Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 731: All of the following are true regarding satellite DNA EXCEPT?

A. It consists of repeated DNA sequences arranged in tandem.
B. It is typically clustered around the centromeres.
C. It is often found clustered around the telomeres.
D. It is transcriptionally active. (Correct Answer)

Explanation: ### Explanation **Satellite DNA** refers to highly repetitive DNA sequences that form a significant portion of the eukaryotic genome. Understanding its characteristics is crucial for molecular biology and forensic genetics. **Why Option D is the Correct Answer (The Exception):** Satellite DNA is primarily **transcriptionally inactive**. It is composed of non-coding sequences that do not translate into proteins. Structurally, it is categorized as **constitutive heterochromatin**, which is highly condensed and chemically modified (e.g., methylated) to prevent gene expression. Therefore, the statement that it is transcriptionally active is incorrect. **Analysis of Incorrect Options:** * **Option A:** Satellite DNA consists of short sequences (ranging from a few to hundreds of base pairs) that are **repeated in tandem** (head-to-tail). This distinguishes it from interspersed repeats like SINEs and LINEs. * **Option B:** Large blocks of satellite DNA (Macro-satellites) are characteristically found at the **centromeres**, where they play a structural role in spindle fiber attachment during mitosis. * **Option C:** Satellite DNA is also frequently clustered at the **telomeres** (the ends of chromosomes) and the secondary constrictions of acrocentric chromosomes, providing genomic stability. **High-Yield Facts for NEET-PG:** 1. **Buoyant Density:** The name "satellite" comes from the fact that these sequences have a different G-C content than the rest of the genome, causing them to form a separate "satellite" band during **cesium chloride (CsCl) density gradient centrifugation**. 2. **VNTRs and STRs:** Minisatellites (VNTRs) and Microsatellites (STRs) are types of satellite DNA used in **DNA Fingerprinting** due to their high degree of polymorphism. 3. **Trinucleotide Repeats:** Microsatellite expansions are responsible for diseases like **Huntington’s chorea** and **Fragile X syndrome**.

Question 732: Which molecular method is used to locate a known gene locus?

A. FISH (Fluorescence in situ hybridization) (Correct Answer)
B. CGH (Comparative genomic hybridization)
C. Chromosome painting
D. RT-PCR (Reverse transcription polymerase chain reaction)

Explanation: **Explanation:** **1. Why FISH is the correct answer:** Fluorescence in situ hybridization (FISH) is a cytogenetic technique used to detect and **locate a specific DNA sequence** on a chromosome. It utilizes fluorescently labeled DNA probes that are complementary to the known gene locus of interest. When the probe binds (hybridizes) to its target sequence on the patient's chromosome, it can be visualized under a fluorescence microscope. This makes it the gold standard for identifying the physical location of a known gene or detecting specific microdeletions/translocations. **2. Why the other options are incorrect:** * **CGH (Comparative Genomic Hybridization):** This technique is used to detect **copy number variations** (gains or losses of DNA) across the entire genome. It compares patient DNA with control DNA but does not pinpoint the specific locus of a single known gene as precisely as FISH. * **Chromosome Painting:** This is a variation of FISH that uses a cocktail of probes to color an **entire chromosome**. It is excellent for identifying complex structural rearrangements or identifying the origin of "marker chromosomes," but it is not used to locate a specific individual gene locus. * **RT-PCR:** This is a molecular technique used to measure **gene expression** by converting RNA into cDNA. It is used for quantification and detection of mRNA, not for locating a gene's physical position on a chromosome. **Clinical Pearls for NEET-PG:** * **FISH** is the investigation of choice for **microdeletion syndromes** (e.g., DiGeorge Syndrome - 22q11.2 deletion). * **Rapid Diagnosis:** FISH can be performed on interphase nuclei, meaning it does not require cell culture (unlike traditional karyotyping). * **HER2/neu status** in breast cancer is often confirmed via FISH if immunohistochemistry (IHC) results are equivocal (2+).

Question 733: Which type of RNA contains codons for specific amino acids?

A. tRNA
B. rRNA
C. mRNA (Correct Answer)
D. None of the above

Explanation: **Explanation:** The correct answer is **mRNA (Messenger RNA)**. In molecular biology, the "Central Dogma" describes the flow of genetic information from DNA to RNA to protein. **mRNA** serves as the intermediary template. It is transcribed from DNA and carries the genetic blueprint from the nucleus to the ribosomes. The sequence of nucleotides on mRNA is organized into **codons**—triplets of bases that each specify a particular amino acid or a stop signal during translation. **Why other options are incorrect:** * **tRNA (Transfer RNA):** tRNA does not contain codons; instead, it contains the **anticodon**. Its role is to act as an adapter molecule that recognizes the mRNA codon via its anticodon and delivers the corresponding amino acid to the ribosome. * **rRNA (Ribosomal RNA):** This is a structural and catalytic component of the ribosome. It ensures the proper alignment of mRNA and tRNA and catalyzes peptide bond formation (peptidyl transferase activity), but it does not carry the genetic code for amino acids. **High-Yield NEET-PG Pearls:** * **mRNA** is the most heterogeneous type of RNA in terms of size and sequence, but it is the **least abundant** (~5% of total cellular RNA). * **rRNA** is the **most abundant** (~80%) and **tRNA** is the **smallest** (75-95 nucleotides). * **Post-transcriptional modifications** of eukaryotic mRNA include 5' capping (7-methylguanosine), 3' polyadenylation (Poly-A tail), and splicing (removal of introns). * **Start Codon:** AUG (codes for Methionine); **Stop Codons:** UAA, UAG, UGA.

Question 734: What is the primary function of RNA polymerase?

A. DNA dependent RNA polymerase (Correct Answer)
B. RNA dependent DNA polymerase
C. DNA dependent DNA polymerase
D. RNA dependent RNA polymerase

Explanation: **Explanation:** The primary function of RNA polymerase (specifically in the context of transcription) is to synthesize an RNA strand using a DNA template. 1. **Why Option A is Correct:** Transcription is the process where genetic information from **DNA** is copied into **RNA**. The enzyme responsible, RNA polymerase, "reads" the DNA template to catalyze the synthesis of RNA. Therefore, it is **DNA-dependent** (requires a DNA template) and an **RNA polymerase** (produces RNA). In eukaryotes, there are three main types: RNA Pol I (rRNA), Pol II (mRNA), and Pol III (tRNA). 2. **Why Other Options are Incorrect:** * **Option B (RNA-dependent DNA polymerase):** Also known as **Reverse Transcriptase**. This enzyme synthesizes DNA from an RNA template (e.g., in retroviruses like HIV or during telomere maintenance by telomerase). * **Option C (DNA-dependent DNA polymerase):** This is **DNA Polymerase**, the primary enzyme involved in **DNA Replication**, where a new DNA strand is synthesized from an existing DNA template. * **Option D (RNA-dependent RNA polymerase):** This enzyme synthesizes RNA from an RNA template. It is primarily found in certain RNA viruses (e.g., Poliovirus, SARS-CoV-2) to replicate their genetic material. **High-Yield Clinical Pearls for NEET-PG:** * **α-Amanitin:** A toxin from the *Amanita phalloides* mushroom that potently inhibits **RNA Polymerase II**, leading to severe liver failure. * **Rifampicin:** An antitubercular drug that inhibits **bacterial DNA-dependent RNA polymerase**. * **Promoter Region:** The specific DNA sequence (like the TATA box) where RNA polymerase binds to initiate transcription. * **Mnemonic:** RNA Pol **I, II, and III** synthesize **r**RNA, **m**RNA, and **t**RNA respectively (Order: **R-M-T** / "**R**ead **M**y **T**ext").

Question 735: What is the initial amino acid incorporated in prokaryotic protein synthesis?

A. Arginine
B. Methionine
C. Formyl-methionine (Correct Answer)
D. Alanine

Explanation: ### Explanation In prokaryotes (such as bacteria), protein synthesis is initiated by a specific modified amino acid: **N-formylmethionine (fMet)**. **Why Option C is Correct:** The initiation of translation in prokaryotes requires a specialized initiator tRNA ($tRNA_f^{met}$). This tRNA carries methionine, which is subsequently formylated by the enzyme **transformylase** using N10-formyl tetrahydrofolate as a donor. This formyl group mimics a peptide bond, helping the initiator tRNA bind directly to the **P-site** of the ribosome, whereas all subsequent aminoacyl-tRNAs enter at the A-site. **Why Other Options are Incorrect:** * **Option B (Methionine):** While methionine is the universal start codon (AUG) product, in prokaryotes, it must be formylated. Non-formylated methionine is the initiator for **eukaryotes**. * **Options A & D (Arginine & Alanine):** These are standard amino acids incorporated during the elongation phase but never serve as the primary initiator amino acid. --- ### High-Yield Clinical Pearls for NEET-PG * **Mitochondrial Connection:** Because mitochondria evolved from prokaryotic endosymbionts, human mitochondrial protein synthesis also starts with **N-formylmethionine**. * **Chemotaxis:** Human neutrophils have receptors (FPR1) that recognize fMet-containing peptides. This allows the immune system to identify and migrate toward sites of bacterial infection (fMet acts as a **PAMP** - Pathogen-Associated Molecular Pattern). * **Deformylation:** In many mature bacterial proteins, the formyl group (and sometimes the methionine itself) is removed post-translationally by peptide deformylase. * **Shine-Dalgarno Sequence:** In prokaryotes, the 16S rRNA of the 30S subunit recognizes this purine-rich sequence to correctly position the AUG start codon for fMet-tRNA binding.

Question 736: Which of the following is an example of a non-coding RNA?

A. siRNA (Correct Answer)
B. miRNA
C. tRNA
D. mRNA

Explanation: **Explanation:** The question asks for an example of **non-coding RNA (ncRNA)**. While the provided key marks **siRNA** as the correct answer, it is important to note that technically, **siRNA, miRNA, and tRNA** are all types of non-coding RNA. However, in the context of gene regulation and silencing—a high-yield area for NEET-PG—siRNA and miRNA are the primary focus. **Why siRNA is correct:** Non-coding RNAs are functional RNA molecules that are **not translated into proteins**. **Small interfering RNA (siRNA)** is a double-stranded RNA molecule (20-25 nucleotides) that plays a crucial role in the **RNA interference (RNAi)** pathway. It binds to specific messenger RNA (mRNA) molecules and induces their cleavage, thereby "silencing" gene expression post-transcriptionally. **Analysis of other options:** * **miRNA (MicroRNA):** Also a non-coding RNA. It inhibits translation or promotes mRNA degradation. (In many exams, if both siRNA and miRNA are present, the question may be looking for the specific mediator of exogenous RNA interference). * **tRNA (Transfer RNA):** A classic non-coding RNA. It acts as an adapter molecule during translation but does not code for proteins itself. * **mRNA (Messenger RNA):** This is **Coding RNA**. It carries the genetic blueprint from DNA to the ribosome to be translated into a polypeptide chain. **NEET-PG High-Yield Pearls:** 1. **RNA Interference (RNAi):** Discovered by Fire and Mello (Nobel Prize). It is a defense mechanism against viral genomes. 2. **siRNA vs. miRNA:** siRNA is usually **exogenous** (derived from viruses/transposons) and requires perfect base pairing, while miRNA is **endogenous** and often involves imperfect pairing. 3. **Clinical Application:** siRNA-based drugs (e.g., **Patisiran**) are used clinically to treat hereditary transthyretin-mediated amyloidosis by silencing the mutant gene. 4. **Long non-coding RNA (lncRNA):** Example is **XIST**, which is essential for X-chromosome inactivation (Lyonization).

Question 737: Which of the following cellular bodies is NOT found in the nucleus?

A. P-bodies (Correct Answer)
B. Nucleolus
C. Cajal bodies
D. Interchromatin granule clusters

Explanation: **Explanation:** The correct answer is **A. P-bodies (Processing bodies)**. **1. Why P-bodies are the correct answer:** P-bodies are distinct, membrane-less granules found exclusively in the **cytoplasm**. They are composed of translationally repressed mRNAs and enzymes involved in mRNA turnover. Their primary functions include mRNA decay (decapping and degradation), mRNA storage, and translational repression. They serve as "processing centers" for mRNA before it is either degraded or returned to the translation machinery. **2. Why the other options are incorrect:** * **Nucleolus:** The most prominent structure in the nucleus; it is the site of ribosomal RNA (rRNA) synthesis and ribosome biogenesis. * **Cajal bodies:** Small sub-nuclear organelles involved in the maturation of small nuclear ribonucleoproteins (snRNPs) and telomerase assembly. * **Interchromatin granule clusters (Speckles):** These are nuclear domains enriched in pre-mRNA splicing factors. They serve as storage and modification sites for splicing machinery. **3. High-Yield Clinical Pearls for NEET-PG:** * **P-bodies vs. Stress Granules:** Both are cytoplasmic. While P-bodies are involved in mRNA decay, **Stress Granules** form specifically during cellular stress to protect mRNA and initiate survival pathways. * **Spinal Muscular Atrophy (SMA):** This condition is linked to the loss of the SMN protein, which is localized in **Gems** (Gemini of Cajal bodies), highlighting the clinical importance of nuclear bodies. * **Nucleolus and Cancer:** Hypertrophy of the nucleolus is a classic histopathological marker of malignant cells due to increased protein synthesis requirements.

Question 738: Protein synthesis occurs in which organelle?

A. Ribosome (Correct Answer)
B. Golgi apparatus
C. Lysosomes
D. Endosomes

Explanation: **Explanation:** **Correct Option: A. Ribosome** Ribosomes are the primary sites of **translation** (protein synthesis) in the cell. They are complex molecular machines composed of ribosomal RNA (rRNA) and proteins. They facilitate the decoding of messenger RNA (mRNA) into a polypeptide chain by catalyzing the formation of peptide bonds between amino acids. Ribosomes exist either freely in the cytosol (synthesizing proteins for internal use) or attached to the Rough Endoplasmic Reticulum (RER) (synthesizing proteins for secretion or membrane insertion). **Why Incorrect Options are Wrong:** * **B. Golgi apparatus:** Its primary function is the **post-translational modification**, sorting, and packaging of proteins into vesicles. It acts as the "shipping center" of the cell. * **C. Lysosomes:** These are the "suicide bags" of the cell containing hydrolytic enzymes. They are involved in **intracellular digestion** and degradation of macromolecules, not synthesis. * **D. Endosomes:** These are membrane-bound compartments involved in **endocytosis**. They sort internalized material and direct it toward recycling or degradation in lysosomes. **High-Yield NEET-PG Pearls:** * **Eukaryotic Ribosome:** 80S (composed of 60S and 40S subunits). * **Prokaryotic Ribosome:** 70S (composed of 50S and 30S subunits). This difference is the basis for the selective toxicity of many antibiotics (e.g., Aminoglycosides, Macrolides). * **Peptidyl transferase:** The specific ribozyme activity within the large subunit that forms peptide bonds. * **Mitochondrial Ribosomes:** Humans have 55S ribosomes within mitochondria, which resemble bacterial ribosomes—a fact relevant to certain drug toxicities (e.g., Chloramphenicol causing bone marrow suppression).

Question 739: Which of the following statements is true regarding Okazaki fragments?

A. Segments of RNA attached to an RNA initiator component
B. Related to the leading strand
C. Several Okazaki fragments must be sequentially synthesized for each replication fork (Correct Answer)
D. Helicase acts on the leading strand to unwind dsDNA

Explanation: ### Explanation **1. Why Option C is Correct:** DNA replication is **semi-discontinuous** [3]. DNA polymerase can only synthesize DNA in the **5' to 3' direction** [3]. At the replication fork, the **leading strand** is synthesized continuously toward the fork. However, the **lagging strand** template runs in the opposite direction (3' to 5'). To accommodate this, DNA polymerase must synthesize DNA in short, discrete segments called **Okazaki fragments** moving away from the fork [2]. Multiple fragments are synthesized sequentially and later joined by **DNA ligase** to complete the lagging strand [1]. **2. Analysis of Incorrect Options:** * **Option A:** Okazaki fragments are segments of **DNA**, not RNA [1]. While they are initiated by a short RNA primer (synthesized by Primase), the bulk of the fragment consists of deoxyribonucleotides [2]. * **Option B:** Okazaki fragments are exclusively related to the **lagging strand** [1]. The leading strand is synthesized continuously and does not require fragmentation. * **Option D:** While Helicase does unwind dsDNA, it acts at the **replication fork junction**, not specifically on the "leading strand." Furthermore, this statement does not describe Okazaki fragments. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme involved:** **DNA Ligase** forms the final phosphodiester bond between Okazaki fragments (requires ATP in eukaryotes/NAD+ in prokaryotes) [1]. * **Length:** Okazaki fragments are significantly shorter in eukaryotes (100–200 nucleotides) compared to prokaryotes (1000–2000 nucleotides) [1]. * **Clinical Correlation:** Deficiencies in DNA repair/ligation (e.g., **Bloom Syndrome** or **Ataxia-Teleangiectasia**) involve defects in handling DNA intermediates during replication, leading to genomic instability. * **RNA Primer Removal:** In eukaryotes, **RNase H** and **FEN1** remove the RNA primers before fragments are joined.

Question 740: Which of the following is NOT an aminoacyl-tRNA?

A. Proline
B. Lysine
C. Hydroxy lysine (Correct Answer)
D. Methionine

Explanation: **Explanation:** The core concept here lies in the distinction between **standard (proteinogenic) amino acids** and **post-translational modifications**. **Why Hydroxylysine is the correct answer:** For an amino acid to be part of an "aminoacyl-tRNA" complex, it must be one of the 20 standard amino acids encoded by the genetic code. These amino acids are attached to their specific tRNA molecules by the enzyme **aminoacyl-tRNA synthetase** during the process of translation. **Hydroxylysine** is a non-standard amino acid. It is formed by the post-translational hydroxylation of Lysine residues *after* the polypeptide chain has already been synthesized on the ribosome. Since there is no genetic codon or specific tRNA for Hydroxylysine, it cannot form an aminoacyl-tRNA complex. **Analysis of incorrect options:** * **Proline, Lysine, and Methionine** are all standard, proteinogenic amino acids. * They each have specific codons (e.g., AUG for Methionine) and corresponding tRNA molecules. * They are "charged" onto their respective tRNAs to participate directly in protein synthesis at the ribosome. **Clinical Pearls for NEET-PG:** * **Post-translational Modification:** Hydroxylation of Lysine and Proline occurs primarily in **Collagen** synthesis. * **Enzyme & Cofactors:** This process is catalyzed by *lysyl hydroxylase*, which requires **Vitamin C (Ascorbic acid)**, Ferrous iron ($Fe^{2+}$), and $\alpha$-ketoglutarate. * **Clinical Correlation:** Deficiency of Vitamin C leads to **Scurvy** due to defective hydroxylation, resulting in unstable collagen triple helices and fragile connective tissue. * **Exceptions:** While most non-standard amino acids are post-translational, **Selenocysteine** (the 21st amino acid) is a rare exception that has its own specialized tRNA mechanism.

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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