Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 401: What is the function of restriction endonucleases?

A. To cut double-stranded DNA at specific recognition sequences. (Correct Answer)
B. To cut RNA at specific sequences.
C. To cut single-stranded DNA at specific sequences.
D. To break peptide bonds in proteins.

Explanation: **Explanation:** **Restriction Endonucleases (REs)**, often referred to as "molecular scissors," are enzymes primarily derived from bacteria. Their physiological role is to protect bacteria from viral (bacteriophage) infections by identifying and cleaving foreign DNA. 1. **Why Option A is Correct:** Restriction endonucleases recognize specific, usually palindromic, sequences (4–8 base pairs long) and catalyze the hydrolysis of phosphodiester bonds on both strands of **double-stranded DNA (dsDNA)**. This cleavage results in either "sticky ends" (overhangs) or "blunt ends," which are fundamental for recombinant DNA technology and gene cloning. 2. **Why Other Options are Incorrect:** * **Option B & C:** REs are highly specific for double-stranded DNA. Enzymes that degrade RNA are called **Ribonucleases (RNases)**, and those that degrade single-stranded DNA are specific **nucleases** or **S1 nucleases**. * **Option D:** Enzymes that break peptide bonds in proteins are called **Proteases** or **Peptidases**, not nucleases. **High-Yield Clinical Pearls for NEET-PG:** * **Nomenclature:** The first letter comes from the Genus, the next two from the species (e.g., *EcoRI* from *Escherichia coli*). * **Type II REs:** These are the most commonly used in labs because they cut exactly at or near the recognition site and do not require ATP. * **Methylation:** Bacteria protect their own DNA from these enzymes by methylating their own recognition sites using **DNA Methyltransferase**. * **Applications:** Essential for **Restriction Fragment Length Polymorphism (RFLP)** analysis, used in forensic medicine (DNA fingerprinting) and diagnosing genetic diseases like Sickle Cell Anemia.

Question 402: What is considered the most important tool used in genetic engineering?

A. Genes
B. Enzymes (Correct Answer)
C. Ribozymes
D. Peptidyl-transferase

Explanation: **Explanation:** In genetic engineering (Recombinant DNA Technology), **Enzymes** are considered the most important tools because they act as the "molecular machinery" required to manipulate DNA. Without these biological catalysts, the precise cutting, joining, and replication of genetic material would be impossible. The two most critical classes of enzymes are: 1. **Restriction Endonucleases (Molecular Scissors):** These recognize specific palindromic sequences and cut DNA at precise locations. 2. **DNA Ligases (Molecular Glue):** These join DNA fragments together by catalyzing the formation of phosphodiester bonds. Other essential enzymes include DNA Polymerases (for synthesis/PCR) and Reverse Transcriptase (for cDNA libraries). **Analysis of Incorrect Options:** * **A. Genes:** These are the *targets* or the "raw material" being manipulated, not the tools used to perform the manipulation. * **C. Ribozymes:** These are RNA molecules with catalytic activity (e.g., snRNAs in splicing). While biological catalysts, they are not the primary tools used in standard recombinant DNA protocols. * **D. Peptidyl-transferase:** This is a specific ribozyme activity of the 28S rRNA (in eukaryotes) or 23S rRNA (in prokaryotes) involved in peptide bond formation during translation. It has no role in the engineering of DNA. **High-Yield Clinical Pearls for NEET-PG:** * **Restriction Enzymes:** Type II restriction enzymes are most commonly used in labs because they cut within the recognition site. * **HindII:** The first restriction endonuclease to be isolated. * **EcoRI:** A classic example that produces "sticky ends" (cohesive ends), which facilitate easier ligation compared to "blunt ends." * **Taq Polymerase:** A heat-stable DNA polymerase derived from *Thermus aquaticus*, essential for the Polymerase Chain Reaction (PCR).

Question 403: How many stop codons are present in the genetic code?

A. 1
B. 2
C. 3 (Correct Answer)
D. 4

Explanation: ### Explanation **1. Why Option C is Correct:** The genetic code consists of 64 codons. Out of these, 61 are **sense codons** (coding for amino acids) and **3 are nonsense or stop codons**. These stop codons signal the termination of protein synthesis during translation. They do not code for any amino acid because there are no corresponding tRNA molecules with matching anticodons for them. Instead, they are recognized by **Release Factors (RFs)**. The three stop codons are: * **UAA** (Ochre) * **UAG** (Amber) * **UGA** (Opal) **2. Why Other Options are Incorrect:** * **Option A & B:** These represent an incomplete set. While specific organisms or organelles (like mitochondria) may occasionally use stop codons differently, the standard universal genetic code strictly utilizes three. * **Option D:** There are only three sequences that lack a corresponding amino acid in the standard code. A fourth codon would imply a different termination mechanism not found in human biochemistry. **3. NEET-PG High-Yield Facts & Clinical Pearls:** * **Mnemonic:** To remember the stop codons: **U** **A**re **A**way (**UAA**), **U** **A**re **G**one (**UAG**), **U** **G**o **A**way (**UGA**). * **Nonsense Mutation:** A point mutation that changes a sense codon into a stop codon, leading to premature termination of the polypeptide chain and often resulting in a non-functional protein (e.g., in some forms of β-thalassemia). * **Exceptions:** In human **mitochondria**, the code varies slightly; **UGA** codes for Tryptophan (not stop), while **AGA** and **AGG** act as stop codons (instead of Arginine). * **The 21st Amino Acid:** **Selenocysteine** is encoded by the stop codon **UGA** when a specific insertion sequence (SECIS element) is present in the mRNA.

Question 404: Which Nobel Prize-winning discovery is related to RNA interference?

A. RNA interference (Correct Answer)
B. Lipoxin
C. T beta transcription factor
D. Mitochondrial DNA

Explanation: **Explanation:** The correct answer is **RNA interference (RNAi)**. This groundbreaking discovery was made by **Andrew Fire and Craig Mello**, who were awarded the **Nobel Prize in Physiology or Medicine in 2006**. **1. Why RNA Interference is Correct:** RNA interference is a natural biological process where double-stranded RNA (dsRNA) molecules inhibit gene expression or translation by neutralizing specific mRNA molecules. This "gene silencing" mechanism involves two main types of small RNA molecules: **microRNA (miRNA)** and **small interfering RNA (siRNA)**. In the cell, the enzyme **Dicer** cleaves dsRNA into fragments, which are then incorporated into the **RISC (RNA-induced silencing complex)** to degrade target mRNA. **2. Why Other Options are Incorrect:** * **Lipoxins:** These are anti-inflammatory lipid mediators derived from arachidonic acid. While clinically significant in resolving inflammation, their discovery did not involve the RNAi Nobel Prize. * **T beta transcription factor:** Transcription factors regulate the conversion of DNA to RNA, but they are distinct from the post-transcriptional silencing mechanism of RNAi. * **Mitochondrial DNA:** Discovered much earlier (1963), mtDNA follows maternal inheritance and codes for oxidative phosphorylation components; it is unrelated to the mechanism of RNA interference. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Dicer (ribonuclease) → siRNA/miRNA → RISC complex → mRNA degradation. * **Therapeutic Potential:** RNAi is being used to develop drugs for "undruggable" targets, such as **Patisiran** (the first FDA-approved RNAi drug) for hereditary transthyretin-mediated amyloidosis. * **Diagnostic Use:** It is a vital tool in functional genomics to study "loss-of-function" phenotypes in cell lines.

Question 405: Which statement is false regarding eukaryotic protein synthesis?

A. N-formylmethionine is the initiator tRNA in eukaryotes. (Correct Answer)
B. mRNA is read in the 5' to 3' direction.
C. EF2 (Elongation Factor 2) cycles between GDP and GTP bound states.
D. Capping facilitates the attachment of mRNA to the 40S ribosomal subunit.

Explanation: **Explanation** In eukaryotic protein synthesis, the initiator tRNA carries **Methionine (Met)**, not N-formylmethionine. This is a fundamental distinction between prokaryotic and eukaryotic translation. 1. **Why Option A is the correct (False) statement:** In **prokaryotes** (and mitochondria), the initiator tRNA is charged with **N-formylmethionine (fMet)**. In **eukaryotes**, the initiator tRNA is charged with unmodified **Methionine (Met-tRNAi)**. Therefore, stating that N-formylmethionine is the initiator in eukaryotes is incorrect. 2. **Analysis of other options:** * **Option B:** In all organisms, mRNA is read in the **5' to 3' direction**, and the protein is synthesized from the N-terminal to the C-terminal. * **Option C:** **EF2** is the eukaryotic translocase. It utilizes the energy from **GTP hydrolysis** to move the ribosome along the mRNA. It cycles between an active GTP-bound state and an inactive GDP-bound state. * **Option D:** The **7-methylguanosine cap** at the 5' end of eukaryotic mRNA is recognized by the eIF4F complex (cap-binding complex), which is essential for the recruitment of the **40S ribosomal subunit**. **High-Yield Clinical Pearls for NEET-PG:** * **Diphtheria toxin** and **Pseudomonas Exotoxin A** inhibit protein synthesis by catalyzing the ADP-ribosylation of **EF2**, leading to cell death. * **Shiga toxin** and **Ricin** (from castor beans) inhibit the **60S subunit** by removing an adenine residue from 28S rRNA (depurination). * **Mitochondrial translation** resembles prokaryotic translation; hence, it uses N-formylmethionine and is susceptible to certain antibiotics (e.g., chloramphenicol).

Question 406: If there were 4 nucleotides per codon instead of 3, how many possible amino acids could be formed?

A. 16
B. 21
C. 256 (Correct Answer)
D. 64

Explanation: ### Explanation **1. Understanding the Concept (The Power Rule)** The genetic code is based on combinations of the four nitrogenous bases (Adenine, Guanine, Cytosine, and Uracil). The number of possible codons is determined by the formula: **$n^k$** *(where **n** = number of different nucleotides available, and **k** = number of nucleotides per codon)*. * **In nature (Triplet Code):** $4^3 = 64$ possible codons. * **In this hypothetical scenario (Quadruplet Code):** $4^4 = 4 \times 4 \times 4 \times 4 = \mathbf{256}$ possible codons. Since each codon theoretically codes for an amino acid (or a stop signal), a 4-nucleotide system could support up to 256 unique combinations. **2. Analysis of Incorrect Options** * **Option A (16):** This represents a doublet code ($4^2$). This would be insufficient to code for the 20 standard amino acids found in humans. * **Option B (21):** This is a distractor often confused with the 20 standard amino acids plus Selenocysteine (the 21st amino acid). * **Option D (64):** This is the standard number of codons in the human genome based on the triplet ($4^3$) system. **3. NEET-PG High-Yield Clinical Pearls** * **Degeneracy/Redundancy:** The genetic code is "degenerate," meaning multiple codons can code for the same amino acid (e.g., Leucine has 6 codons). This provides a buffer against point mutations. * **Non-Ambiguity:** While one amino acid can have many codons, **one codon never codes for more than one amino acid.** * **Exceptions to Universality:** The genetic code is nearly universal, but **Mitochondrial DNA (mtDNA)** shows variations (e.g., UGA codes for Tryptophan in mitochondria instead of acting as a Stop codon). * **21st and 22nd Amino Acids:** Selenocysteine (coded by UGA + SECIS element) and Pyrrolysine (coded by UAG in some archaea).

Question 407: Which of the following statements regarding nucleic acids is not true?

A. mRNA is synthesized from the template strand of DNA.
B. mRNA is synthesized from the non-coding strand of DNA.
C. tRNA does not contain thymine as one of its pyrimidine bases. (Correct Answer)
D. Two strands of DNA are antiparallel in nature.

Explanation: **Explanation** This question tests the fundamental understanding of nucleic acid structure and transcription. **Why Option C is the correct (false) statement:** While it is a general rule that RNA contains Uracil and DNA contains Thymine, **tRNA is a notable exception.** tRNA undergoes extensive post-transcriptional modifications. One of the most characteristic features of the tRNA "TψC arm" (T-loop) is the presence of **Ribothymidine (T)**, which is formed by the methylation of uracil. Therefore, stating that tRNA does not contain thymine is factually incorrect. **Analysis of other options:** * **Option A & B:** These are **true**. During transcription, mRNA is synthesized using the **Template strand** (3'→5'). This strand is also known as the **Non-coding strand** or **Antisense strand**. The resulting mRNA sequence matches the "Coding/Sense" strand (except U replaces T). * **Option D:** This is **true**. According to the Watson-Crick model, the two strands of the DNA double helix run in opposite directions; one runs 5'→3' and the other 3'→5'. **NEET-PG High-Yield Pearls:** * **Unusual Bases in tRNA:** Besides Ribothymidine, tRNA contains Pseudouridine (ψ) and Dihydrouridine (D). * **The TψC Arm:** Responsible for binding the tRNA to the **ribosome** (specifically the 5S rRNA of the large subunit). * **The DHU Arm:** Contains Dihydrouracil and is recognized by the specific **Aminoacyl-tRNA synthetase**. * **Coding vs. Template:** Always remember: **mRNA sequence = Coding strand sequence** (with U instead of T). mRNA is complementary to the Template strand.

Question 408: Which enzyme is responsible for removing RNA primers from both the leading and lagging strands during DNA replication?

A. DNA Polymerase I (Correct Answer)
B. DNA Polymerase II
C. DNA Polymerase III
D. DNA Ligase

Explanation: **Explanation:** The correct answer is **DNA Polymerase I**. In prokaryotic DNA replication, RNA primers are necessary to provide a 3'-OH group for DNA synthesis. However, these primers must be removed and replaced with DNA to ensure genomic integrity. 1. **Why DNA Polymerase I is correct:** It is the only polymerase that possesses **5' to 3' exonuclease activity**. This unique property allows it to "chew away" the RNA primer in front of it while simultaneously synthesizing DNA to fill the gap (a process known as **Nick Translation**). It functions on both the leading strand (at the start) and the lagging strand (at the beginning of every Okazaki fragment). 2. **Why other options are incorrect:** * **DNA Polymerase II:** Primarily involved in **DNA repair** mechanisms when the replication fork stalls; it does not play a major role in primer removal. * **DNA Polymerase III:** The primary enzyme for **elongation**. While it has 3' to 5' proofreading activity, it lacks the 5' to 3' exonuclease activity required to remove primers. * **DNA Ligase:** Its role is to catalyze the formation of a **phosphodiester bond** to seal the "nicks" between DNA fragments after the primer has already been replaced. **High-Yield Clinical Pearls for NEET-PG:** * **Eukaryotic Equivalent:** In eukaryotes, RNA primers are removed by **RNase H** and **Flap Endonuclease 1 (FEN1)**, as eukaryotic polymerases lack 5' to 3' exonuclease activity. * **Klenow Fragment:** This is a proteolytic product of DNA Pol I that retains polymerase and 3' to 5' exonuclease activity but **loses** the 5' to 3' exonuclease (primer removal) activity. * **Directionality:** Always remember: **Synthesis** is 5'→3'; **Proofreading** is 3'→5'; **Primer removal** is 5'→3'.

Question 409: A mutation that completely disrupts the function of a gene is utilized in which of the following techniques?

A. Gene knockout (Correct Answer)
B. Nonsense mutation
C. Restriction fragment length polymorphism
D. Targeted gene disruption

Explanation: **Explanation:** The correct answer is **Gene knockout (Option A)**. **1. Why Gene Knockout is Correct:** A **gene knockout** is a genetic engineering technique where a specific gene is rendered entirely inoperative ("knocked out"). This is achieved by replacing or disrupting the endogenous gene sequence, typically using homologous recombination or CRISPR/Cas9 technology. The primary goal is to observe the resulting phenotype in the absence of the gene product, which helps researchers determine the gene's biological function. **2. Analysis of Incorrect Options:** * **Nonsense mutation (Option B):** This is a type of point mutation where a single nucleotide change results in a premature stop codon (UAG, UAA, or UGA). While it often leads to a non-functional protein, it is a *type* of mutation, not a *technique* used to study gene function. * **Restriction Fragment Length Polymorphism (RFLP) (Option C):** This is a laboratory technique used to exploit variations in homologous DNA sequences (polymorphisms). It involves cutting DNA with restriction enzymes and analyzing the fragment lengths. It is used for genetic mapping and fingerprinting, not for disrupting gene function. * **Targeted gene disruption (Option D):** While this term is often used interchangeably with gene knockout, in the context of standardized exams like NEET-PG, **Gene Knockout** is the specific, established term for the methodology used to create "null alleles" in model organisms (like knockout mice). **High-Yield Clinical Pearls for NEET-PG:** * **Knock-in:** A technique where a functional gene is inserted into a specific locus (e.g., replacing a mutated gene with a healthy one). * **RNA Interference (RNAi):** Known as **Gene Knockdown**; it reduces gene expression at the mRNA level rather than deleting the DNA. * **Nobel Prize Connection:** Mario Capecchi, Martin Evans, and Oliver Smithies won the 2007 Nobel Prize for their work on gene modifications in mice using embryonic stem cells.

Question 410: Which of the following techniques is used for detection of variations in DNA sequence and gene expression?

A. Hypothalamus (Correct Answer)
B. Thalamus
C. Putamen
D. Limbic cortex

Explanation: **Explanation** The question asks for the technique used to detect variations in DNA sequence and gene expression. However, there appears to be a **discrepancy between the question and the provided options**, as the options list neuroanatomical structures rather than molecular biology techniques. In the context of **Molecular Biology**, the correct answer to the question "Which technique detects variations in DNA sequence and gene expression?" should be **Microarray (DNA Microarray)**. Microarrays allow for the simultaneous analysis of thousands of genes to detect Single Nucleotide Polymorphisms (SNPs) and quantify mRNA expression levels. Regarding the provided options (Neuroanatomy context): * **Hypothalamus (Correct per key):** While not a molecular technique, the hypothalamus is the master regulator of homeostasis, controlling the endocrine system, autonomic nervous system, thirst, hunger, and temperature. * **Thalamus:** Acts as the primary sensory relay station for all senses except olfaction. * **Putamen:** A part of the basal ganglia involved in regulating movements and learning. * **Limbic Cortex:** Involved in emotion, memory, and behavior. **High-Yield NEET-PG Pearls:** 1. **DNA Microarray:** Uses nucleic acid hybridization to study the "transcriptome." 2. **Southern Blot:** Detects specific DNA sequences. 3. **Northern Blot:** Detects RNA (gene expression). 4. **Western Blot:** Detects proteins using antibodies. 5. **RT-PCR:** The gold standard for measuring specific gene expression (mRNA levels). 6. **Sanger Sequencing:** The "gold standard" for determining the exact DNA sequence.

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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