Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 461: Which biomedical tool used in DNA technology utilizes an oligomer with a single base pair substitution?

A. Polymerase Chain Reaction (PCR)
B. Restriction Fragment Length Polymorphism (RFLP) (Correct Answer)
C. Error coded mutation analysis
D. Fluorescence In Situ Hybridization (FISH)

Explanation: ### Explanation **Correct Option: B. Restriction Fragment Length Polymorphism (RFLP)** RFLP is a technique used to detect variations in DNA sequences. The underlying principle relies on the fact that a **single base pair substitution** (a Single Nucleotide Polymorphism or SNP) can either create or abolish a specific recognition site for a **restriction endonuclease**. When the DNA is digested with these enzymes, the resulting fragments differ in length between individuals. These fragments are then separated by electrophoresis and hybridized with a labeled **oligomer (DNA probe)** to visualize the specific polymorphic regions. This tool is classic for linkage analysis and identifying genetic disease carriers (e.g., Sickle Cell Anemia). **Analysis of Incorrect Options:** * **A. PCR:** This is an amplification technique using primers to exponentially increase DNA quantity. While it can be used *prior* to RFLP, the PCR process itself does not inherently rely on single base substitutions to function. * **C. Error coded mutation analysis:** This is a distractor term. While "Allele-Specific Oligonucleotide (ASO)" probes detect mutations, they are not the standard definition for the process described in the context of traditional RFLP. * **D. FISH:** This technique uses fluorescent probes to detect the presence, absence, or location of specific **large DNA sequences** or whole genes on chromosomes. It is used for gross chromosomal abnormalities (e.g., Trisomy 21, BCR-ABL translocation) rather than single base substitutions. **Clinical Pearls for NEET-PG:** * **Sickle Cell Anemia:** The classic RFLP example where a mutation in the $\beta$-globin gene destroys the *MstII* restriction site. * **VNTRs:** Variable Number Tandem Repeats are the basis for DNA fingerprinting, often analyzed via RFLP. * **Southern Blotting:** The laboratory technique required to visualize RFLP fragments.

Question 462: Which of the following methods is most suited to assess the function of a gene?

A. Southern blot
B. Northern blot
C. Gene knockout animals (Correct Answer)
D. Transgenic animals

Explanation: To assess the **function** of a gene, we must observe the physiological consequences when that gene is absent. This is the principle of "Reverse Genetics." ### Why "Gene Knockout Animals" is Correct A **Gene Knockout** involves the deliberate inactivation or "deletion" of a specific gene within an organism’s genome. By comparing the phenotype of the knockout animal with a wild-type (normal) animal, researchers can determine exactly what biological processes that gene controls. For example, knocking out the *LDL receptor* gene in mice leads to hypercholesterolemia, confirming its function in cholesterol clearance. ### Why Other Options are Incorrect * **Southern Blot:** Used for the detection of a specific **DNA sequence** in a sample. It identifies the presence or size of a gene but does not provide information about its biological function. * **Northern Blot:** Used to study **gene expression** by detecting specific **RNA** molecules. While it shows if a gene is "active," it doesn't prove what the resulting protein actually does in the body. * **Transgenic Animals:** These animals have a **foreign gene inserted** into their genome (Gain-of-function). While useful, adding a gene can sometimes cause non-specific effects or overexpression artifacts, making "Knockout" (Loss-of-function) the gold standard for defining a gene's essential baseline function. ### High-Yield NEET-PG Pearls * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-**O**, **W**estern-**P**rotein. * **Knock-in:** A variation where a mutated gene is substituted for the normal gene to study specific diseases (e.g., Sickle Cell models). * **RNA Interference (RNAi):** Another method to study function by "silencing" mRNA, often called a "Gene Knockdown." * **CRISPR-Cas9:** The most modern and efficient tool used to create knockout models.

Question 463: Polypeptide chain termination is enhanced by:

A. Stop codon (Correct Answer)
B. Promoter
C. Ribosomal unit
D. All

Explanation: ### Explanation **Why the Correct Answer is Right:** Translation termination occurs when the ribosome encounters one of the three **stop codons** (UAA, UAG, or UGA) on the mRNA. Unlike other codons, stop codons do not code for an amino acid and are not recognized by tRNA. Instead, they are recognized by **Release Factors (RFs)**. These factors trigger the peptidyl transferase to catalyze the addition of a water molecule instead of an amino acid, leading to the hydrolysis of the bond between the completed polypeptide chain and the tRNA, thereby releasing the protein. **Analysis of Incorrect Options:** * **B. Promoter:** This is a DNA sequence located upstream of a gene where RNA polymerase binds to initiate **transcription**, not translation. It has no role in polypeptide chain termination. * **C. Ribosomal unit:** While the ribosome is the machinery where translation occurs, the units themselves (40S/60S in eukaryotes) do not "enhance" termination; they dissociate *after* the stop codon is recognized by release factors. * **D. All:** Incorrect, as only the stop codon serves as the specific signal for termination. **NEET-PG High-Yield Pearls:** * **Mnemonic for Stop Codons:** **U** **A**re **A**way (UAA), **U** **A**re **G**one (UAG), **U** **G**o **A**way (UGA). * **Release Factors:** In Prokaryotes, RF1 (UAA, UAG) and RF2 (UAA, UGA) are used. In Eukaryotes, a single factor, **eRF1**, recognizes all three stop codons. * **Nonsense Mutation:** A point mutation that results in a premature stop codon, leading to a truncated, usually non-functional protein (e.g., in some forms of Beta-thalassemia). * **Energy Requirement:** Termination is an energy-dependent process requiring **GTP** hydrolysis.

Question 464: A mutation that converts an amino acid codon to a stop codon is a:

A. Nonsense mutation (Correct Answer)
B. Transversion
C. Silent mutation
D. Frame shift mutation

Explanation: ### **Explanation** **Correct Answer: A. Nonsense mutation** **Mechanism:** A **nonsense mutation** is a type of point mutation where a single nucleotide substitution results in the transformation of an amino acid-coding codon into a **premature stop codon** (UAG, UAA, or UGA). This leads to the premature termination of translation, resulting in a truncated, usually non-functional protein. --- ### **Analysis of Incorrect Options:** * **B. Transversion:** This refers to a specific type of substitution where a **purine is replaced by a pyrimidine** (e.g., A → C) or vice versa. While a transversion *could* result in a nonsense mutation, the term describes the chemical nature of the base change, not the functional outcome on the protein. * **C. Silent mutation:** This occurs when a nucleotide change does not alter the amino acid sequence due to the **degeneracy of the genetic code** (e.g., GAA and GAG both code for Glutamate). The protein remains unchanged. * **D. Frame shift mutation:** This is caused by the **insertion or deletion** of nucleotides (not in multiples of three). This shifts the reading frame of the mRNA, completely changing the amino acid sequence downstream and often creating a premature stop codon eventually, but it is not defined by a single codon-to-stop conversion. --- ### **High-Yield Clinical Pearls for NEET-PG:** 1. **Stop Codons:** Remember the mnemonic: **U** **A**re **G**one (UAG), **U** **A**re **A**way (UAA), **U** **G**o **A**way (UGA). 2. **Clinical Example:** Nonsense mutations are frequently seen in severe forms of **β-thalassemia (β⁰)** and **Duchenne Muscular Dystrophy (DMD)**. 3. **Nonsense-Mediated Decay (NMD):** The cell often recognizes and degrades mRNAs containing premature stop codons to prevent the accumulation of potentially toxic truncated proteins. 4. **Transition vs. Transversion:** Transitions (Purine to Purine, e.g., A↔G) are more common than Transversions (Purine to Pyrimidine).

Question 465: Genomic imprinting is seen in which of the following conditions?

A. Prader-Willi syndrome (Correct Answer)
B. Marfan syndrome
C. Ehlers-Danlos syndrome
D. Osteogenesis imperfecta

Explanation: **Explanation:** **Genomic Imprinting** is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. While most autosomal genes are expressed from both alleles, imprinted genes are "silenced" (via DNA methylation) on either the maternal or paternal chromosome. **Why Prader-Willi Syndrome (PWS) is correct:** PWS is a classic example of genomic imprinting involving chromosome **15q11-q13**. In normal individuals, the PWS gene is active on the paternal chromosome and silenced (imprinted) on the maternal one. PWS occurs when the **paternal** contribution is lost—either through microdeletion (70%), Maternal Uniparental Disomy (25%), or imprinting defects. Conversely, loss of the **maternal** contribution in the same region leads to **Angelman Syndrome**. **Why the other options are incorrect:** * **Marfan Syndrome:** An autosomal dominant disorder caused by mutations in the *FBN1* gene (Fibrillin-1). It follows Mendelian inheritance, not imprinting. * **Ehlers-Danlos Syndrome:** A heterogeneous group of connective tissue disorders (mostly autosomal dominant or recessive) involving collagen synthesis defects. * **Osteogenesis Imperfecta:** Primarily an autosomal dominant "brittle bone" disease caused by mutations in *COL1A1* or *COL1A2* genes. **High-Yield Clinical Pearls for NEET-PG:** * **PWS Mnemonic:** **P**aternal **D**eletion = **P**rader-Willi (presents with hyperphagia, obesity, hypogonadism, and hypotonia). * **Angelman Mnemonic:** **M**aternal **D**eletion = **A**ngelman (presents with inappropriate laughter/“Happy Puppet,” seizures, and ataxia). * **Other Imprinting Disorders:** Beckwith-Wiedemann Syndrome (Chromosome 11p15) and Silver-Russell Syndrome. * **Mechanism:** Imprinting occurs during **gametogenesis** and is maintained throughout mitosis in somatic cells.

Question 466: Which of the following is NOT a feature of the genetic code?

A. Ambiguous (Correct Answer)
B. Non-overlapping
C. Commaless
D. Specific

Explanation: The genetic code is a set of rules used by living cells to translate information encoded within genetic material into proteins. Understanding its properties is fundamental for medical biochemistry. ### **Why "Ambiguous" is the Correct Answer** The genetic code is **unambiguous**, not ambiguous. This means that **one specific codon always codes for one specific amino acid**. For example, the codon UUU always codes for Phenylalanine and nothing else. If the code were "ambiguous," a single codon could code for multiple different amino acids, which would lead to the synthesis of unpredictable and non-functional proteins. ### **Explanation of Incorrect Options** * **B. Non-overlapping:** The code is read sequentially, three bases at a time. A single base is part of only one codon and is not shared between adjacent codons. * **C. Commaless:** There are no "punctuation marks" or spacers between codons. Once translation begins at the start codon (AUG), the mRNA is read continuously until a stop codon is reached. * **D. Specific:** This is a synonym for "unambiguous." It reinforces that a particular codon is dedicated to a specific amino acid. ### **High-Yield Clinical Pearls for NEET-PG** * **Degeneracy (Redundancy):** While one codon codes for only one amino acid (unambiguous), **one amino acid can be coded by multiple codons** (e.g., Leucine has six codons). This provides protection against silent mutations. * **Universality:** The code is the same in almost all organisms. **Exception:** Mitochondrial DNA (e.g., UGA codes for Tryptophan in mitochondria, but is a Stop codon in the cytosol). * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why the third base of a codon can sometimes vary without changing the amino acid, allowing one tRNA to recognize multiple codons.

Question 467: What is true about the coding strand of DNA?

A. Minus strand
B. Template strand
C. Runs in the 3'-5' direction
D. Runs in the 5'-3' direction (Correct Answer)

Explanation: **Explanation:** In molecular biology, DNA transcription involves two strands with distinct roles. The **coding strand** (also known as the **sense strand** or **plus (+) strand**) is the DNA strand whose base sequence corresponds directly to the sequence of the RNA transcript produced (with Thymine replaced by Uracil). **Why Option D is Correct:** By convention, the coding strand is always written in the **5' to 3' direction**. This matches the direction of the newly synthesized mRNA and the direction in which the ribosome reads the genetic code during translation. Because it "codes" for the protein sequence in the same orientation as the mRNA, it is designated as the 5'-3' strand. **Analysis of Incorrect Options:** * **Option A (Minus strand):** The coding strand is the **plus (+)** strand. The "minus" (-) strand refers to the template strand. * **Option B (Template strand):** The template strand (or antisense strand) is the one actually read by RNA polymerase to synthesize RNA via complementary base pairing. The coding strand is the non-template strand. * **Option C (Runs in the 3'-5' direction):** This describes the **template strand**. RNA polymerase moves along the template strand in a 3' to 5' direction so that it can synthesize the new RNA molecule in a 5' to 3' direction. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** DNA synthesis (Replication), RNA synthesis (Transcription), and Protein synthesis (Translation) all occur in the **5' → 3' direction**. * **Sequence Identity:** The mRNA sequence is identical to the **coding strand** (except U for T) and complementary to the **template strand**. * **Promoters:** Regulatory sequences like the TATA box are described based on their position on the **coding strand**.

Question 468: What are the main components of a chromosome?

A. DNA (Correct Answer)
B. Transfer RNA (tRNA)
C. Messenger RNA (mRNA)
D. Ribosomal RNA (rRNA)

Explanation: ### Explanation **1. Why DNA is the Correct Answer:** Chromosomes are the organized structures of genetic material found in the nucleus of eukaryotic cells. The primary chemical component of a chromosome is **Deoxyribonucleic Acid (DNA)**, which carries the hereditary information. In humans, DNA is tightly coiled around basic proteins called **histones** to form nucleosomes (the "beads on a string" structure), which further condense into chromatin and eventually chromosomes during cell division. **2. Why the Other Options are Incorrect:** * **tRNA (Transfer RNA):** These are small RNA molecules (70-90 nucleotides) found in the **cytosol**. Their role is to transport specific amino acids to the ribosome during translation; they are not structural components of chromosomes. * **mRNA (Messenger RNA):** This is a transient molecule synthesized in the nucleus (via transcription) that carries genetic code to the cytoplasm for protein synthesis. It does not form the structural framework of a chromosome. * **rRNA (Ribosomal RNA):** This is the catalytic component of **ribosomes**. While synthesized in the nucleolus, it functions in the cytoplasm to facilitate protein synthesis. **3. NEET-PG High-Yield Clinical Pearls:** * **Nucleosome Core:** Consists of an octamer of histones (**H2A, H2B, H3, and H4**) with 146 base pairs of DNA wrapped around it. **Histone H1** acts as the "linker" protein. * **Euchromatin vs. Heterochromatin:** Euchromatin is transcriptionally active (loose), while heterochromatin is inactive (dense). * **Clinical Correlation:** Drugs like **Valproic acid** act as Histone Deacetylase (HDAC) inhibitors, altering chromosome condensation to affect gene expression. * **Karyotyping:** Chromosomes are best visualized during the **Metaphase** stage of mitosis using Colchicine to arrest the cell cycle.

Question 469: What is an Okazaki fragment?

A. Synthesized along the leading strand
B. Synthesized by DNA polymerase delta in eukaryotes (Correct Answer)
C. Joined by Flap endonuclease I
D. Synthesized by DNA polymerase I in prokaryotes

Explanation: **Explanation:** DNA replication is **semi-discontinuous**. While the leading strand is synthesized continuously, the **lagging strand** is synthesized in short, discontinuous segments known as **Okazaki fragments**. **Why Option B is correct:** In eukaryotes, DNA replication involves specific polymerases. While **Pol α** (alpha) initiates synthesis with an RNA primer, **DNA Polymerase δ (delta)** is the primary enzyme responsible for the elongation of the lagging strand (Okazaki fragments). It possesses high processivity and 3'→5' exonuclease activity for proofreading. **Analysis of Incorrect Options:** * **Option A:** Okazaki fragments are synthesized exclusively along the **lagging strand** (3' to 5' template direction) because DNA polymerase can only synthesize DNA in the 5' to 3' direction. * **Option C:** While Flap endonuclease I (FEN1) removes the RNA primers in eukaryotes, the actual "joining" or sealing of the nicks between fragments is performed by **DNA Ligase I**. * **Option D:** In prokaryotes (like *E. coli*), Okazaki fragments are synthesized by **DNA Polymerase III**. DNA Polymerase I is responsible for removing the RNA primer and filling the resulting gaps. **High-Yield Clinical Pearls for NEET-PG:** * **Polymerase Switching:** The transition from Pol α to Pol δ/ε is called polymerase switching. * **Eukaryotic Polymerases:** Remember **"α, δ, ε"**: * **α (Alpha):** Primase activity (starts the chain). * **δ (Delta):** Lagging strand synthesis. * **ε (Epsilon):** Leading strand synthesis. * **γ (Gamma):** Mitochondrial DNA replication. * **DNA Ligase:** Uses ATP in eukaryotes but NAD+ in some bacteria to form the phosphodiester bond.

Question 470: Restriction fragment length polymorphism is used for?

A. Analysis of chromosome structures (Correct Answer)
B. DNA estimation
C. Synthesis of nucleic acid
D. Detecting proteins in a cell

Explanation: **Explanation:** **Restriction Fragment Length Polymorphism (RFLP)** is a molecular technique used to detect variations in homologous DNA sequences. It relies on the use of **Restriction Endonucleases**, which act as "molecular scissors" to cut DNA at specific recognition sites. 1. **Why Option A is Correct:** RFLP is used for the **analysis of chromosome structures** because mutations (substitutions, insertions, or deletions) within a DNA sequence can create or abolish recognition sites for restriction enzymes. This results in DNA fragments of varying lengths. By comparing these fragment patterns, scientists can map genes, identify chromosomal rearrangements, and detect genetic polymorphisms associated with specific diseases (e.g., Sickle Cell Anemia). 2. **Why Other Options are Incorrect:** * **B. DNA estimation:** This is typically done using **Spectrophotometry** (measuring absorbance at 260 nm) or fluorometry, not RFLP. * **C. Synthesis of nucleic acid:** This refers to processes like **PCR (Polymerase Chain Reaction)** or automated DNA synthesis, whereas RFLP is an analytical/diagnostic tool. * **D. Detecting proteins:** Protein detection is achieved through **Western Blotting** or ELISA. RFLP is strictly a DNA-based technique. **High-Yield Clinical Pearls for NEET-PG:** * **Southern Blotting:** RFLP analysis is traditionally performed using the Southern Blot technique. * **Sickle Cell Anemia:** A classic application of RFLP is detecting the loss of the *MstII* restriction site in the β-globin gene. * **DNA Fingerprinting:** RFLP was the original method used for forensic DNA profiling and paternity testing (though now largely replaced by STR analysis). * **Mapping:** It is a primary tool for **Linkage Analysis** to locate disease-causing genes on specific chromosomes.

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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