hereditary nonpolyposis colon cancer

Question 2

What are the components of the 50S ribosomal subunit?

Accepted Answer

23S RNA & 5S RNA

Answer

5S RNA, 5.8S & 28S RNA

Answer

16S RNA & 23S RNA

Answer

23S RNA & 5.8S RNA

Question 3

Which gene product is transcribed but not translated?

Accepted Answer

t-RNA

Answer

Glycosyl transferase

Answer

Keratin

Answer

Histone

Question 4

Multiple proteins from a single gene may be generated by:

Accepted Answer

Alternative splicing

Answer

Mutations in the promoter of the gene

Answer

RNA interference

Answer

mRNA capping

Question 5

Friedreich's ataxia is caused due to triplet repeats of which nucleotide sequence?

Accepted Answer

GAA

Answer

AGG

Answer

UAA

Answer

AUG

Question 6

Which of the following is a classic example of a missense mutation?

Accepted Answer

Sickle cell disease

Answer

Thalassemia

Answer

Sideroblastic anemia

Answer

Hemochromatosis

Question 7

Which of the following is NOT a function of restriction endonucleases?

Accepted Answer

They are non-specific to base pairs.

Answer

They cut both strands of double-stranded DNA.

Answer

The cut ends produced by restriction endonucleases can be sticky.

Answer

The cut ends produced by restriction endonucleases can be blunt.

Question 8

Genomic imprinting is observed in which of the following conditions?

Accepted Answer

Angelman syndrome

Answer

Klinefelter's syndrome

Answer

Down's syndrome

Answer

Hydatidiform mole

Question 9

Which of the following is required for certain types of eukaryotic protein synthesis but not for prokaryotic protein synthesis?

Accepted Answer

Signal recognition particle

Answer

Ribosomal RNA

Answer

Messenger RNA

Answer

Peptidyl transferase

Question 10

Signal peptides are synthesized in which cellular component?

Accepted Answer

Membrane-bound ribosomes

Answer

Endoplasmic Reticulum (ER)

Answer

Free ribosomes

Answer

Golgi apparatus

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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