Translation: Protein Synthesis — MCQs
Which of the following statements regarding collagen synthesis is incorrect?
Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography
What is the primary function of the sigma subunit of prokaryotic RNA polymerase?
During eukaryotic protein synthesis, phosphorylation of which of the following is enhanced by insulin?
miRNA binds to which part of the mRNA to inhibit translation?
Which of the following is a function of ribozymes?
Which bacterium is particularly notorious for producing extended-spectrum beta-lactamases (ESBLs) and carbapenemases, contributing to significant antibiotic resistance in hospital settings?
Which of the following is active in dephosphorylated state?
The Shine-Dalgarno sequence is primarily associated with which biological process?
What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?
Translation: Protein Synthesis Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following statements regarding collagen synthesis is incorrect?
- A. Hydroxylation of lysine occurs in ER
- B. Synthesized in ribosomes as preprocollagen
- C. Triple helix assembly occurs in ER
- D. Hydroxylation of proline occurs in Golgi apparatus (Correct Answer)
Explanation: ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.
Question 2: Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography
- A. 2,1,3,5,4 (Correct Answer)
- B. 2,4,5,3,1
- C. 1,2,4,3,5
- D. 1,5,2,4,3
Explanation: ***2,1,3,5,4*** - This sequence accurately reflects the typical order of operations in **recombinant protein isolation**: first, the gene is introduced into bacteria, then protein is expressed and cells lysed, followed by **SDS-PAGE as an intermediate quality check** to confirm protein expression before proceeding to purification steps (column chromatography and elution). - The process starts with gene incorporation, includes an analytical checkpoint after lysis, and ends with purified protein elution. *2,4,5,3,1* - This sequence is incorrect because **protein elution (4)** and **column chromatography (5)** are purification steps that occur *after* protein expression and cell lysis. - **Lysis (1)** cannot happen after elution, as cells must be lysed first to release the protein for purification. *1,2,4,3,5* - This sequence is incorrect because **expression and lysis (1)** must occur *after* the gene has been **incorporated into bacteria (2)** - the gene must be present before it can be expressed. - Additionally, **protein elution (4)** should follow **column chromatography (5)**, as elution is the step where protein is collected from the chromatography column. *1,5,2,4,3* - This sequence is incorrect because **incorporation of genes (2)** must be the first step - the gene needs to be in the bacteria before any expression, lysis, or purification can occur. - Starting with **expression and lysis (1)** before gene incorporation is impossible.
Question 3: What is the primary function of the sigma subunit of prokaryotic RNA polymerase?
- A. Is inhibited by α-amanitin
- B. Specifically recognizes the promoter site (Correct Answer)
- C. Is part of the core enzyme
- D. Inhibits the activity of RNA polymerase
Explanation: ***Specifically recognizes the promoter site*** - The **sigma subunit** is crucial for **transcription initiation** in prokaryotes, enabling the RNA polymerase holoenzyme to specifically bind to **promoter sequences** on the DNA. - This specific recognition ensures that transcription begins at the correct start site, making it a key component for accurate gene expression. *Inhibits the activity of RNA polymerase* - The sigma subunit does not inhibit RNA polymerase; rather, it **facilitates** its activity by guiding it to the correct transcription start sites. - After initiation, the sigma subunit often **dissociates** from the core enzyme, allowing the core polymerase to proceed with elongation. *Is inhibited by α-amanitin* - **α-amanitin** is a toxin that primarily inhibits **eukaryotic RNA polymerases**, particularly RNA polymerase II, and is not known to inhibit prokaryotic RNA polymerase or its sigma subunit. - Prokaryotic RNA polymerase has a different structure and mechanism, rendering it **insensitive** to α-amanitin. *Is part of the core enzyme* - The sigma subunit is **not considered part of the core enzyme**; the core enzyme consists of the α, β, β', and ω subunits. - Together with the core enzyme, the sigma subunit forms the **RNA polymerase holoenzyme**, which is responsible for initiating transcription.
Question 4: During eukaryotic protein synthesis, phosphorylation of which of the following is enhanced by insulin?
- A. eIF2
- B. eIF4A
- C. eIF4G
- D. eIF4E (Correct Answer)
Explanation: ***eIF4E*** - Insulin activates the **mTOR pathway**, which leads to activation of **Mnk1/2 kinases** that phosphorylate eIF4E at **Ser209**. - This phosphorylation enhances eIF4E's **affinity for the 5' cap structure** and increases **cap-dependent translation initiation** efficiency. *eIF4G* - While eIF4G is essential for **eIF4F complex formation**, its phosphorylation is not the primary target enhanced by insulin signaling. - Insulin's effect on eIF4G is mainly **indirect through 4E-BP1 phosphorylation**, which releases eIF4E to bind eIF4G. *eIF2* - **eIF2 phosphorylation** by kinases like **PERK, PKR, and GCN2** inhibits translation initiation during stress conditions. - This is **opposite to insulin's anabolic effects**, as insulin signaling typically promotes conditions that reduce eIF2 phosphorylation. *eIF4A* - eIF4A functions as an **RNA helicase** in the eIF4F complex, unwinding mRNA secondary structures. - While important for translation, **direct phosphorylation enhancement by insulin** is not a primary mechanism for eIF4A regulation.
Question 5: miRNA binds to which part of the mRNA to inhibit translation?
- A. Gene promoter
- B. 3'UTR (Correct Answer)
- C. Gene body
- D. 5'UTR
Explanation: ***3'UTR*** - MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression. - They primarily bind to the **3' untranslated region (3'UTR)** of messenger RNA (mRNA) molecules, leading to translational repression or mRNA degradation. *Gene promoter* - The **gene promoter** is a region of DNA located upstream of a gene, where regulatory proteins bind to initiate transcription. - miRNAs do not directly bind to gene promoters to inhibit translation. *Gene body* - The **gene body** refers to the entire transcribed region of a gene, including exons and introns. - While some regulatory elements can be found within the gene body, the primary binding site for miRNAs to exert translational control is the 3'UTR. *5'UTR* - The **5' untranslated region (5'UTR)** is located at the 5' end of an mRNA molecule, upstream of the start codon. - While the 5'UTR can play a role in regulating translation initiation, it is not the primary target for miRNA binding to inhibit translation.
Question 6: Which of the following is a function of ribozymes?
- A. Peptidyl transferase activity (Correct Answer)
- B. Cut DNA at specific site
- C. GTPase activity
- D. Participate in DNA synthesis
Explanation: ***Peptidyl transferase activity*** - The **ribosome's large subunit**, which contains **ribosomal RNA (rRNA)**, catalyzes the formation of peptide bonds during protein synthesis. - This **rRNA enzyme**, known as a **ribozyme**, exhibits **peptidyl transferase activity**. *Cut DNA at specific site* - This function is primarily carried out by **restriction enzymes**, which are **proteins**, not ribozymes. - **Ribozymes** are **RNA molecules** with catalytic activity and do not typically cleave DNA. *Participate in DNA synthesis* - **DNA synthesis** is mediated by **DNA polymerases** and other **protein enzymes**, not ribozymes. - Ribozymes' primary roles involve **RNA processing** and **peptide bond formation**. *GTPase activity* - **GTPase activity** is characteristic of **G-proteins**, which are **protein enzymes** involved in signal transduction and cell regulation. - While some ribosomal activities are **GTP-dependent**, the **GTPase itself is a protein**, not the ribozyme component.
Question 7: Which bacterium is particularly notorious for producing extended-spectrum beta-lactamases (ESBLs) and carbapenemases, contributing to significant antibiotic resistance in hospital settings?
- A. Pseudomonas
- B. Staphylococcus
- C. Streptococcus
- D. Klebsiella (Correct Answer)
Explanation: ***Klebsiella*** - **_Klebsiella pneumoniae_** is particularly well-known for producing a wide range of beta-lactamases, including both **Extended-Spectrum Beta-Lactamases (ESBLs)** and **carbapenemases**, making it a significant cause of **hospital-acquired infections** that are difficult to treat. - The presence of these enzymes allows it to hydrolyze and inactivate many commonly used antibiotics, leading to **multidrug resistance**. *Pseudomonas* - While *Pseudomonas aeruginosa* can produce various resistance mechanisms, including **carbapenemases (e.g., VIM, IMP)** and **AmpC beta-lactamases**, it is not as frequently associated with ESBL production as *Klebsiella*. - *Pseudomonas* is notorious for its intrinsic resistance to many antibiotics and its ability to form **biofilms**. *Staphylococcus* - **_Staphylococcus aureus_** is well-known for **Methicillin-Resistant *Staphylococcus aureus* (MRSA)**, which is resistant to beta-lactam antibiotics due to the acquisition of the **_mecA_ gene**, encoding altered penicillin-binding proteins. - However, *Staphylococcus* species are not commonly associated with the production of ESBLs or carbapenemases in the same way Gram-negative bacteria like *Klebsiella* are. *Streptococcus* - While some streptococcal species can develop resistance to antibiotics like **penicillin and macrolides**, they are not typically associated with the production of ESBLs or carbapenemases. - Resistance in *Streptococcus pneumoniae*, for example, often involves **alterations in penicillin-binding proteins**, similar to MRSA.
Question 8: Which of the following is active in dephosphorylated state?
- A. PEPCK
- B. Pyruvate Carboxylase
- C. Glycogen Synthase (Correct Answer)
- D. Glycogen Phosphorylase
Explanation: ***Glycogen Synthase*** - **Glycogen synthase** is primarily active in its **dephosphorylated state**, which is promoted by insulin and signals glycogen synthesis. - Dephosphorylation relieves the inhibitory effect of phosphorylation, allowing the enzyme to efficiently add glucose units to a **growing glycogen chain**. *PEPCK* - **Phosphoenolpyruvate carboxykinase (PEPCK)** activity is primarily regulated at the transcriptional level, not typically by phosphorylation state for activation. - Its expression is induced by **glucagon** and **cortisol** during gluconeogenesis. *Pyruvate Carboxylase* - **Pyruvate carboxylase** is allosterically activated by **acetyl-CoA** and its activity is not directly regulated by phosphorylation/dephosphorylation in the same manner as glycogen synthase. - This enzyme plays a key role in **gluconeogenesis** by converting pyruvate to oxaloacetate. *Glycogen Phosphorylase* - **Glycogen phosphorylase** is active in its **phosphorylated state**, particularly the 'a' form, which is promoted by glucagon and adrenaline for glycogen breakdown. - Phosphorylation activates the enzyme, leading to the **breakdown of glycogen** into glucose-1-phosphate.
Question 9: The Shine-Dalgarno sequence is primarily associated with which biological process?
- A. Transcription
- B. Translation (Correct Answer)
- C. DNA replication
- D. RNA splicing
Explanation: ***Translation*** - The **Shine-Dalgarno sequence** is a **ribosome-binding site** in prokaryotic messenger RNA (mRNA) that helps recruit the ribosome to the mRNA to initiate protein synthesis. - Its interaction with the **16S rRNA** of the small ribosomal subunit positions the start codon (AUG) correctly for **translation initiation**. - This sequence is located approximately **8 base pairs upstream** of the start codon in bacterial mRNA. *Transcription* - **Transcription** is the process of synthesizing RNA from a DNA template. - It involves elements like **promoters** and **enhancers**, not the Shine-Dalgarno sequence. *DNA replication* - **DNA replication** is the process by which DNA makes a copy of itself. - This process involves origins of replication, helicases, and DNA polymerases, with no role for the Shine-Dalgarno sequence. *RNA splicing* - **RNA splicing** is a eukaryotic process that removes introns from pre-mRNA. - The Shine-Dalgarno sequence is found in **prokaryotes**, which lack splicing machinery and introns.
Question 10: What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?
- A. 3' TAC 5' (Correct Answer)
- B. 3' TAG 5'
- C. 3' TAA 5'
- D. 3' ATG 5'
Explanation: ***3' TAC 5'*** - The **start codon** for protein synthesis on **mRNA** is **5'-AUG-3'**, which codes for **methionine** (or N-formylmethionine in prokaryotes) and signals the initiation of translation. - To produce an mRNA codon of **5'-AUG-3'**, the complementary sequence on the **template DNA strand** must be **3'-TAC-5'** (adenine pairs with uracil/thymine, guanine pairs with cytosine, and the strands are antiparallel). - During transcription, RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes mRNA in the 5' to 3' direction. *3' TAG 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUC-3'**, which codes for **isoleucine**, not methionine. - Therefore, this sequence does not correspond to the first amino acid inserted into a protein. *3' TAA 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUU-3'**, which also codes for **isoleucine**, not methionine. - This is not the initiation codon sequence. *3' ATG 5'* - While **ATG** appears in this sequence, when presented as the **template strand** in the 3' to 5' orientation, it would be transcribed to produce mRNA **5'-UAC-3'**, which codes for **tyrosine**, not methionine. - The sequence **ATG** on the **coding strand** (non-template strand) corresponds to the start codon, but this option incorrectly presents it as the template strand sequence.
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