Molecular Biology and Genomics Practice Questions

Q: What is true about DNA methylation?

All of the above.. DNA methylation is a key **epigenetic modification** involving the addition of a methyl group to the 5th carbon of the cytosine ring, typically within **CpG islands**. **Explanation of Options:** * **A. Alters gene expression:** Methylation of promoter regions generally leads to **gene silencing**. It recruits methyl-CpG-binding domain proteins (MBDs) and histone deacetylases, which condense chromatin (heterochromatin), making it inaccessible to transcription factors. * **B. Genetic code remains intact:** This is the hallmark of epigenetics. Methylation changes the *phenotype* (expression) without altering the *genotype* (DNA sequence). The underlying nucleotide sequence remains unchanged. * **C. Carcinogenesis and Restriction Endonucleases:** * **In Cancer:** Global hypomethylation (causing genomic instability) and focal hypermethylation of **tumor suppressor genes** (e.g., *BRCA1, RB1*) are classic drivers of malignancy. * **In Bacteria:** The **Restriction-Modification (R-M) system** uses methylation to mark "self" DNA. This protects the bacterial genome from being degraded by its own restriction endonucleases, which are designed to cleave unmethylated "foreign" viral DNA. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme:** DNA methyltransferase (DNMT) uses **S-adenosylmethionine (SAM)** as the methyl donor. * **Genomic Imprinting:** Methylation is the mechanism behind Prader-Willi and Angelman syndromes (Chromosome 15). * **Fragile X Syndrome:** Characterized by hypermethylation of the *FMR1* gene due to CGG triplet repeat expansion. * **Lyonization:** X-chromosome inactivation in females occurs via extensive DNA methylation.

Q: What is true about next-generation sequencing?

All of the above.. **Explanation:** Next-Generation Sequencing (NGS), also known as high-throughput sequencing, represents a paradigm shift from traditional Sanger sequencing by utilizing **massive parallel processing**. This allows millions of DNA fragments to be sequenced simultaneously rather than one at a time. * **Option A is correct:** Due to its high-throughput nature, NGS has drastically reduced the time required for genomic analysis. While the first Human Genome Project took 13 years, modern NGS platforms can sequence an entire human genome in **under 24 hours**. * **Option B is correct:** NGS is significantly more sensitive than Sanger sequencing. It can detect **low-frequency variants** (somatic mutations) and mosaicism that Sanger sequencing (which has a detection limit of ~15-20%) would typically miss. * **Option C is correct:** NGS is not limited to DNA. **RNA-Seq** (Transcriptome sequencing) allows for the discovery of novel RNA variants, alternative splicing patterns, and gene fusion events, which are critical in oncology and personalized medicine. Since all individual statements are accurate descriptions of NGS capabilities, **Option D (All of the above)** is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **Sanger Sequencing:** Still the "Gold Standard" for clinical validation of single gene variants but is low-throughput. * **Library Preparation:** The first step in NGS involving fragmentation of DNA and ligation of adapters. * **Bridge Amplification:** A key feature of Illumina sequencing (the most common NGS technology). * **Clinical Application:** NGS is the preferred method for **Liquid Biopsies** (detecting circulating tumor DNA) and diagnosing rare genetic disorders via **Whole Exome Sequencing (WES)**.

Q: What part of mRNA is removed during protein synthesis?

Interon. ### Explanation The correct answer is **A. Intron** (spelled as "Interon" in some question banks). **1. Why Introns are the Correct Answer:** In eukaryotes, the initial product of transcription is **pre-mRNA** (hnRNA). This precursor contains both coding sequences (**Exons**) and non-coding intervening sequences (**Introns**). Before translation (protein synthesis) can occur, the pre-mRNA undergoes **Post-transcriptional modification**. A critical step is **Splicing**, where introns are excised and removed by the spliceosome complex, and exons are joined together to form the mature mRNA. Therefore, introns are the parts removed to ensure only the functional genetic code reaches the ribosome. **2. Analysis of Incorrect Options:** * **B. Codon:** These are sequences of three nucleotides that correspond to a specific amino acid. They are the fundamental units of the genetic code and are *read* during protein synthesis, not removed. * **C. Exon:** These are the "expressed" sequences. They carry the actual information for the protein sequence and are retained and joined together after splicing. * **D. Suistron (Cistron):** A cistron is a genetic unit synonymous with a gene (a segment of DNA that encodes a single polypeptide). It is a structural unit, not a part removed during processing. **3. NEET-PG High-Yield Clinical Pearls:** * **Spliceosome:** Composed of **snRNPs** (small nuclear ribonucleoproteins). Autoantibodies against these (Anti-Smith antibodies) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **Alternative Splicing:** A process where different combinations of exons are joined, allowing one single gene to code for multiple different proteins (e.g., membrane-bound vs. secreted antibodies). * **Splicing Mutations:** Mutations at splice sites are a common cause of genetic diseases like **β-Thalassemia**.

Q: Angelman syndrome is due to what genetic mechanism?

Uniparental disomy. **Explanation:** Angelman syndrome (AS) is a classic example of **Genomic Imprinting**, where the expression of a gene depends on whether it is inherited from the mother or the father. It is caused by the loss of function of the **UBE3A gene** on chromosome 15 (15q11-q13). While the most common cause is a maternal deletion, **Paternal Uniparental Disomy (UPD)**—where a child inherits two copies of chromosome 15 from the father and none from the mother—accounts for approximately 3–7% of cases. Since the paternal UBE3A gene is normally silenced (imprinted), UPD results in a total lack of active UBE3A expression in the brain. **Analysis of Options:** * **Mitochondrial inheritance (A):** This involves genes located in the mitochondrial DNA, passed exclusively from the mother (e.g., MELAS, LHON). AS involves nuclear DNA on chromosome 15. * **Chimerism (B):** This refers to an individual composed of cells from two different zygotes. It is not the mechanism for imprinting disorders. * **Mosaicism (C):** This is the presence of two or more populations of cells with different genotypes in one individual (e.g., some cells having Trisomy 21 and others being normal). While rare mosaic forms of AS exist, UPD is a primary recognized mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **Prader-Willi Syndrome (PWS):** The "sister" condition caused by **Maternal UPD** (or paternal deletion) of the same 15q11-q13 region. * **Mnemonic:** **M**aternal deletion/Paternal UPD = **A**ngelman (**M**appy Child: **M**aternal **A**ngelman, **P**uppet-like gait, **P**aroxysms of laughter). * **Clinical Features of AS:** "Happy Puppet" syndrome—inappropriate laughter, seizures, ataxia, and severe intellectual disability.

Q: Which enzyme produces single-strand nicks in DNA?

DNase I. **Explanation:** **DNase I (Deoxyribonuclease I)** is an endonuclease that cleaves phosphodiester bonds in DNA. Its primary characteristic is that it acts on double-stranded DNA (dsDNA) to produce **single-strand nicks** (random internal breaks) rather than cutting both strands simultaneously. In molecular biology, this property is exploited for "Nick Translation" and "DNase Footprinting" to study DNA-protein interactions. **Analysis of Incorrect Options:** * **DNA Polymerase I:** This enzyme is primarily involved in DNA replication and repair. While it has 5'→3' and 3'→5' exonuclease activity to remove RNA primers and proofread, its main function is **synthesis** of DNA, not the production of nicks. * **Polynucleotide Kinase (PNK):** This enzyme catalyzes the transfer of a phosphate group from ATP to the **5'-hydroxyl terminus** of DNA or RNA. It is used for end-labeling nucleic acids, not for cleaving the phosphodiester backbone. * **5'-Exonuclease:** Exonucleases remove nucleotides one by one from the **ends** of a DNA molecule. They cannot produce internal nicks in a circular or intact linear dsDNA molecule as they require a pre-existing free end to function. **High-Yield Clinical Pearls for NEET-PG:** * **DNase I Clinical Use:** Recombinant human DNase (Dornase alfa) is used in **Cystic Fibrosis** to reduce the viscosity of sputum by digesting the extracellular DNA released from degenerating neutrophils. * **Topoisomerase I:** Another enzyme that produces transient single-strand nicks to relieve torsional strain (supercoiling) during replication. * **Restriction Endonucleases:** Unlike DNase I, these produce specific double-strand breaks at palindromic sequences.

Q: Which of the following amino acid substitutions is considered a conservative mutation?

Alanine-leucine. ### Explanation **Concept Overview:** A **conservative mutation** is a type of missense mutation where one amino acid is replaced by another with **similar physicochemical properties** (e.g., charge, hydrophobicity, or size). Because the new amino acid "mimics" the original, the overall tertiary structure and function of the protein often remain largely unaffected. **Why Option C is Correct:** * **Alanine and Leucine** are both **non-polar, hydrophobic, branched-chain aliphatic amino acids**. * Substituting Alanine with Leucine preserves the hydrophobic nature of the protein core. While the side chain size differs slightly, their chemical behavior is identical, making this a classic example of a conservative substitution. **Analysis of Incorrect Options:** * **A. Glutamic acid (Acidic/Negative) to Glutamine (Neutral/Polar):** This changes a charged residue to an uncharged one, potentially disrupting ionic bonds (salt bridges). * **B. Histidine (Basic/Positive) to Glycine (Smallest/Neutral):** This is a non-conservative change. Glycine provides extreme conformational flexibility and lacks the bulky, charged imidazole ring of Histidine. * **D. Arginine (Basic/Positive) to Aspartic acid (Acidic/Negative):** This is a **radical substitution**. Reversing the charge from positive to negative drastically alters protein folding and enzymatic activity. **NEET-PG High-Yield Pearls:** * **Sickle Cell Anemia:** A classic **non-conservative** mutation where Glutamic acid (polar) is replaced by Valine (non-polar) at the 6th position of the $\beta$-globin chain. * **Transition vs. Transversion:** Transitions (Purine to Purine) are more likely to result in conservative mutations than Transversions (Purine to Pyrimidine). * **Silent Mutation:** A point mutation that changes the codon but results in the *same* amino acid (due to degeneracy of the genetic code).

Q: What sequence is present at the 3' end of tRNA?

CCA sequence. **Explanation:** The correct answer is **B. CCA sequence**. Transfer RNA (tRNA) is a cloverleaf-shaped molecule essential for protein synthesis. The **3' end** of all mature tRNA molecules terminates in the highly conserved sequence **5'-CCA-3'**. This is known as the **acceptor stem**. The amino acid is covalently attached to the hydroxyl (-OH) group of the adenosine (A) residue in this CCA sequence by the enzyme *aminoacyl-tRNA synthetase*. **Analysis of Incorrect Options:** * **A. Poly 'A' tail:** This is a long chain of adenine nucleotides added to the **3' end of mRNA** (not tRNA) to enhance stability and facilitate nuclear export. * **C. Anticodon:** This is a triplet of nucleotides located in the **Anticodon Loop** (at the opposite end of the acceptor stem). It base-pairs with the complementary codon on mRNA to ensure the correct amino acid is incorporated. * **D. D arm:** This is a lateral loop of the tRNA containing **Dihydrouridine**. Its primary function is the recognition of the specific aminoacyl-tRNA synthetase enzyme. **High-Yield NEET-PG Pearls:** 1. **Post-transcriptional modification:** In eukaryotes, the CCA sequence is not coded by the DNA but is added enzymatically by **tRNA nucleotidyltransferase**. 2. **TψC Arm:** Contains Pseudouridine and is responsible for binding the tRNA to the **ribosome** (specifically the 5S rRNA). 3. **Smallest RNA:** tRNA is the smallest (73–93 nucleotides) and is also known as **Soluble RNA (sRNA)**. 4. **Charging:** The process of adding an amino acid to the CCA end is called "charging" or aminoacylation, which requires ATP.

Q: Highly repetitive DNA is seen in which of the following locations?

All of these. ### Explanation The human genome is categorized into **Single-copy DNA** (coding sequences) and **Repetitive DNA** (non-coding sequences). Repetitive DNA is further divided based on the length of the repeat unit and its location. **Highly repetitive DNA**, often referred to as **Satellite DNA**, consists of short sequences repeated thousands to millions of times. **Why "All of these" is correct:** Highly repetitive DNA is characteristically found in specific structural regions of the chromosome: * **Centromeres:** These contain **Alpha-satellite DNA** (171 bp repeats). This repetitive sequence is essential for kinetochore assembly and proper spindle attachment during cell division. * **Telomeres:** These consist of hexameric repeats (**TTAGGG** in humans). These repetitive "caps" protect the ends of chromosomes from degradation and fusion. * **Microsatellites (Short Tandem Repeats - STRs):** These are small repetitive sequences (2–6 bp) scattered throughout the genome. They are highly polymorphic between individuals. **Analysis of Options:** * **Option A (Microsatellites):** While these are scattered, they are a classic example of repetitive DNA used in DNA fingerprinting. * **Option B (Telomere):** These are specialized repetitive structures at chromosomal ends. * **Option C (Centromere):** This is the primary site for constitutive heterochromatin, which is composed almost entirely of highly repetitive DNA. **NEET-PG High-Yield Pearls:** 1. **Satellite DNA** is separated from bulk DNA during **density gradient centrifugation** as distinct "satellite" bands due to differing G-C content. 2. **Minisatellites (VNTRs):** Used in DNA fingerprinting (Southern Blotting). 3. **Microsatellites (STRs):** Used in forensic analysis and paternity testing via PCR. 4. **Telomerase:** A reverse transcriptase enzyme that maintains telomere length; its activity is high in cancer cells and germ cells but low in somatic cells.

Q: Which of the following blood cells contains drumsticks?

Neutrophils. **Explanation:** The correct answer is **Neutrophils**. **Why Neutrophils are correct:** In females, the **drumstick** (also known as a Davidson body) is a small, pedunculated nuclear appendage found in approximately 1–6% of mature **neutrophils**. This structure represents the inactivated X chromosome, known as **sex chromatin** or a **Barr body**. While Barr bodies are typically seen as dense masses against the nuclear membrane in buccal mucosal cells, they appear as "drumsticks" in neutrophils due to the unique multilobed nature of the polymorphonuclear leukocyte nucleus. **Why the other options are incorrect:** * **Lymphocytes & Monocytes:** These are mononuclear cells. While they do contain the inactivated X chromosome in females, it does not manifest as a "drumstick" appendage. In these cells, the Barr body is usually indistinguishable from other chromatin clumps within the nucleus. * **Platelets:** These are anucleated cell fragments derived from megakaryocytes. Since they lack a nucleus, they cannot possess nuclear appendages like drumsticks or Barr bodies. **High-Yield Clinical Pearls for NEET-PG:** * **Lyon’s Hypothesis:** Explains that X-inactivation is random, fixed, and occurs early in embryonic development. * **Formula:** The number of Barr bodies/drumsticks = (Total number of X chromosomes – 1). * **Clinical Correlation:** * **Turner Syndrome (45, XO):** Zero drumsticks (Male-like pattern). * **Klinefelter Syndrome (47, XXY):** One drumstick present (Female-like pattern). * **Diagnostic Note:** While historically used for sex determination, chromosomal analysis (Karyotyping) or PCR for the SRY gene are now the gold standards.

Question 1

What is true about DNA methylation?

Accepted Answer

All of the above.

Answer

It alters gene expression.

Answer

The genetic code remains intact.

Answer

It plays a role in carcinogenesis and acts as a protective mechanism against cleavage by restriction endonucleases.

Question 2

What is true about next-generation sequencing?

Accepted Answer

All of the above.

Answer

The entire human genome can be sequenced within a single day.

Answer

It is more sensitive than Sanger sequencing.

Answer

It utilizes RNA sequencing to discover novel RNA variants.

Question 3

What part of mRNA is removed during protein synthesis?

Accepted Answer

Interon

Answer

Codon

Answer

Exon

Answer

Suistron

Question 4

Angelman syndrome is due to what genetic mechanism?

Accepted Answer

Uniparental disomy

Answer

Mitochondrial inheritance

Answer

Chimerism

Answer

Mosaicism

Question 5

Which enzyme produces single-strand nicks in DNA?

Accepted Answer

DNase I

Answer

DNA polymerase I

Answer

Polynucleotide kinase

Answer

5'-exonuclease

Question 6

Which of the following amino acid substitutions is considered a conservative mutation?

Accepted Answer

Alanine-leucine

Answer

Glutamic acid-glutamine

Answer

Histidine-glycine

Answer

Arginine-aspartic acid

Question 7

What sequence is present at the 3' end of tRNA?

Accepted Answer

CCA sequence

Answer

Poly 'A' tail

Answer

Anticodon

Answer

D arm

Question 8

Highly repetitive DNA is seen in which of the following locations?

Accepted Answer

All of these

Answer

Microsatellite DNA

Answer

Telomere

Answer

Centromere

Question 9

Radio-labeled DNA was allowed to replicate twice in a non-radioactive environment. Which of the following is true?

Accepted Answer

Half of the DNA will have no radioactivity

Answer

All the strands will have radioactivity

Answer

No strands will have radioactivity

Answer

Three-fourth of the DNA replicated will have radioactivity

Question 10

Which of the following blood cells contains drumsticks?

Accepted Answer

Neutrophils

Answer

Lymphocytes

Answer

Monocytes

Answer

Platelets

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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