Molecular Biology and Genomics Practice Questions

Q: Which of the following is an example of post-translational modification?

N-glycosylation of proteins. ### Explanation **Post-translational modification (PTM)** refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis. These modifications occur after the messenger RNA (mRNA) has been translated into a polypeptide chain. **Correct Option: B. N-glycosylation of proteins** Glycosylation is the process of adding carbohydrate chains to proteins. **N-glycosylation** occurs in the **Lumen of the Rough Endoplasmic Reticulum (RER)**, where a carbohydrate is attached to the nitrogen atom of an **Asparagine** residue. This is a classic example of a PTM essential for protein folding, stability, and cell signaling. **Analysis of Incorrect Options:** * **A. Gamma carboxylation of glutamate residues:** While this is indeed a post-translational modification (crucial for clotting factors II, VII, IX, and X), the question likely seeks the most representative or "textbook" example of PTM in a general cellular context. *Note: In many standardized exams, if multiple PTMs are listed, focus on the site of occurrence (ER/Golgi).* * **C. 7-methylguanosine capping:** This is a **post-transcriptional** modification. It occurs in the nucleus on the 5' end of the pre-mRNA to protect it from nuclease degradation. * **D. Poly(A) tail addition:** This is also a **post-transcriptional** modification where a tail of adenine nucleotides is added to the 3' end of the mRNA. **High-Yield Clinical Pearls for NEET-PG:** * **N-glycosylation** occurs in the **RER**, while **O-glycosylation** (attachment to Serine/Threonine) occurs in the **Golgi apparatus**. * **I-Cell Disease:** Caused by a deficiency in the enzyme phosphotransferase, leading to a failure of mannose-6-phosphate tagging (a PTM) on lysosomal enzymes. * **Vitamin K** is a co-factor for the **gamma-carboxylation** of glutamate residues; inhibition of this PTM is the mechanism of action for Warfarin.

Q: Which of the following proteins is synthesized by free ribosomes?

Cytosolic proteins. In eukaryotic cells, protein synthesis occurs in two distinct locations based on the ultimate destination of the protein. This concept is central to the **Signal Hypothesis**. ### 1. Why Cytosolic Proteins are Correct **Free ribosomes** (ribosomes suspended in the cytosol) are responsible for synthesizing proteins that function within the intracellular environment. These include: * **Cytosolic proteins** (e.g., enzymes of glycolysis like Hexokinase). * **Nuclear proteins** (e.g., Histones, DNA polymerase). * **Mitochondrial and Peroxisomal proteins.** These proteins lack a signal peptide for the Endoplasmic Reticulum (ER) and are released directly into the cytosol after translation. ### 2. Why Other Options are Incorrect * **Secretory Proteins (B) & Membrane Proteins (C):** These are synthesized by **membrane-bound ribosomes** on the **Rough Endoplasmic Reticulum (RER)**. These proteins possess an N-terminal **Signal Peptide** that is recognized by the **Signal Recognition Particle (SRP)**, which docks the ribosome-mRNA complex to the RER membrane. * *Secretory proteins* (e.g., Insulin, Albumin) are sequestered into the ER lumen. * *Membrane proteins* (e.g., Receptors, Ion channels) are integrated into the ER membrane during synthesis. ### High-Yield Clinical Pearls for NEET-PG * **Signal Recognition Particle (SRP):** A cytosolic ribonucleoprotein that inhibits premature folding and facilitates docking to the RER. * **I-Cell Disease:** A defect in the post-translational modification (mannose-6-phosphate tagging) of lysosomal enzymes, which are synthesized on the RER but fail to reach the lysosomes. * **Chaperones:** Proteins like **Hsp70** help in the correct folding of proteins synthesized by both free and bound ribosomes.

Q: Which of the following is necessarily present in an expression vector but not in a cloning vector?

Ribosomal entry site. ### Explanation The fundamental difference between a **cloning vector** and an **expression vector** lies in their purpose: a cloning vector is designed to simply replicate and store a DNA fragment, whereas an expression vector is designed to produce the protein encoded by that DNA. **Why "Ribosomal Entry Site" is Correct:** For a protein to be synthesized (translation), the mRNA transcribed from the vector must bind to a ribosome. In prokaryotic expression vectors, this requires a **Shine-Dalgarno sequence**, and in eukaryotic systems, an **Internal Ribosome Entry Site (IRES)** or a Kozak sequence. Since cloning vectors only aim to replicate DNA and not translate it into protein, they do not require these translation-initiation signals. **Analysis of Incorrect Options:** * **Origin of Replication (ori):** Essential for both. It allows the plasmid to replicate independently within the host cell. * **Restriction Site:** Essential for both. These are specific sequences where restriction endonucleases cut the DNA to allow the insertion of the target gene (Multiple Cloning Site). * **Selectable Marker:** Essential for both. These are typically antibiotic resistance genes (e.g., Ampicillin resistance) that allow researchers to identify and select host cells that have successfully taken up the vector. **High-Yield NEET-PG Pearls:** * **Expression Vectors** must also contain a **strong promoter** (like *tac* or *T7*) and a **transcription termination signal**, which are absent in basic cloning vectors. * **Shine-Dalgarno Sequence:** A purine-rich sequence (AGGAGG) located 8-13 nucleotides upstream of the start codon (AUG) in prokaryotes. * **cDNA Requirement:** When using expression vectors in bacteria to produce human proteins (like Insulin), **cDNA** must be used because bacteria cannot perform post-transcriptional splicing to remove introns.

Q: Acetylation of histones results in which of the following modification?

Increased mRNA production. **Explanation:** **1. Why Option B is Correct:** Histone acetylation is a key epigenetic modification that regulates gene expression. Histones are rich in basic amino acids (Lysine and Arginine), giving them a positive charge that binds tightly to the negatively charged DNA phosphate backbone [5]. * **Mechanism:** The enzyme **Histone Acetyltransferase (HAT)** adds acetyl groups to the lysine residues on histone tails [3]. * **Result:** This neutralizes the positive charge of the histones, weakening their affinity for DNA. The chromatin shifts from a tightly packed state (Heterochromatin) to a relaxed, open state (**Euchromatin**). This allows RNA polymerase and transcription factors access to the DNA, leading to **increased mRNA production (transcription)** [1], [4]. **2. Why Other Options are Incorrect:** * **Option A:** Acetylation *decreases* heterochromatin formation by promoting the formation of euchromatin [4]. * **Option C:** Acetylation is the *result* of HAT activity. Deacetylases (HDACs) perform the opposite function—they remove acetyl groups to silence genes [2]. * **Option D:** Arginine methylation is a distinct modification. While both are epigenetic markers, acetylation specifically targets lysine residues to activate transcription [3]. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **A**cetylation **A**ctivates transcription; **M**ethylation **M**utes (usually) or **M**odifies DNA. * **HDAC Inhibitors:** Drugs like **Vorinostat** and **Valproic acid** inhibit Histone Deacetylases, keeping chromatin in an acetylated (active) state [2]. These are used in treating certain cancers (e.g., Cutaneous T-cell lymphoma) to reactivate tumor suppressor genes. * **DNA Methylation:** Unlike histone acetylation, DNA methylation (usually at CpG islands) typically leads to **gene silencing**.

Q: A patient presents with a condition highly suggestive of a defect in which of the following cellular mechanisms?

Peptidyl transferase activity. ***Peptidyl transferase activity*** - This **ribosomal enzyme** located in the **23S rRNA** of the large ribosomal subunit (60S in eukaryotes) catalyzes **peptide bond formation** during protein synthesis. - Clinical relevance: antibiotics like **chloramphenicol** and **linezolid** specifically inhibit peptidyl transferase, leading to impaired protein synthesis and cellular dysfunction. *Cutting DNA at a specific site* - This describes **restriction endonucleases** that cleave DNA at specific recognition sequences, primarily used in molecular biology techniques. - These enzymes are not involved in routine cellular protein synthesis or common pathological conditions affecting ribosomal function. *Participating in DNA synthesis* - This refers to **DNA polymerases** that synthesize new DNA strands during replication and repair processes. - Defects in DNA synthesis would present with different clinical features related to **DNA repair disorders** or **replication errors**, not protein synthesis defects. *GTPase activity* - This involves enzymes that hydrolyze **GTP to GDP**, including **elongation factors** (eEF1A, eEF2) and **signal transduction proteins**. - While GTPases participate in protein synthesis, the primary defect suggested by the clinical presentation points to the **peptide bond formation step** rather than GTP hydrolysis.

Q: Defect in snRNPs causes which of the following?

Defect in splicing. ### Explanation **Correct Answer: C. Defect in splicing** **Concept:** Small nuclear ribonucleoproteins (**snRNPs**, pronounced "snurps") are the core components of the **Spliceosome**. In eukaryotes, primary mRNA transcripts (pre-mRNA) contain non-coding sequences called **introns** and coding sequences called **exons**. Splicing is the process of removing introns and joining exons to form mature mRNA. The spliceosome consists of five small nuclear RNAs (U1, U2, U4, U5, and U6) complexed with proteins to form snRNPs. These complexes recognize specific consensus sequences at the 5' donor site (GU) and 3' acceptor site (AG) of the intron. A defect in snRNPs prevents the formation of the spliceosome, leading to the retention of introns and the production of non-functional proteins. **Analysis of Incorrect Options:** * **A. Defect in 5' capping:** This process involves the addition of a 7-methylguanosine cap to the 5' end of the mRNA. It is catalyzed by **guanylyltransferase** and **methyltransferase**, not snRNPs. * **B. Defect in addition of poly-A tail:** Polyadenylation at the 3' end is performed by the enzyme **Poly-A polymerase** after cleavage by specific endonucleases. * **D. Defect in terminal addition of nucleotide:** This refers to processes like the addition of "CCA" to the 3' end of tRNA (by **tRNA nucleotidyltransferase**) or telomere lengthening (by **telomerase**). **Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often produce **Anti-Smith (Anti-Sm) antibodies**, which are highly specific for SLE. These antibodies are directed against the proteins associated with snRNPs. * **Spinal Muscular Atrophy (SMA):** Caused by a mutation in the *SMN1* gene, leading to defective assembly of snRNPs and subsequent motor neuron degeneration. * **Splicing Rule:** Introns almost always begin with **GU** and end with **AG** (the "GU-AG rule").

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

Angelman syndrome. **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" (usually via methylation) in either the maternal or paternal germline. **Why Angelman Syndrome is Correct:** Angelman syndrome and Prader-Willi syndrome are the classic examples of imprinting defects involving the **15q11-q13** region. * **Angelman Syndrome ("Happy Puppet"):** Occurs when the **maternal** allele is lost (deletion) or silenced, and the paternal allele is already imprinted (silenced). It is characterized by intellectual disability, seizures, ataxia, and frequent laughter. * **Prader-Willi Syndrome:** Occurs when the **paternal** allele is lost/silenced. **Analysis of Incorrect Options:** * **Turner Syndrome (45, XO):** A chromosomal numerical abnormality (monosomy) caused by nondisjunction, not imprinting. * **Fragile-X Syndrome:** Caused by a **trinucleotide repeat expansion (CGG)** in the *FMR1* gene. While it involves DNA methylation, it is classified as a triplet repeat disorder, not classic genomic imprinting. * **Noonan Syndrome:** An autosomal dominant disorder caused by mutations in the RAS-MAPK pathway (most commonly the *PTPN11* gene). **High-Yield NEET-PG Pearls:** 1. **Uniparental Disomy (UPD):** If a child inherits two copies of Chromosome 15 from the father (and none from the mother), it results in Angelman Syndrome. 2. **Other Imprinting Disorders:** Beckwith-Wiedemann Syndrome (Chromosome 11p15) and Silver-Russell Syndrome. 3. **Mnemonic:** **P**ader-Willi = **P**aternal deletion; **A**ngelman = **M**aternal deletion (**M**aple Syrup is **S**weet - **M**aternal **A**ngelman).

Question 1

Which of the following statements is not true regarding the 5' cap of mRNA?

Accepted Answer

The cap is attached to the 5' end of mRNA through a 5' to 5' phosphodiester bond.

Answer

The 5' terminal of mRNA is capped by 7-methylguanosine triphosphate.

Answer

The 5' cap contains 2'-O-methyl ribonucleoside adjacent to it.

Answer

7-methylguanosine is attached to mRNA through three intervening phosphate groups.

Question 2

Ultraviolet light can damage a DNA strand causing what type of alteration?

Accepted Answer

Two adjacent pyrimidine residues to form a covalently bonded dimer.

Answer

Two adjacent purine residues to form a covalently bonded dimer.

Answer

Disruption of phosphodiesterase linkage.

Answer

Disruption of non-covalent linkage.

Question 3

Which of the following is an example of post-translational modification?

Accepted Answer

N-glycosylation of proteins

Answer

Gamma carboxylation of glutamate residues

Answer

7-methylguanosine capping

Answer

Poly(A) tail addition

Question 4

Which of the following proteins is synthesized by free ribosomes?

Accepted Answer

Cytosolic proteins

Answer

Secretory proteins

Answer

Membrane proteins

Answer

None of the above

Question 5

Which of the following is necessarily present in an expression vector but not in a cloning vector?

Accepted Answer

Ribosomal entry site

Answer

Origin of replication

Answer

Restriction site

Answer

Selectable marker

Question 6

Acetylation of histones results in which of the following modification?

Accepted Answer

Increased mRNA production

Answer

Increased Heterochromatin formation

Answer

Activate Deacetylases

Answer

Arginine Methylation

Question 7

A patient presents with a condition highly suggestive of a defect in which of the following cellular mechanisms?

Accepted Answer

Peptidyl transferase activity

Answer

Cutting DNA at a specific site

Answer

Participating in DNA synthesis

Answer

GTPase activity

Question 8

Defect in snRNPs causes which of the following?

Accepted Answer

Defect in splicing

Answer

Defect in 5' capping

Answer

Defect in addition of poly-A tail

Answer

Defect in terminal addition of nucleotide

Question 9

What property of DNA synthesis does the Sanger method of DNA sequencing take advantage of to generate a sequencing ladder?

Accepted Answer

The addition of nucleotides requires a free 3'-hydroxyl group on the DNA strand.

Answer

DNA possesses a free 5'-phosphoryl group.

Answer

DNA polymerase exhibits proofreading capabilities.

Answer

Nucleotides are linked via phosphodiester bonds.

Question 10

Genomic imprinting is seen in which of the following conditions?

Accepted Answer

Angelman syndrome

Answer

Turner syndrome

Answer

Fragile-X syndrome

Answer

Noonan syndrome

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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