Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 601: Thymidylated RNA is present in which of the following?

A. mRNA
B. rRNA
C. tRNA (Correct Answer)
D. 16S rRNA

Explanation: **Explanation:** The presence of **Thymidine** (5-methyluridine) is a unique feature of **tRNA** (Transfer RNA). While Thymine is typically exclusive to DNA and Uracil to RNA, tRNA undergoes extensive post-transcriptional modifications. One such modification is the methylation of Uracil to form Thymidine, which occurs specifically in the **TψC loop** (T-loop) of the tRNA molecule. This loop is essential for the binding of tRNA to the ribosomal surface during protein synthesis. **Analysis of Options:** * **tRNA (Correct):** Contains the "TψC arm," where 'T' stands for Ribothymidine, 'ψ' for Pseudouridine, and 'C' for Cytidine. This is the only major RNA species where thymine is a standard structural component. * **mRNA:** Primarily consists of Adenine, Guanine, Cytosine, and Uracil. Its modifications involve the 5' capping (7-methylguanosine) and 3' polyadenylation, but not thymidylation. * **rRNA (and 16S rRNA):** While ribosomal RNA contains many modified bases (like pseudouridine and methylated bases), thymidylated RNA is not a characteristic structural hallmark of rRNA in the same way it is for the T-loop of tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **The TψC Loop:** Responsible for recognition and binding to the **ribosome** (specifically the 5S rRNA of the large subunit). * **The DHU Loop:** Contains Dihydrouridine; it is responsible for recognition by the specific **Aminoacyl tRNA synthetase** enzyme. * **The Anticodon Loop:** Recognizes the specific codon on the mRNA. * **3' End:** Always ends in the sequence **CCA**, which is the attachment site for the amino acid (at the 3'-OH group). * **Unusual Bases in tRNA:** Include Inosine, Pseudouridine, Dihydrouridine, and Ribothymidine.

Question 602: What is a microsatellite sequence?

A. Small satellite
B. Extra chromosomal DNA
C. Short tandem repeat (STR) DNA (Correct Answer)
D. Looped DNA

Explanation: ### Explanation **Correct Answer: C. Short tandem repeat (STR) DNA** **Understanding the Concept:** Microsatellites, also known as **Short Tandem Repeats (STRs)**, are short sequences of DNA (typically 2–6 base pairs long) that are repeated multiple times in tandem at specific loci throughout the genome. They are a subset of repetitive DNA. Because the number of repeats varies significantly between individuals, they are highly **polymorphic**, making them the "gold standard" markers for DNA fingerprinting, paternity testing, and linkage analysis. **Analysis of Incorrect Options:** * **A. Small satellite:** While the name "microsatellite" implies size, it refers to the density of the DNA during centrifugation (forming a "satellite" band), not the physical size of a chromosome. "Small satellite" is not a standard genomic term. * **B. Extra chromosomal DNA:** This refers to DNA found outside the nucleus, such as **Mitochondrial DNA (mtDNA)** or plasmids in bacteria. Microsatellites are located on nuclear chromosomes. * **D. Looped DNA:** This refers to the structural organization of chromatin (e.g., during transcription or DNA packaging) and is unrelated to repetitive sequence classification. **High-Yield Clinical Pearls for NEET-PG:** 1. **Microsatellite Instability (MSI):** This is a critical clinical concept. MSI occurs when there is a defect in the **Mismatch Repair (MMR)** genes (e.g., *MLH1, MSH2*). 2. **Lynch Syndrome:** Also known as Hereditary Non-Polyposis Colorectal Cancer (HNPCC), it is characterized by MSI due to germline mutations in MMR genes. 3. **Trinucleotide Repeat Disorders:** These are a specific type of microsatellite expansion. Examples include **Huntington’s Disease** (CAG), **Fragile X Syndrome** (CGG), and **Friedreich Ataxia** (GAA). 4. **Forensics:** STR analysis is the primary method used by CODIS (Combined DNA Index System) for human identification.

Question 603: DNA from RNA is synthesized by?

A. Topoisomerase
B. Helicase
C. Reverse transcriptase (Correct Answer)
D. DNA dependent DNA polymerase

Explanation: ### Explanation **Correct Answer: C. Reverse transcriptase** **Concept:** In the standard "Central Dogma" of molecular biology, information flows from DNA to RNA (Transcription). However, certain viruses and cellular processes perform **Reverse Transcription**, where a DNA strand is synthesized using an RNA template. This process is catalyzed by the enzyme **Reverse Transcriptase** (also known as RNA-dependent DNA polymerase). **Why the other options are incorrect:** * **A. Topoisomerase:** These enzymes regulate the overwinding or underwinding of DNA. They relieve torsional strain (supercoiling) during replication and transcription by creating transient breaks in the DNA backbone. * **B. Helicase:** This enzyme is responsible for unwinding the DNA double helix at the replication fork by breaking the hydrogen bonds between complementary nitrogenous bases. It does not synthesize new strands. * **D. DNA-dependent DNA polymerase:** This is the primary enzyme for standard DNA replication (e.g., DNA Pol III in prokaryotes). It synthesizes a new DNA strand using an existing **DNA** template, not an RNA template. --- ### High-Yield Clinical Pearls for NEET-PG: * **Retroviruses:** The most clinically significant use of reverse transcriptase is by **HIV**. It converts its viral RNA genome into proviral DNA, which then integrates into the host genome. * **Telomerase:** This is a specialized reverse transcriptase (containing an internal RNA template) that maintains the length of telomeres. It is highly active in cancer cells and germ cells. * **Laboratory Use:** Reverse transcriptase is a critical component of **RT-PCR** (Reverse Transcription Polymerase Chain Reaction), used to detect RNA viruses like SARS-CoV-2 or to measure gene expression (mRNA). * **Drug Target:** Nucleoside Reverse Transcriptase Inhibitors (**NRTIs**) like Zidovudine (AZT) and Tenofovir are cornerstones of HAART therapy for HIV.

Question 604: Where does translation occur in a cell?

A. Ribosomes (Correct Answer)
B. Mitochondria
C. Nucleus
D. Cytoplasm

Explanation: **Explanation:** **1. Why Ribosomes are the Correct Answer:** Translation is the process of protein synthesis where the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids. The **ribosome** is the definitive site of translation. It acts as a complex molecular machine (composed of rRNA and proteins) that facilitates the binding of tRNA anticodons to mRNA codons and catalyzes peptide bond formation via its peptidyl transferase activity. **2. Analysis of Incorrect Options:** * **Nucleus (C):** This is the site of **Transcription** (DNA to mRNA) and DNA replication. Translation cannot occur here because the nuclear envelope separates the genetic material from the translational machinery. * **Cytoplasm (D):** While ribosomes are located *within* the cytoplasm (either free-floating or attached to the Rough ER), the cytoplasm itself is the medium. The specific functional unit performing the synthesis is the ribosome. In NEET-PG, if "Ribosome" is an option, it is the most specific and correct answer. * **Mitochondria (B):** While mitochondria do have their own ribosomes (mitoribosomes) and perform translation for their own 13 proteins, they are not the primary site for general cellular protein synthesis. **3. NEET-PG High-Yield Pearls:** * **Prokaryotic Ribosome:** 70S (50S + 30S). * **Eukaryotic Ribosome:** 80S (60S + 40S). * **Clinical Correlation:** Many antibiotics target translation. For example, **Aminoglycosides** and **Tetracyclines** bind to the 30S subunit, while **Macrolides** and **Chloramphenicol** bind to the 50S subunit. * **Peptidyl Transferase:** In eukaryotes, this ribozyme activity is associated with the **28S rRNA** of the 60S subunit.

Question 605: What percentage of the human genome is considered coding DNA?

A. 0.50%
B. 1%
C. 1.50% (Correct Answer)
D. 2%

Explanation: **Explanation:** The human genome consists of approximately **3.2 billion base pairs**, but only a tiny fraction is dedicated to the synthesis of proteins. **1. Why 1.5% is Correct:** The "coding DNA" refers to the **exome**—the portion of the genome composed of exons that are transcribed into mRNA and subsequently translated into proteins. According to the Human Genome Project and ENCODE data, protein-coding genes account for roughly **1.5%** of the total genomic sequence. The remaining 98.5% consists of non-coding elements, including introns, regulatory sequences (promoters/enhancers), repetitive elements (LINEs, SINEs), and structural regions like centromeres and telomeres. **2. Analysis of Incorrect Options:** * **0.5% and 1% (Options A & B):** These values are underestimates. While the number of protein-coding genes is lower than initially predicted (approx. 20,000), they consistently occupy about 1.5% of the sequence. * **2% (Option D):** While some older textbooks approximate this value to "less than 2%," the specific consensus figure used in competitive exams like NEET-PG is 1.5%. **3. NEET-PG High-Yield Pearls:** * **The Exome:** Although it represents only 1.5% of the genome, it is estimated that **85% of disease-causing mutations** occur within these coding regions. * **Repetitive DNA:** Nearly **50%** of the human genome consists of repetitive sequences (e.g., Alu elements). * **Introns vs. Exons:** Introns (non-coding intervening sequences) are much larger than exons. The average gene contains significantly more intronic DNA than exonic DNA. * **Intergenic DNA:** The majority of the non-coding DNA is "intergenic," located between genes, previously dismissed as "junk DNA" but now known to have vital regulatory roles.

Question 606: Which of the following can be detected using RFLP (Restriction Fragment Length Polymorphism)?

A. Mutations
B. Trinucleotide repeats
C. Deletions
D. All of the above (Correct Answer)

Explanation: **Explanation:** **Restriction Fragment Length Polymorphism (RFLP)** is a molecular technique that exploits variations in homologous DNA sequences. It relies on the principle that specific genetic changes alter the recognition sites for **restriction endonucleases** (enzymes that cut DNA at specific sequences). 1. **Why "All of the above" is correct:** * **Mutations (Point Mutations):** If a single nucleotide change occurs within a restriction site, the enzyme may no longer recognize it, or a new site may be created. This changes the number and length of DNA fragments produced. (e.g., Sickle Cell Anemia diagnosis using *MstII*). * **Trinucleotide Repeats:** An expansion of repeats (as seen in Huntington’s or Fragile X) increases the distance between two flanking restriction sites, resulting in a significantly larger (longer) DNA fragment on gel electrophoresis. * **Deletions (and Insertions):** If a segment of DNA is deleted between two restriction sites, the resulting fragment will be shorter than the wild-type. 2. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sickle Cell Anemia:** The classic RFLP example. The mutation (GAG → GTG) abolishes the *MstII* restriction site, leading to a larger DNA fragment compared to the normal β-globin gene. * **Paternity Testing & Forensics:** RFLP was the gold standard for DNA fingerprinting before being largely replaced by PCR-based STR (Short Tandem Repeat) analysis. * **Requirement:** RFLP requires a large amount of high-quality DNA and is more time-consuming than PCR. * **Key Step:** It involves DNA digestion, gel electrophoresis, Southern blotting, and hybridization with a labeled probe.

Question 607: Which component of the lac operon acts as an inducer?

A. Repressor
B. Inducer (Correct Answer)
C. Operator
D. Activator

Explanation: **Explanation:** The **lac operon** is a classic model of gene regulation in prokaryotes, specifically designed for the metabolism of lactose. **Why the Correct Answer is Right:** The **Inducer** (specifically **allolactose**, an isomer of lactose) is the molecule that initiates gene expression. In the absence of lactose, the lac repressor protein binds to the operator, blocking RNA polymerase. When lactose enters the cell, it is converted to allolactose, which binds to the repressor, causing a conformational change. This prevents the repressor from binding to the operator, thereby "inducing" the transcription of structural genes (*lacZ, lacY, lacA*). **Analysis of Incorrect Options:** * **A. Repressor:** This is a protein product of the *lacI* gene. It acts as a negative regulator by binding to the operator to inhibit transcription. * **C. Operator:** This is a DNA sequence located between the promoter and the structural genes. It serves as the binding site for the repressor; it is a regulatory element, not an inducer. * **D. Activator:** In the lac operon, the **CAP-cAMP complex** acts as an activator. It enhances RNA polymerase binding when glucose levels are low, but it is not the primary inducer. **NEET-PG High-Yield Pearls:** * **Gratuitous Inducer:** **IPTG** (Isopropyl β-D-1-thiogalactopyranoside) is a synthetic inducer used in labs that induces the operon but is not metabolized by the enzymes. * **Diauxic Growth:** Bacteria prefer glucose over lactose. If both are present, glucose is used first. This is mediated by **Catabolite Repression**. * **Structural Genes:** *lacZ* (β-galactosidase), *lacY* (Permease), and *lacA* (Transacetylase). * The lac operon is an example of **negative inducible control**.

Question 608: Polymerase Chain Reaction (PCR) is primarily used for what purpose?

A. Resolving medicolegal cases
B. Amplification of specific DNA sequences
C. Identification of pathogenic organisms
D. All of the above (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to produce millions of copies of a specific DNA segment from a minute starting sample. It relies on thermal cycling, consisting of denaturation, annealing, and extension, catalyzed by a heat-stable DNA polymerase (e.g., **Taq polymerase**). **Why "All of the above" is correct:** PCR is a versatile molecular tool with broad clinical and forensic applications: * **Amplification of DNA (Option B):** This is the fundamental purpose of PCR. It allows scientists to take a trace amount of DNA and amplify it to a quantity sufficient for analysis (e.g., sequencing or blotting). * **Identification of Pathogens (Option C):** PCR is the "gold standard" for diagnosing infectious diseases, especially for organisms that are slow-growing or difficult to culture (e.g., *M. tuberculosis*, HIV, and SARS-CoV-2). It detects the presence of specific microbial genomic sequences. * **Medicolegal Cases (Option A):** In forensics, PCR is used for **DNA profiling** (Fingerprinting). By amplifying Short Tandem Repeats (STRs) from biological samples (blood, hair, semen), individuals can be identified with high precision in paternity disputes or criminal investigations. **High-Yield Clinical Pearls for NEET-PG:** * **Components:** Requires a DNA template, primers (forward and reverse), dNTPs, and Taq Polymerase (derived from *Thermus aquaticus*). * **RT-PCR:** Reverse Transcriptase PCR is used to amplify **RNA** (e.g., for RNA viruses like HIV or COVID-19) by first converting it into cDNA. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green). * **Sensitivity:** PCR can detect a single molecule of DNA, making it superior to traditional culture methods for early diagnosis.

Question 609: Which of the following is not required for protein synthesis (translation) in eukaryotes?

A. RNA polymerase
B. Ribosomes
C. Peptidyl transferase
D. Amino acyl tRNA synthetase (Correct Answer)

Explanation: **Explanation:** The question asks which component is **not** required for the process of **translation** (protein synthesis) in eukaryotes. **Why Option D is the Correct Answer:** **Aminoacyl tRNA synthetase** is essential for the "charging" of tRNA (attaching an amino acid to its specific tRNA). While this step is a prerequisite for translation, it occurs in the **cytoplasm as a preliminary step** before the actual translation process begins on the ribosome. Translation technically starts with the formation of the initiation complex involving the mRNA, the already-charged initiator tRNA, and ribosomal subunits. Therefore, in the context of the specific stages of translation (Initiation, Elongation, Termination), aminoacyl tRNA synthetase is considered a pre-translational enzyme. **Analysis of Incorrect Options:** * **A. RNA Polymerase:** This is the most common distractor. In eukaryotes, **RNA Polymerase III** is required to transcribe 5S rRNA and tRNAs, while **RNA Polymerase I** transcribes other rRNAs. Without these, the machinery for translation cannot exist. (Note: If the question implies the *act* of translation, RNA Polymerase is not directly involved; however, in many standardized exams, Aminoacyl tRNA synthetase is the "more" correct answer as it is a preparatory step). * **B. Ribosomes:** These are the structural sites of protein synthesis (the "protein factories") where mRNA is read and polypeptide chains are assembled. * **C. Peptidyl transferase:** This is the ribozyme activity of the large ribosomal subunit (28S rRNA in eukaryotes) that catalyzes the formation of peptide bonds between amino acids. **NEET-PG High-Yield Pearls:** * **Ribozyme:** Peptidyl transferase is not a protein; it is an RNA enzyme (28S rRNA in eukaryotes, 23S rRNA in prokaryotes). * **Energy Source:** Translation requires **GTP** for initiation, translocation, and termination, while **ATP** is used by aminoacyl tRNA synthetase for tRNA charging. * **Inhibitors:** Diphtheria toxin and Pseudomonas exotoxin A inhibit eukaryotic translation by inactivating **EF-2** (Elongation Factor 2) via ADP-ribosylation.

Question 610: Chromosome mutations can be detected by all methods, except:

A. Single-strand conformation polymorphism (SSCP)
B. Dideoxy chain termination method (Sanger sequencing)
C. Agarose gel electrophoresis (Correct Answer)
D. Denaturing gradient gel electrophoresis (DGGE)

Explanation: **Explanation:** The detection of chromosomal mutations (specifically point mutations or small sequence variations) requires high-resolution techniques capable of distinguishing differences at the single-nucleotide level. **Why Agarose Gel Electrophoresis is the Correct Answer:** Agarose gel electrophoresis is a technique used to separate DNA fragments based on **size and charge**, typically ranging from 100 bp to 25 kb. While it is excellent for identifying large structural changes (like large deletions or insertions), it lacks the resolution to detect **point mutations** (single-base substitutions). Since most "chromosome mutations" in a molecular context refer to sequence variations, agarose gel is insufficient for their detection. **Analysis of Other Options:** * **SSCP (Single-strand conformation polymorphism):** This method detects mutations based on the principle that single-stranded DNA folds into specific 3D conformations. A single base change alters this folding, resulting in different migration speeds during non-denaturing electrophoresis. * **Sanger Sequencing (Dideoxy method):** This is the **Gold Standard** for mutation detection. It allows for the direct reading of the nucleotide sequence, identifying the exact nature and position of any mutation. * **DGGE (Denaturing gradient gel electrophoresis):** This technique uses a gradient of chemical denaturants. DNA fragments with different sequences (even by one base) will denature (melt) at different points, significantly altering their mobility in the gel. **Clinical Pearls for NEET-PG:** * **Gold Standard for Mutation Detection:** DNA Sequencing (Sanger). * **RFLP (Restriction Fragment Length Polymorphism):** Used to detect mutations that create or abolish a restriction site (e.g., Sickle Cell Anemia). * **Resolution Power:** Polyacrylamide Gel Electrophoresis (PAGE) has much higher resolution than Agarose and can separate DNA fragments differing by only 1 bp. * **Karyotyping:** Used for large-scale chromosomal abnormalities (aneuploidy/translocations), not sequence mutations.

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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