Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 361: Which of the following is true about telomerase or telomeres?

A. They are present at the ends of eukaryotic chromosomes.
B. Increased telomerase activity favors cancer cells. (Correct Answer)
C. DNA dependent RNA polymerase.
D. DNA polymerase.

Explanation: ### Explanation **Correct Answer: B. Increased telomerase activity favors cancer cells.** **Concept:** Telomeres are repetitive DNA sequences (TTAGGG) located at the ends of linear eukaryotic chromosomes. Due to the "end-replication problem," DNA polymerase cannot fully replicate the extreme 3' end of the lagging strand, causing telomeres to shorten with every cell division. Once telomeres reach a critical length, the cell undergoes senescence or apoptosis (the Hayflick limit). **Cancer Correlation:** Cancer cells achieve "immortality" by upregulating **telomerase**, an enzyme that maintains telomere length. This prevents the cell from entering senescence, allowing for indefinite proliferation and tumor growth. **Analysis of Incorrect Options:** * **Option A:** While telomeres are indeed at the ends of eukaryotic chromosomes, the question asks for the *best* true statement among the choices. In many competitive exams like NEET-PG, if multiple statements are technically true, the one describing a functional or clinical mechanism (like cancer immortality) is often prioritized. However, in a strict sense, A is also a fact; but B is the hallmark pathological significance. * **Option C:** Telomerase is an **RNA-dependent DNA polymerase** (Reverse Transcriptase). It carries its own RNA template to synthesize DNA. It is *not* a DNA-dependent RNA polymerase (which makes mRNA). * **Option D:** While telomerase is a specialized type of DNA polymerase, simply calling it "DNA polymerase" is imprecise and usually refers to the standard replicative enzymes (α, δ, ε) which cannot extend telomeres. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Telomerase consists of a protein component (**TERT** - Telomerase Reverse Transcriptase) and an RNA template (**TERC**). * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as DNA double-strand breaks. * **Progeria (Hutchinson-Gilford Syndrome):** Characterized by accelerated telomere shortening, leading to premature aging. * **Cells with high telomerase:** Germ cells, stem cells, and cancer cells. Somatic cells generally have low or no telomerase activity.

Question 362: What is the function of mitochondrial DNA?

A. Encodes proteins of the cell membrane
B. Encodes proteins of the respiratory chain (Correct Answer)
C. Helps in cell replication
D. Formation of rRNA

Explanation: ### Explanation **Correct Answer: B. Encodes proteins of the respiratory chain** Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule (approx. 16.5 kb) that follows maternal inheritance. Its primary function is to encode essential components of the **Oxidative Phosphorylation (OXPHOS)** system. Specifically, human mtDNA encodes **13 polypeptides**, all of which are subunits of the respiratory chain complexes (Complex I, III, IV, and V). It also encodes 2 rRNAs and 22 tRNAs required for their translation within the mitochondria. **Why other options are incorrect:** * **Option A:** Proteins of the cell membrane are encoded by **nuclear DNA** and synthesized by ribosomes in the cytosol or on the rough endoplasmic reticulum. * **Option C:** While mitochondria replicate independently of the cell cycle, the overall process of cell replication (mitosis/meiosis) is governed by nuclear DNA and regulatory proteins like cyclins and CDKs. * **Option D:** While mtDNA does form its own specific mitochondrial rRNAs (12S and 16S), the vast majority of cellular rRNA (which forms the cytoplasmic ribosomes) is synthesized in the **nucleolus** by RNA Polymerase I. **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother; thus, mitochondrial diseases (e.g., **MELAS, LHON, MERRF**) affect both sons and daughters, but only daughters can pass the trait to the next generation. * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (normal and mutated mtDNA) within a cell. This explains the variable clinical severity of mitochondrial diseases. * **High Mutation Rate:** mtDNA lacks histones and has less efficient repair mechanisms compared to nuclear DNA, making it 10 times more prone to mutations.

Question 363: Which of the following statements describing restriction endonucleases is true?

A. They always yield overhanging single-stranded ends
B. They recognize methylated DNA sequences
C. They recognize triplet repeats
D. They cleave both strands in duplex DNA (Correct Answer)

Explanation: **Explanation:** **Restriction Endonucleases (REs)**, often called "molecular scissors," are bacterial enzymes used in recombinant DNA technology. The correct answer is **D** because these enzymes function by recognizing specific palindromic sequences and catalyzing the hydrolysis of phosphodiester bonds on **both strands** of the DNA duplex. **Why the other options are incorrect:** * **Option A:** While many REs (like *EcoRI*) create "sticky" or overhanging ends, others (like *SmaI*) cut straight across the duplex to produce **blunt ends**. Therefore, they do not *always* yield overhanging ends. * **Option B:** REs generally **do not** recognize methylated DNA. In fact, bacteria use **DNA methylation** (via methyltransferases) to protect their own genome from being cleaved by their own restriction enzymes. This is known as the Restriction-Modification System. * **Option C:** REs recognize specific **palindromic sequences** (typically 4–8 base pairs long that read the same 5'→3' on both strands), not triplet repeats. Triplet repeats are associated with genetic disorders like Huntington’s disease. **High-Yield Clinical Pearls for NEET-PG:** * **Type II REs** are the most commonly used in labs because they cut DNA at specific, predictable sites and do not require ATP. * **Restriction Fragment Length Polymorphism (RFLP):** A technique using REs to detect genetic variations (e.g., in sickle cell anemia, where a mutation abolishes a specific *MstII* recognition site). * **Palindromes:** Remember that REs look for symmetry. Example: 5'-GAATTC-3' (the complementary strand is also 5'-GAATTC-3').

Question 364: DNA repair defects are seen in which of the following conditions?

A. Xeroderma pigmentosa (Correct Answer)
B. Bloom's syndrome
C. Ataxia telangiectasia
D. Li-Fraumeni syndrome

Explanation: **Explanation:** The correct answer is **Xeroderma Pigmentosa (XP)**. This condition is the classic example of a defect in **Nucleotide Excision Repair (NER)**. In healthy individuals, NER removes pyrimidine dimers (usually thymine dimers) caused by UV light. In XP patients, the inability to repair this DNA damage leads to extreme photosensitivity, severe sunburns, and a 2000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **Analysis of Options:** * **Bloom’s Syndrome:** While it involves genomic instability, it is specifically caused by a mutation in the *BLM* gene, which encodes a **DNA Helicase** enzyme. It is characterized by short stature, telangiectatic erythema, and "sister chromatid exchanges." * **Ataxia Telangiectasia:** This is caused by a mutation in the *ATM* gene. The ATM protein is a kinase responsible for detecting **double-strand breaks** and signaling cell cycle arrest. It is not primarily classified under the standard DNA repair pathways like NER. * **Li-Fraumeni Syndrome:** This is caused by a germline mutation in the **TP53** gene (tumor suppressor). While p53 triggers DNA repair or apoptosis, the syndrome itself is defined as a cell cycle regulation/checkpoint defect rather than a primary DNA repair pathway defect. **High-Yield Clinical Pearls for NEET-PG:** * **HNPCC (Lynch Syndrome):** Defect in **Mismatch Repair (MMR)** genes (*MSH2, MLH1*). * **Fanconi Anemia:** Defect in repair of **DNA inter-strand cross-links**. * **BRCA 1/2 Mutations:** Defect in **Homologous Recombination** (double-strand break repair). * **Cockayne Syndrome:** A subtype of NER defect (transcription-coupled repair) presenting with "bird-like" facies and premature aging, but *without* an increased risk of skin cancer.

Question 365: Which of the following is not a cause of point mutation?

A. Paracentric inversion (Correct Answer)
B. Deletion
C. Substitution
D. Insertion

Explanation: **Explanation:** The core of this question lies in distinguishing between **Gene (Point) Mutations** and **Chromosomal Aberrations**. **1. Why Paracentric Inversion is the Correct Answer:** A **point mutation** involves a change in a single nucleotide base pair or a very small segment of DNA within a single gene. **Paracentric inversion**, however, is a large-scale **chromosomal mutation**. It occurs when a segment of a chromosome breaks at two points, flips 180 degrees, and reinserts itself *without* involving the centromere. Because this involves the rearrangement of large chromosomal segments rather than a single base change, it is not classified as a point mutation. **2. Analysis of Incorrect Options:** * **Substitution (Option C):** This is the classic point mutation where one base is replaced by another (e.g., Transition or Transversion). It can lead to silent, missense, or nonsense mutations. * **Deletion (Option B) & Insertion (Option D):** When these involve only one or two nucleotides, they are considered point mutations. These are particularly significant because they often cause a **Frameshift Mutation**, altering the entire downstream reading frame of the mRNA, usually resulting in a non-functional protein. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Transition vs. Transversion:** Transition (Purine to Purine) is more common than Transversion (Purine to Pyrimidine). * **Sickle Cell Anemia:** A classic example of a point mutation (Missense) where Glutamic acid is replaced by Valine at the 6th position of the β-globin chain. * **Inversions:** Do not result in loss of genetic material (balanced rearrangement) but can lead to infertility or abnormal offspring due to crossover difficulties during meiosis. * **Frameshift Mutations:** Often lead to premature **stop codons** (UAA, UAG, UGA).

Question 366: Apolipoprotein B48 and Apolipoprotein B100 are synthesized from the same mRNA due to a difference in what process?

A. Splicing and chemical modification
B. Splicing
C. Differential RNA processing (Correct Answer)
D. All of the above

Explanation: ### Explanation The synthesis of Apolipoprotein B48 (Apo B48) and Apolipoprotein B100 (Apo B100) from a single gene is a classic example of **RNA Editing**, which falls under the broader category of **Differential RNA processing**. **Why the correct answer is right:** Both proteins are encoded by the *APOB* gene. In the liver, the mRNA is translated directly to produce **Apo B100** (the full-length protein). However, in the small intestine, a specific enzyme called **Cytidine Deaminase** acts on the mRNA. It converts a Cytosine (C) to Uracil (U) at codon 2153. This change transforms the original glutamine codon (**CAA**) into a premature stop codon (**UAA**). Consequently, translation terminates early, producing **Apo B48**, which represents the N-terminal 48% of the full protein. This tissue-specific modification of the mRNA sequence after transcription is a form of differential RNA processing. **Why incorrect options are wrong:** * **Splicing (Options A & B):** Splicing involves the removal of introns and joining of exons. While "Alternative Splicing" can create different proteins from one gene (e.g., Calcitonin/CGRP), the Apo B diversity is specifically due to **base substitution (editing)**, not the rearrangement of exons. * **Chemical modification (Option A):** While RNA editing is a chemical change (deamination), "chemical modification" usually refers to 5' capping or 3' polyadenylation, which do not change the coding sequence to create different protein isoforms in this context. **NEET-PG High-Yield Pearls:** * **Apo B100:** Found in VLDL, IDL, and LDL; synthesized in the **Liver**. * **Apo B48:** Found in Chylomicrons; synthesized in the **Small Intestine**. * **Enzyme:** Cytidine Deaminase (C $\rightarrow$ U editing). * **Memory Aid:** **L**iver = **L**ong (B100); **S**mall Intestine = **S**hort (B48).

Question 367: Which enzyme is involved in peptide chain synthesis?

A. Topoisomerase
B. Transformylase
C. RNA polymerase
D. Peptidyl transferase (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Peptidyl transferase** **Why it is correct:** Peptidyl transferase is the primary enzyme responsible for **peptide bond formation** during the elongation phase of translation (protein synthesis). It catalyzes the reaction between the carboxyl group of the amino acid in the P-site and the amino group of the new amino acid in the A-site. * **Key Concept:** In both prokaryotes and eukaryotes, this is not a protein-based enzyme but a **ribozyme** (catalytic RNA). In prokaryotes, it is the **23S rRNA** (of the 50S subunit), and in eukaryotes, it is the **28S rRNA** (of the 60S subunit). **Why the other options are incorrect:** * **A. Topoisomerase:** These enzymes regulate DNA supercoiling during replication and transcription by creating transient breaks in the DNA backbone. They are not involved in protein synthesis. * **B. Transformylase:** This enzyme is involved in the initiation of translation in prokaryotes. It transfers a formyl group to methionyl-tRNA to form **fMet-tRNA**, which serves as the initiator tRNA. It does not synthesize the peptide chain itself. * **C. RNA polymerase:** This enzyme is central to **transcription**, responsible for synthesizing RNA (mRNA, tRNA, rRNA) from a DNA template, not for synthesizing peptide chains from amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Antibiotic Target:** Several antibiotics inhibit the 50S subunit to block peptidyl transferase activity, most notably **Chloramphenicol**. * **Ribozyme Nature:** Remember that the ribosome is a ribozyme; the catalytic activity resides in the RNA, not the ribosomal proteins. * **Energy Source:** While peptide bond formation itself is catalyzed by peptidyl transferase, the overall process of translation requires energy in the form of **GTP**.

Question 368: What is the rapid method for chromosome identification in interphase?

A. FISH (Correct Answer)
B. PCR
C. SSCP
D. Karyotyping

Explanation: **Explanation:** The correct answer is **FISH (Fluorescent In Situ Hybridization)**. **Why FISH is the correct answer:** FISH is a molecular cytogenetic technique that uses fluorescently labeled DNA probes to bind to specific complementary DNA sequences on a chromosome. Its primary advantage in this context is that it **does not require cell culture or metaphase spread**. Because the probes can hybridize to DNA while it is still in the decondensed state within the nucleus, it allows for the rapid identification of numerical and structural chromosomal abnormalities directly in **interphase**. This is particularly useful for rapid prenatal diagnosis (e.g., Trisomy 21) or identifying gene amplifications (e.g., HER2/neu). **Why the other options are incorrect:** * **PCR (Polymerase Chain Reaction):** While rapid, PCR is used to amplify specific DNA sequences to detect mutations or presence of pathogens; it does not provide a visual "identification" of a whole chromosome or its location. * **SSCP (Single-Strand Conformation Polymorphism):** This is a screening method used to detect small sequence variations or point mutations based on the folding patterns of single-stranded DNA; it is not a chromosomal identification tool. * **Karyotyping:** This is the "gold standard" for chromosomal analysis but requires cells to be arrested in **metaphase**. This necessitates time-consuming cell culture (often 48–72 hours), making it much slower than interphase FISH. **High-Yield NEET-PG Pearls:** * **Sky-high yield:** FISH is the method of choice for detecting **microdeletions** (e.g., DiGeorge Syndrome, 22q11.2) that are too small to be seen on a standard karyotype. * **Comparative Genomic Hybridization (CGH):** Used to detect copy number variations (CNVs) across the entire genome but cannot detect balanced translocations. * **Spectral Karyotyping (SKY):** A variation of FISH where every chromosome pair is painted a different color; excellent for identifying complex translocations and marker chromosomes.

Question 369: Which of the following is TRUE regarding the ribozyme, the first recognized "molecular machine"?

A. Peptidyl transferase activity (Correct Answer)
B. Cut DNA at a specific site
C. Participates in DNA synthesis
D. GTPase activity

Explanation: **Explanation:** A **ribozyme** is a ribonucleic acid (RNA) molecule capable of acting as an enzyme. This discovery challenged the traditional dogma that all biological catalysts are proteins. **Why Option A is Correct:** The most clinically significant ribozyme in human biology is the **23S rRNA** (in prokaryotes) or **28S rRNA** (in eukaryotes) located within the large ribosomal subunit. This RNA component possesses **peptidyl transferase activity**, which catalyzes the formation of peptide bonds during protein synthesis (translation). Because the catalytic site is composed entirely of RNA, the ribosome is fundamentally a ribozyme. **Why the Other Options are Incorrect:** * **Option B:** Cutting DNA at specific sites is the function of **Restriction Endonucleases** (protein enzymes), not ribozymes. (Note: While some RNA-guided systems like CRISPR-Cas9 involve RNA, the "cutting" is performed by the Cas9 protein). * **Option C:** DNA synthesis is catalyzed by **DNA Polymerase**, which is a protein-based enzyme. * **Option D:** GTPase activity in translation is associated with protein factors like **EF-G and EF-Tu**, which provide energy for translocation, not the ribozyme itself. **High-Yield Facts for NEET-PG:** * **Examples of Ribozymes:** Peptidyl transferase, RNase P (processes tRNA), and SnRNAs in Spliceosomes (remove introns). * **Clinical Correlation:** Many antibiotics, such as **Macrolides** (Erythromycin) and **Chloramphenicol**, work by binding to the ribosomal subunits and inhibiting this specific ribozyme-mediated peptidyl transferase activity. * **Evolutionary Significance:** The existence of ribozymes supports the **"RNA World" hypothesis**, suggesting RNA preceded DNA and proteins in evolution.

Question 370: Which of the following eukaryotic transcription factors recognizes the promoter?

A. Rho
B. Pribnow box
C. Sigma
D. TFIID (Correct Answer)

Explanation: ### Explanation In eukaryotes, RNA Polymerase II cannot initiate transcription alone; it requires a group of proteins called **General Transcription Factors (GTFs)**. **1. Why TFIID is correct:** **TFIID** is the first component of the basal transcription apparatus to bind to the promoter. It is a multi-protein complex consisting of the **TATA-binding protein (TBP)** and TBP-associated factors (TAFs). TBP specifically recognizes and binds to the **TATA box** (located approximately 25-30 base pairs upstream of the transcription start site), effectively "marking" the promoter for the recruitment of RNA Polymerase II and other GTFs (TFIIA, TFIIB, TFIIE, TFIIF, and TFIIH). **2. Why the other options are incorrect:** * **Rho (A):** This is a protein involved in **prokaryotic** transcription termination (Rho-dependent termination). It is not involved in promoter recognition. * **Pribnow box (B):** This is a **DNA sequence** (TATAAT) found in **prokaryotic** promoters (the -10 element). It is a site, not a transcription factor. * **Sigma (C):** The **Sigma factor ($\sigma$)** is a subunit of the **prokaryotic** RNA polymerase holoenzyme. It is responsible for promoter recognition in bacteria, not eukaryotes. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **TFIIH** is unique because it has both **helicase** activity (to unwind DNA) and **kinase** activity (to phosphorylate the CTD of RNA Polymerase II). * Defects in TFIIH subunits are associated with clinical conditions like **Xeroderma Pigmentosum** and **Cockayne Syndrome** due to its role in Nucleotide Excision Repair (NER). * **Alpha-amanitin** (from the *Amanita phalloides* mushroom) specifically inhibits **RNA Polymerase II**, leading to severe hepatotoxicity.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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