Molecular Biology and Genomics Practice Questions

Q: Which amino acid is coded for by the initiator codon?

Methionine. **Explanation:** In molecular biology, the translation of mRNA into a protein begins at a specific sequence known as the **initiator codon**, which is almost universally **AUG**. This codon specifies the amino acid **Methionine**. 1. **Why Methionine is Correct:** The genetic code is read in triplets (codons). The AUG codon serves a dual function: it acts as the "Start" signal for the ribosome to begin protein synthesis and simultaneously codes for Methionine. In eukaryotes, the initiator tRNA carries unmodified methionine, whereas in prokaryotes (and mitochondria), it carries **N-formylmethionine (fMet)**. 2. **Why Other Options are Incorrect:** * **Cystine (A):** This is a dimer of two Cysteine molecules. Cysteine is coded by UGU and UGC. * **Arginine (B):** A basic amino acid coded by six different codons (CGU, CGC, CGA, CGG, AGA, AGG), none of which function as start signals. * **Lysine (D):** A basic amino acid coded by AAA and AAG. **High-Yield Clinical Pearls for NEET-PG:** * **Kozak Consensus Sequence:** In eukaryotes, the efficiency of translation initiation is increased if the AUG is embedded within a specific sequence (ACCAUGG). * **Shine-Dalgarno Sequence:** In prokaryotes, this purine-rich sequence helps the 16S rRNA of the small ribosomal subunit align correctly with the AUG start codon. * **Non-Standard Start Codons:** Occasionally, GUG or UUG can act as initiator codons in prokaryotes, but they still recruit fMet-tRNA. * **Post-translational Modification:** The initial methionine is often enzymatically removed from the N-terminus of the mature protein.

Q: What is the function of the poly 'A' tail attached at the 3' end of mRNA?

Stabilization of mRNA. **Explanation:** The **poly-A tail** is a stretch of 20–250 adenine residues added post-transcriptionally to the 3' end of eukaryotic mRNA. This process, known as polyadenylation, is catalyzed by the enzyme **Poly(A) Polymerase**. **Why the correct answer is right:** The primary function of the poly-A tail is to **stabilize the mRNA molecule**. It protects the mRNA from degradation by 3' exonucleases in the cytoplasm, thereby increasing its half-life. Additionally, it facilitates the export of mRNA from the nucleus to the cytoplasm and enhances translation efficiency by interacting with Poly(A) Binding Proteins (PABP), which help form the translation initiation complex. **Why the other options are incorrect:** * **A. Unwinding of mRNA:** This is typically performed by helicases during translation or RNA-induced silencing complexes, not by the poly-A tail. * **C. Polymerization of mRNA:** This refers to the synthesis of the RNA strand itself, which is performed by **RNA Polymerase II** during transcription. * **D. Transcription of mRNA:** This is the process of copying DNA into RNA. The poly-A tail is added *after* the transcription of the main gene body is complete (post-transcriptional modification). **High-Yield Clinical Pearls for NEET-PG:** * **The Signal:** The consensus sequence for polyadenylation is **AAUAAA**, located upstream of the cleavage site. * **Exception:** **Histone mRNAs** are the only eukaryotic mRNAs that do **not** have a poly-A tail; they use a stem-loop structure instead. * **Laboratory Utility:** In molecular biology, oligo(dT) primers are used to isolate mRNA from total cellular RNA by binding to the poly-A tail.

Q: Which among the following is a transcription inhibitor?

Amanitin. **Explanation:** The correct answer is **Amanitin** (specifically $\alpha$-amanitin), a potent toxin found in the *Amanita phalloides* (death cap) mushroom. **1. Why Amanitin is Correct:** Transcription is the process of synthesizing RNA from a DNA template. **$\alpha$-amanitin** acts as a specific inhibitor of **RNA Polymerase II** in eukaryotes. By binding to the enzyme, it prevents the synthesis of mRNA, leading to a cessation of protein synthesis and eventual cell death (primarily hepatotoxicity). At higher concentrations, it can also inhibit RNA Polymerase III. **2. Why the Other Options are Incorrect:** The other three options are inhibitors of **Translation** (protein synthesis), not transcription: * **Chloramphenicol:** Inhibits the bacterial **50S** ribosomal subunit by blocking peptidyl transferase activity. * **Streptomycin:** An aminoglycoside that binds to the bacterial **30S** ribosomal subunit, causing misreading of mRNA and inhibition of initiation. * **Puromycin:** Acts as a structural analog of aminoacyl-tRNA; it causes **premature chain termination** in both prokaryotes and eukaryotes. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **RNA Polymerase Sensitivity:** RNA Pol I (rRNA) is insensitive to $\alpha$-amanitin; RNA Pol II (mRNA) is highly sensitive; RNA Pol III (tRNA) is moderately sensitive. * **Rifampicin:** A key antitubercular drug that inhibits **prokaryotic** transcription by binding to the $\beta$-subunit of bacterial DNA-dependent RNA polymerase. * **Actinomycin D:** Inhibits transcription in both prokaryotes and eukaryotes by intercalating between DNA bases. * **Mushroom Poisoning:** Clinical presentation of *Amanita* ingestion typically involves a latent period followed by severe GI distress and fulminant liver failure.

Q: Which of the following is NOT a mechanism of epigenetics?

Non-heritable changes. **Explanation:** Epigenetics refers to the study of heritable changes in gene expression that occur **without** altering the underlying DNA sequence. The fundamental hallmark of an epigenetic change is that it is stable enough to be passed down through cell divisions (mitosis) and sometimes across generations (meiosis), yet it remains reversible. **Why Option A is correct:** By definition, epigenetic modifications are **heritable**. Therefore, "Non-heritable changes" is a contradiction to the core concept of epigenetics. These changes create a "cellular memory" that ensures a daughter cell maintains the same phenotype as the parent cell. **Analysis of Incorrect Options:** * **B. Acetylation of Histone:** This is a classic epigenetic mechanism. Histone Acetyltransferases (HATs) add acetyl groups to lysine residues, neutralizing their positive charge. This relaxes the chromatin (euchromatin), making DNA accessible for transcription ("Acetylation Activates"). * **C. Hereditary changes:** As stated, epigenetics involves the transmission of gene expression patterns from parent to progeny cells. Examples include genomic imprinting (e.g., Prader-Willi and Angelman syndromes). * **D. Methylation of DNA:** This typically occurs at CpG islands in promoter regions. DNA Methyltransferase (DNMT) adds a methyl group to cytosine, which usually leads to gene silencing ("Methylation Mutes"). **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** A specialized form of epigenetics where only one allele (either maternal or paternal) is expressed. * **Drugs targeting epigenetics:** **5-Azacytidine** (DNA Methyltransferase inhibitor) and **Vorinostat** (HDAC inhibitor) are used in treating certain malignancies. * **Chromatin States:** Heterochromatin (densely packed, transcriptionally inactive) vs. Euchromatin (loosely packed, active).

Q: Which of the following is NOT a nucleic acid test?

Western blot. **Explanation:** The core concept tested here is the identification of specific macromolecules using blotting techniques. **Nucleic acid tests (NATs)** are methods used to detect specific sequences of DNA or RNA. **Why Western Blot is the correct answer:** Western blot is used for the detection of specific **proteins**, not nucleic acids. In this technique, proteins are separated by electrophoresis (usually SDS-PAGE), transferred to a membrane (nitrocellulose or PVDF), and probed using labeled **antibodies**. Since it identifies amino acid chains rather than nucleotide sequences, it is not a nucleic acid test. **Why the other options are incorrect:** * **Southern Blot:** Used for the detection of specific **DNA** sequences. It involves DNA digestion by restriction endonucleases and hybridization with a DNA probe. (Mnemonic: **S**outhern = **D**NA). * **Northern Blot:** Used for the detection of specific **RNA** sequences (usually mRNA) to study gene expression. (Mnemonic: **N**orthern = **R**NA). * **Microarray:** A high-throughput nucleic acid technique where thousands of DNA or RNA probes are fixed to a solid surface to study gene expression or genetic variations (SNPs) simultaneously. **High-Yield Clinical Pearls for NEET-PG:** * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-O, **W**estern-**P**rotein. * **Clinical Use of Western Blot:** Historically the confirmatory test for **HIV** (detecting antibodies against p24, gp120, and gp41) and Lyme disease. * **Southwestern Blot:** A hybrid technique used to identify **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Dot Blot:** A simplified version of blotting where the sample is applied directly to the membrane without prior electrophoretic separation.

Q: Highly repetitive DNA is characteristically found in which chromosomal region?

Centromere. **Explanation:** The human genome is categorized based on the frequency of sequence repetition. **Highly repetitive DNA** (also known as Satellite DNA) consists of short sequences (2–10 bp) repeated millions of times. These sequences are non-coding and are characteristically concentrated in the **centromeres** and pericentromeric regions of chromosomes. They play a structural role in maintaining chromosomal integrity and facilitating spindle fiber attachment during cell division. **Analysis of Options:** * **A. Centromere (Correct):** This region contains **Alpha-satellite DNA**, a classic example of highly repetitive DNA. It is essential for the formation of the kinetochore. * **B. Microsatellite DNA:** While repetitive, these are classified as **moderately repetitive DNA** (specifically Short Tandem Repeats or STRs). They are dispersed throughout the genome rather than being localized to a specific chromosomal landmark like the centromere. * **C. Telomere:** Telomeres contain repetitive sequences (TTAGGG), but they are categorized as **telomeric repeats**. While repetitive, the term "Highly Repetitive DNA" in classical biochemistry specifically refers to the bulk satellite DNA found at the centromeres. * **D. All of the above:** Incorrect, as the classification of "highly repetitive" specifically targets satellite DNA found at centromeres in standard medical nomenclature. **NEET-PG High-Yield Pearls:** * **Satellite DNA** can be separated from bulk DNA using **density gradient centrifugation**, appearing as separate "satellite" bands due to different buoyant densities. * **Minisatellites (VNTRs)** and **Microsatellites (STRs)** are highly polymorphic and are the basis for **DNA Fingerprinting**. * **Trinucleotide repeats** (a type of microsatellite) are associated with diseases like Huntington’s chorea and Fragile X syndrome.

Q: Splicing activity is a function of which type of RNA?

snRNA. **Explanation:** The correct answer is **snRNA (Small nuclear RNA)**. **Why snRNA is correct:** Splicing is the process of removing non-coding sequences (**introns**) and joining coding sequences (**exons**) from the primary transcript (pre-mRNA). This process is mediated by the **Spliceosome**, a large complex composed of **snRNAs** (U1, U2, U4, U5, and U6) and specific proteins. These are collectively called **snRNPs** (Small Nuclear Ribonucleoproteins, pronounced "snurps"). The snRNA component is crucial because it recognizes the consensus sequences at the 5' and 3' splice sites through complementary base pairing, facilitating the transesterification reactions required for splicing. **Why other options are incorrect:** * **mRNA (Messenger RNA):** This is the template for protein synthesis. It undergoes splicing but does not perform the catalytic activity itself. * **tRNA (Transfer RNA):** Its primary role is to act as an adapter molecule that carries specific amino acids to the ribosome during translation. * **rRNA (Ribosomal RNA):** This is a structural and catalytic component of ribosomes. While it has ribozyme activity (peptidyl transferase), it is involved in translation, not splicing. **High-Yield Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often develop **anti-Smith (anti-Sm) antibodies**, which are directed against the proteins associated with snRNPs. * **Alternative Splicing:** A mechanism where different combinations of exons are joined, allowing a single gene to code for multiple proteins (increases genetic diversity). * **Splice Site Mutations:** Mutations at splice sites can lead to abnormal proteins, a classic example being certain forms of **β-thalassemia**.

Q: Catabolite Activator Protein (CAP) in the Lac operon functions as a:

Positive regulator. ### Explanation **1. Why Option A is Correct:** The **Catabolite Activator Protein (CAP)**, also known as cAMP Receptor Protein (CRP), acts as a **positive regulator** (activator) of the Lac operon. In the absence of glucose, intracellular levels of **cAMP** rise. cAMP binds to CAP, forming a cAMP-CAP complex. This complex binds to a specific site upstream of the promoter, significantly increasing the affinity of RNA polymerase for the promoter. This "turns up" transcription, ensuring the cell can efficiently metabolize lactose when glucose is unavailable. **2. Why Other Options are Incorrect:** * **Option B (Promoter):** The promoter is a DNA sequence where RNA polymerase binds to initiate transcription. CAP is a protein that binds *near* the promoter, not the promoter itself. * **Option C & D (Repressor/Negative Regulator):** These terms refer to the **Lac Repressor protein** (encoded by the *lacI* gene). The repressor binds to the **operator** to inhibit transcription when lactose is absent. CAP does the opposite; it enhances transcription. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Dual Control:** The Lac operon is under both negative control (by the Lac Repressor) and positive control (by CAP). * **Glucose Effect:** This is also known as **Catabolite Repression**. High glucose levels lead to low cAMP, preventing the CAP complex from forming, which keeps the operon "off" even if lactose is present. * **Inducer:** Allolactose (an isomer of lactose) is the natural inducer that inactivates the repressor. * **IPTG:** A synthetic, non-metabolizable inducer often used in laboratory settings to study the Lac operon. * **Requirement for Expression:** For maximal expression of the Lac operon, two conditions must be met: **High Lactose** (to remove the repressor) and **Low Glucose** (to allow CAP binding).

Question 1

Which amino acid is coded for by the initiator codon?

Accepted Answer

Methionine

Answer

Cystine

Answer

Arginine

Answer

Lysine

Question 2

In a patient with non-Hodgkin B cell lymphoma, a nuclear gene is actively transcribed to mRNA and transported to the cytoplasm, where a protein is translated. This process is associated with up-regulation of BCL2. In a control group without lymphoma, translation of this mRNA does not occur. How is the silencing of this active gene's mRNA most likely to occur?

Accepted Answer

Binding to microRNA (miRNA)

Answer

Absence of transfer RNA (tRNA)

Answer

Methylation of DNA

Answer

Mutation of messenger RNA (mRNA)

Question 3

What is the function of the poly 'A' tail attached at the 3' end of mRNA?

Accepted Answer

Stabilization of mRNA

Answer

Unwinding of mRNA

Answer

Polymerization of mRNA

Answer

Transcription of mRNA

Question 4

Which among the following is a transcription inhibitor?

Accepted Answer

Amanitin

Answer

Chloramphenicol

Answer

Streptomycin

Answer

Puromycin

Question 5

Which of the following is NOT a mechanism of epigenetics?

Accepted Answer

Non-heritable changes

Answer

Acetylation of Histone

Answer

Hereditary changes

Answer

Methylation of DNA

Question 6

A 36-year-old male patient with a family history of dementia is being considered for genetic testing. What type of genetic test is most appropriate in this scenario?

Accepted Answer

Genetic carrier test

Answer

Presymptomatic test

Answer

Post-symptomatic test

Answer

Genetic presymptomatic test

Question 7

Which of the following is NOT a nucleic acid test?

Accepted Answer

Western blot

Answer

Southern blot

Answer

Northern blot

Answer

Microarray

Question 8

Highly repetitive DNA is characteristically found in which chromosomal region?

Accepted Answer

Centromere

Answer

Microsatellite DNA

Answer

Telomere

Answer

All of the above

Question 9

Splicing activity is a function of which type of RNA?

Accepted Answer

snRNA

Answer

mRNA

Answer

tRNA

Answer

rRNA

Question 10

Catabolite Activator Protein (CAP) in the Lac operon functions as a:

Accepted Answer

Positive regulator

Answer

Promoter

Answer

Repressor

Answer

Negative regulator

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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