Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 701: Which of the following is NOT a component of tRNA?

A. D-loop
B. TpsC loop
C. Codon arm (Correct Answer)
D. Variable loop

Explanation: **Explanation:** The correct answer is **C. Codon arm**. In molecular biology, tRNA (transfer RNA) acts as an adapter molecule that translates the genetic code from mRNA into a sequence of amino acids. While tRNA interacts with a codon, it does so via its **Anticodon arm**, which contains a triplet of bases complementary to the mRNA codon. There is no structural component known as a "Codon arm" in tRNA. **Analysis of Options:** * **A. D-loop:** This is a standard component of tRNA containing **Dihydrouridine**. It is primarily responsible for recognition by the enzyme aminoacyl-tRNA synthetase. * **B. TψC loop:** Also known as the T-loop, it contains **Ribothymidine** and **Pseudouridine** (ψ). This loop is essential for binding the tRNA to the ribosomal surface (specifically the 5S rRNA of the large subunit). * **D. Variable loop:** Located between the TψC loop and the Anticodon loop, its length varies among different tRNAs. It is used to classify tRNA into Class I (short loop) and Class II (long loop). **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** tRNA has a **Cloverleaf** secondary structure and an **L-shaped** tertiary structure. * **3' End:** All tRNAs have a **CCA sequence** at the 3' hydroxy terminus, which is the attachment site for the specific amino acid (forming aminoacyl-tRNA). * **Wobble Hypothesis:** The 3rd base of the mRNA codon can form non-standard base pairs with the 1st base of the tRNA anticodon, allowing one tRNA to recognize multiple codons. * **Inosine:** Often found in the anticodon loop, it is a modified base that facilitates "wobbling."

Question 702: Which of the following is NOT true about Ribozymes?

A. It is a protein (Correct Answer)
B. Involved in transesterification
C. Involved in peptide bond formation
D. Catalytic RNA

Explanation: **Explanation:** **Why Option A is correct:** Ribozymes are **catalytic RNA molecules**, not proteins. While most biological catalysts are proteins (enzymes), ribozymes are unique because they consist of ribonucleic acid sequences that fold into complex three-dimensional structures to catalyze specific biochemical reactions. Therefore, the statement "It is a protein" is false. **Analysis of other options:** * **Option B (Transesterification):** Many ribozymes, such as **Group I and II introns** and the **Spliceosome** (specifically U2/U6 snRNA), catalyze transesterification reactions during RNA splicing to remove introns and join exons. * **Option C (Peptide bond formation):** This is a high-yield fact. The **23S rRNA** (in prokaryotes) and **28S rRNA** (in eukaryotes) of the large ribosomal subunit act as a **peptidyl transferase**. This ribozyme activity is responsible for forming peptide bonds during translation. * **Option D (Catalytic RNA):** This is the fundamental definition of a ribozyme. They function by lowering activation energy, similar to protein enzymes, but use nucleotides instead of amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **First Ribozyme Discovered:** RNAse P (involved in tRNA processing) and the self-splicing Group I introns (by Thomas Cech). * **Ribozymes in Medicine:** Artificial ribozymes are being researched as therapeutic agents to "silence" specific genes by cleaving viral RNA or oncogenic mRNA. * **Key Example:** The **Ribosome** is essentially a ribozyme because the catalytic heart of the organelle is composed of RNA, not protein.

Question 703: Which of the following statements about the genetic code is FALSE?

A. It is degenerate.
B. It is universal.
C. It involves punctuation. (Correct Answer)
D. It is non-overlapping.

Explanation: ### Explanation The genetic code is the set of rules by which information encoded in genetic material is translated into proteins. Understanding its properties is fundamental to molecular biology and medical genetics. **Why "It involves punctuation" is FALSE:** The genetic code is **commaless**. Once the translation begins at the start codon (AUG), the "reading frame" is established, and the mRNA is read continuously, three nucleotides at a time, without any skipped bases or "punctuation" marks between codons. There are no internal spacers; if a base is added or deleted, it results in a **frameshift mutation**, altering every subsequent codon. **Analysis of Other Options:** * **A. Degenerate:** Most amino acids are coded by more than one codon (e.g., Leucine has six). This provides "wobble" protection against some point mutations. * **B. Universal:** The code is the same across almost all organisms, from bacteria to humans. *Exception:* Minor variations exist in mitochondrial DNA (e.g., UGA codes for Tryptophan instead of 'Stop'). * **D. Non-overlapping:** Each nucleotide is part of only one codon. In a sequence like ABCDEF, the codons are ABC and DEF, never BCD. **High-Yield Clinical Pearls for NEET-PG:** * **Initiation Codon:** **AUG** (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Stop Codons (Nonsense Codons):** **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These do not code for any amino acid. * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why the third base of a codon can often be changed without changing the amino acid, allowing one tRNA to recognize multiple codons. * **Frameshift Mutations:** These occur due to the "commaless" nature of the code. Examples include certain forms of **Duchenne Muscular Dystrophy** and **Tay-Sachs disease**.

Question 704: Which statement about mitochondrial DNA is true?

A. Mitochondrial DNA has fewer mutations compared to nuclear DNA.
B. Mitochondrial DNA has 3x10^9 base pairs.
C. Mitochondrial DNA receives 23 chromosomes from each parent.
D. Mitochondrial DNA codes for less than 20% of the proteins involved in the respiratory chain. (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** The human mitochondrial genome (mtDNA) is a small, circular molecule containing only **16,569 base pairs** and **37 genes**. While the mitochondria are the "powerhouses" of the cell, the vast majority of the ~1,000–1,500 proteins required for mitochondrial function are encoded by **nuclear DNA** and imported from the cytosol. Specifically, the respiratory chain (Oxidative Phosphorylation) consists of approximately 80–90 subunits; mtDNA codes for only **13** of these subunits (roughly 15%), which is well under the 20% threshold. **2. Why the Other Options are Incorrect:** * **Option A:** mtDNA actually has a **10–20 times higher mutation rate** than nuclear DNA. This is due to the lack of protective histones, limited DNA repair mechanisms, and proximity to the high-ROS (Reactive Oxygen Species) environment of the electron transport chain. * **Option B:** The human **nuclear genome** contains approximately $3 \times 10^9$ base pairs. mtDNA is significantly smaller (16.6 kb). * **Option C:** mtDNA follows **maternal inheritance**. It does not involve 23 chromosomes from each parent; instead, almost all mitochondria are derived from the oocyte. **3. High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** Diseases like LHON (Leber’s Hereditary Optic Neuropathy) and MELAS are passed only from the mother to all her children. * **Heteroplasmy:** The coexistence of mutated and wild-type mtDNA in a single cell, explaining the variable clinical severity of mitochondrial diseases. * **Genetic Code Exceptions:** mtDNA uses a slightly different genetic code (e.g., **UGA** codes for Tryptophan instead of a Stop codon; **AYA** codes for Methionine). * **Replication:** mtDNA replicates independently of the cell cycle (S-phase) using **DNA Polymerase Gamma**.

Question 705: Xeroderma pigmentosum is characterized by a defect in which DNA repair pathway?

A. Mismatch repair
B. Base excision repair
C. Nucleotide excision repair (Correct Answer)
D. Double strand break repair

Explanation: **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by extreme sensitivity to ultraviolet (UV) radiation. The correct answer is **Nucleotide Excision Repair (NER)** because this specific pathway is responsible for identifying and removing bulky DNA lesions, most notably **pyrimidine dimers** (thymine dimers) caused by UV light. In XP patients, mutations in *XP* genes (XPA through XPG) lead to a failure in the endonuclease-mediated excision of these dimers, resulting in the accumulation of mutations and a high risk of skin malignancies. **Analysis of Incorrect Options:** * **Mismatch Repair (MMR):** Corrects errors (mismatched bases) that escape proofreading during DNA replication. Defects in MMR lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). * **Base Excision Repair (BER):** Repairs "small" lesions like deaminated bases (e.g., cytosine to uracil) or oxidized bases. It utilizes specific **glycosylases** and is not the primary pathway for UV damage. * **Double Strand Break Repair:** Involves Non-Homologous End Joining (NHEJ) or Homologous Recombination. Defects here lead to conditions like **Ataxia-telangiectasia** (ATM gene) or **BRCA1/2** related breast cancers. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Photosensitivity, pigmentary changes (poikiloderma), and a 1000-fold increased risk of skin cancers (BCC, SCC, Melanoma). * **Key Enzyme:** The defect is specifically in **UV-specific endonuclease**. * **Variant Form:** A rare variant of XP is caused by a defect in **DNA Polymerase Eta (η)**. * **Associated Condition:** Cockayne syndrome also involves the NER pathway but presents with "Mickey Mouse" facies and dwarfism without a high risk of skin cancer.

Question 706: Which of the following is a component of the 60S subunit of the ribosome?

A. 5.8S rRNA (Correct Answer)
B. 23S rRNA
C. 16S rRNA
D. 18S rRNA

Explanation: **Explanation:** The eukaryotic ribosome is an **80S** particle consisting of two subunits: the **60S (large)** and **40S (small)** subunits. Understanding the specific rRNA components of these subunits is a high-yield topic for NEET-PG. **1. Why Option A is Correct:** The **60S subunit** (large subunit of eukaryotes) is composed of approximately 50 proteins and three distinct strands of ribosomal RNA: **28S, 5.8S, and 5S rRNA**. The 5.8S rRNA is unique to eukaryotes and plays a critical role in protein translocation and the structural integrity of the ribosome. **2. Analysis of Incorrect Options:** * **Option B (23S rRNA):** This is a component of the **50S (large) subunit of Prokaryotes**. It possesses the peptidyl transferase activity (ribozyme). * **Option C (16S rRNA):** This is the structural component of the **30S (small) subunit of Prokaryotes**. It contains the Shine-Dalgarno complementary sequence for translation initiation. * **Option D (18S rRNA):** This is the sole rRNA component of the **40S (small) subunit of Eukaryotes**. **High-Yield Clinical Pearls for NEET-PG:** * **Svedberg Unit (S):** Measures sedimentation rate, which depends on mass and shape (not additive; 60S + 40S = 80S). * **Origin:** In eukaryotes, 28S, 18S, and 5.8S rRNA are transcribed by **RNA Polymerase I** in the nucleolus as a single 45S precursor. The **5S rRNA** is unique because it is transcribed by **RNA Polymerase III** outside the nucleolus. * **Antibiotic Target:** Many antibiotics target specific subunits (e.g., Aminoglycosides/Tetracyclines target 30S; Macrolides/Chloramphenicol target 50S). Eukaryotic 60S is the target for the toxin **Ricin**.

Question 707: Some mRNA exhibits self-splicing activity. The removal of introns is performed by which of the following?

A. Introns that form ribozymes
B. Self-splicing introns
C. Lariat intermediates
D. All of the above (Correct Answer)

Explanation: ### Explanation **Concept Overview:** Splicing is the process of removing non-coding sequences (introns) and joining coding sequences (exons). While most eukaryotic splicing is mediated by the **spliceosome** (snRNPs), certain RNAs possess intrinsic catalytic activity, allowing them to splice themselves without external protein enzymes. These are known as **self-splicing introns**. **Why "All of the Above" is Correct:** 1. **Introns that form ribozymes (Option A):** Self-splicing introns (Group I and Group II) are essentially RNA molecules with catalytic activity, termed **ribozymes**. They fold into specific 3D structures to catalyze the transesterification reactions required for their own excision. 2. **Self-splicing introns (Option B):** This is the literal definition of the process. Group I introns (found in rRNA) and Group II introns (found in organelle mRNA) do not require ATP or spliceosomal machinery to function. 3. **Lariat intermediates (Option C):** In **Group II self-splicing** (and spliceosomal splicing), the intron is removed via a specific branched structure called a **lariat**. An internal adenine residue attacks the 5' splice site, forming a loop. Since the question asks what "performs" or is involved in the removal process, the formation of this intermediate is a fundamental mechanistic step in the self-splicing pathway of Group II introns. **High-Yield Facts for NEET-PG:** * **Group I Introns:** Use a **Guanosine** cofactor (G-nucleotide) as a nucleophile. No lariat is formed. * **Group II Introns:** Use an internal **Adenine** to form a **lariat** (similar to the mechanism used by the spliceosome). * **Ribozyme Examples:** Peptidyl transferase (23S/28S rRNA), RNase P, and self-splicing introns. * **Clinical Correlation:** Mutations in the splicing machinery (spliceosomes) are linked to diseases like **Spinal Muscular Atrophy (SMA)** and **Systemic Lupus Erythematosus (SLE)** (anti-Smith antibodies target snRNPs).

Question 708: What is an enzyme that recognizes a specific palindromic sequence and cuts within a DNA molecule?

A. Exonuclease
B. Methylase
C. Modification enzyme
D. Restriction endonuclease (Correct Answer)

Explanation: **Explanation:** **1. Why Restriction Endonuclease is Correct:** Restriction endonucleases (REs), often called "molecular scissors," are enzymes that recognize specific, short DNA sequences (usually 4–8 base pairs long) known as **palindromes**. A palindrome in DNA reads the same on both strands when read in the 5' to 3' direction (e.g., 5'-GAATTC-3' and its complement 3'-CTTAAG-5'). These enzymes function by cleaving the phosphodiester bonds **within** the DNA molecule, producing either "sticky ends" (overhangs) or "blunt ends." They are essential tools in recombinant DNA technology for gene cloning and mapping. **2. Why Other Options are Incorrect:** * **Exonuclease:** These enzymes remove nucleotides one at a time from the **ends** (termini) of a DNA molecule rather than cutting at specific internal sequences. * **Methylase:** This enzyme adds a methyl group to DNA bases (usually Cytosine or Adenine). In bacteria, methylases protect host DNA from being digested by their own restriction enzymes. * **Modification Enzyme:** This is a general term often referring to enzymes like methylases that modify DNA to protect it. While part of the "Restriction-Modification System," they do not cut the DNA. **High-Yield Clinical Pearls for NEET-PG:** * **Type II Restriction Enzymes** (e.g., EcoRI, HindIII) are the most commonly used in labs because they cut exactly at the recognition site and do not require ATP. * **RFLP (Restriction Fragment Length Polymorphism):** A technique using REs to detect genetic variations, used in forensic medicine and prenatal diagnosis of diseases like Sickle Cell Anemia. * **Blunt end cutters:** *AluI* and *HaeIII*. * **Sticky end cutters:** *EcoRI*, *BamHI*, and *HindIII*.

Question 709: What is the normal role of Micro RNA?

A. Gene Regulation (Correct Answer)
B. RNA splicing
C. Initiation of translation
D. DNA conformational change

Explanation: **Explanation:** **MicroRNAs (miRNAs)** are small, non-coding RNA molecules (typically 21–25 nucleotides long) that play a critical role in **post-transcriptional gene regulation**. They function by binding to the 3' untranslated region (3' UTR) of specific target messenger RNAs (mRNAs). This binding leads to either **mRNA degradation** or **translational repression**, effectively "silencing" the gene expression. Because they regulate the stability and translation of protein-coding genes, they are essential for processes like development, cell differentiation, and apoptosis. **Analysis of Incorrect Options:** * **B. RNA Splicing:** This is primarily the function of **snRNAs** (small nuclear RNAs), which combine with proteins to form the spliceosome (snRNPs) to remove introns from pre-mRNA. * **C. Initiation of Translation:** While miRNAs *inhibit* translation, the initiation of translation is mediated by **eukaryotic Initiation Factors (eIFs)** and the 5' cap-binding complex. * **D. DNA Conformational Change:** Changes in DNA conformation (like supercoiling) are managed by enzymes like **Topoisomerases**, not by microRNAs. **High-Yield Clinical Pearls for NEET-PG:** * **Biogenesis:** miRNAs are transcribed by **RNA Polymerase II** as primary-miRNA (pri-miRNA), processed by the **Drosha** enzyme in the nucleus, and further cleaved by the **Dicer** enzyme in the cytoplasm. * **RISC Complex:** To function, miRNA must be loaded into the **RNA-induced Silencing Complex (RISC)**. * **OncomiRs:** miRNAs that are dysregulated in cancer are called "OncomiRs." For example, downregulation of *let-7* is often seen in lung cancer, leading to overexpression of the RAS oncogene.

Question 710: Which part of eukaryotic DNA contributes to polypeptide synthesis?

A. Exon and Enhancer
B. Exon
C. Exon and tRNA
D. Exon and Leader sequence (Correct Answer)

Explanation: ### Explanation **1. Why "Exon and Leader sequence" is correct:** In eukaryotic gene expression, the DNA is transcribed into pre-mRNA, which undergoes processing to become mature mRNA. * **Exons:** These are the coding regions of the DNA that remain in the mature mRNA after splicing. They contain the codons that are translated into the amino acid sequence of a polypeptide. * **Leader Sequence (5' UTR):** This is the segment of the mRNA located upstream of the initiation codon (AUG). While it is not translated into amino acids, it is **essential for polypeptide synthesis** because it contains the ribosome-binding site and regulatory elements that facilitate the recruitment of the ribosome to the mRNA. Without the leader sequence, translation initiation cannot occur efficiently. **2. Why other options are incorrect:** * **Enhancer (Option A):** Enhancers are regulatory DNA sequences located far from the gene. They increase the rate of transcription but are not transcribed into mRNA and do not contribute to the actual synthesis of the polypeptide chain. * **Exon only (Option B):** While exons provide the primary code, they cannot function in isolation. The leader sequence is a structural requirement for the translation machinery to engage with the mRNA. * **tRNA (Option C):** tRNA is a functional RNA molecule involved in translation, but it is not a "part of DNA" that codes for the polypeptide itself; it acts as an adapter molecule. **3. NEET-PG High-Yield Pearls:** * **Introns:** Non-coding sequences removed by **spliceosomes** (snRNPs). "Introns stay **In** the nucleus; Exons **Exit** and are expressed." * **Shine-Dalgarno Sequence:** The prokaryotic equivalent of the leader sequence (binds to 16S rRNA). * **Kozak Sequence:** The specific sequence within the eukaryotic leader sequence that contains the AUG start codon and increases translation efficiency. * **Poly-A Tail:** Added to the 3' end; it protects mRNA from degradation and assists in translation termination/export.

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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