Translation initiation — MCQs

10 questions

A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?

An 8-year-old girl is brought to the emergency room for a 6-hour history of fever, sore throat, and difficulty swallowing. Physical examination shows pooling of oral secretions and inspiratory stridor. Lateral x-ray of the neck shows thickening of the epiglottis and aryepiglottic folds. Throat culture with chocolate agar shows small, gram-negative coccobacilli. The patient's brother is started on the recommended antibiotic for chemoprophylaxis. Which of the following is the primary mechanism of action of this drug?

In translation, the wobble phenomenon is best illustrated by the fact that:

An investigator is studying the modification of newly formed polypeptides in cultured eukaryotic cells. After the polypeptides are released from the ribosome, a chemically-tagged protein attaches covalently to lysine residues on the polypeptide chain, forming a modified polypeptide. When a barrel-shaped complex is added to the cytoplasm, the modified polypeptide lyses, resulting in individual amino acids and the chemically-tagged proteins. Which of the following post-translational modifications has most likely occurred?

A 12-year-old male presents to the emergency department following several days of facial edema. A urinalysis confirms proteinuria and hematuria. Once admitted, a kidney biopsy is viewed under an electron microscope to confirm the diagnosis of minimal change disease. In the following electron micrograph, what process occurs in the structure marked with an arrow?

An investigator is studying the genotypes of wingless fruit flies using full exome sequencing. Compared to wild-type winged fruit flies, the wingless fruit flies are found to have a point mutation in the gene encoding wing bud formation during embryogenesis. The point mutation in the gene causes the mRNA transcript to have a 'UUG' segment instead of an 'AUG' segment. Which of the following processes is most likely affected by this mutation?

Given the mRNA sequence shown below, if translation were to start at the first base, what would the tRNA anticodon be for the last amino acid translated in the chain? 5'----GCACCGGCCUGACUAUAA---3'

Expression of an mRNA encoding for a soluble form of the Fas protein prevents a cell from undergoing programmed cell death. However, after inclusion of a certain exon, this same Fas pre-mRNA eventually leads to the translation of a protein that is membrane bound, subsequently promoting the cell to undergo apoptosis. Which of the following best explains this finding?

An investigator is studying the rate of wound healing by secondary intention. He performs a biopsy of a surgically debrided wound 1 day and 5 days after the initial surgical procedure. The second biopsy shows wound contraction, endothelial cell proliferation, and accumulation of macrophages. The cells responsible for wound contraction also secrete a protein that assembles in supercoiled triple helices. In which of the following structures does this protein type play an important structural role?

Q10

During protein translation, the triplet code of mRNA is read by a ribosome and assisted by elongation and translation factors until it reaches a stop codon (UAA, UAG, or UGA). When a stop codon is reached, a release factor binds, removing the peptide from the active ribosome and completing translation. What will happen if a mutation causes the recruitment of a release factor prior to the completion of a full peptide?

Translation initiation US Medical PG Practice Questions and MCQs

Question 1: A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?

A. CUA (Correct Answer)
B. CCC
C. UAA
D. CCA
E. AUG

Explanation: ***CUA*** - The **wobble hypothesis** allows for non-standard base pairing at the **third position** of the codon. - Since CUU codes for leucine, a change in the third base to **A (CUA)** can often still code for the same amino acid due to degeneracy of the genetic code. *CCC* - This codon codes for **proline**, not leucine. - A change in the **second letter** of the codon almost always results in a different amino acid. *UAA* - This is one of the **stop codons** (UAA, UAG, UGA), which signals the termination of translation. - It does not code for any amino acid. *CCA* - This codon codes for **proline**, not leucine. - Changing the first or second nucleotide typically results in a different amino acid. *AUG* - This codon codes for **methionine** and also serves as the **start codon** for protein synthesis. - It does not code for leucine.

Question 2: An 8-year-old girl is brought to the emergency room for a 6-hour history of fever, sore throat, and difficulty swallowing. Physical examination shows pooling of oral secretions and inspiratory stridor. Lateral x-ray of the neck shows thickening of the epiglottis and aryepiglottic folds. Throat culture with chocolate agar shows small, gram-negative coccobacilli. The patient's brother is started on the recommended antibiotic for chemoprophylaxis. Which of the following is the primary mechanism of action of this drug?

A. Inhibition of the 50S ribosomal subunit
B. Inhibition of prokaryotic topoisomerase II
C. Inhibition of DNA-dependent RNA-polymerase (Correct Answer)
D. Inhibition of the 30S ribosomal subunit
E. Inhibition of peptidoglycan crosslinking

Explanation: ***Inhibition of DNA-dependent RNA-polymerase*** - The clinical picture strongly suggests **epiglottitis** caused by *Haemophilus influenzae type b* (Hib), characterized by **fever, sore throat, difficulty swallowing, pooling of oral secretions, inspiratory stridor**, and **epiglottic thickening** on X-ray. - **Rifampin** is the recommended antibiotic for chemoprophylaxis in close contacts of Hib patients; its primary mechanism of action is to inhibit bacterial **DNA-dependent RNA polymerase**, thereby preventing **mRNA synthesis**. *Inhibition of the 50S ribosomal subunit* - This mechanism is characteristic of **macrolides** (e.g., azithromycin, erythromycin) and **clindamycin**, which are not the primary choice for Hib chemoprophylaxis. - These drugs prevent **protein synthesis** by interfering with translocation or peptide bond formation on the larger ribosomal subunit. *Inhibition of prokaryotic topoisomerase II* - This is the mechanism of action for **fluoroquinolones** (e.g., ciprofloxacin), which are typically reserved for specific infections due to potential side effects in children. - Fluoroquinolones interfere with **DNA replication** and **transcription** by preventing DNA unwinding and supercoiling. *Inhibition of the 30S ribosomal subunit* - This mechanism is associated with **tetracyclines** and **aminoglycosides** (e.g., doxycycline, gentamicin). - These antibiotics block **protein synthesis** by preventing tRNA attachment or causing misreading of mRNA. *Inhibition of peptidoglycan crosslinking* - This describes the mechanism of **beta-lactam antibiotics** (e.g., penicillin, amoxicillin, cephalosporins), which inhibit bacterial **cell wall synthesis**. - While some beta-lactams are used to treat Hib infections, they are not the primary drug for **chemoprophylaxis**.

Question 3: In translation, the wobble phenomenon is best illustrated by the fact that:

A. Charged tRNA contains energy needed for peptide bonds to form
B. The last nucleotide provides specificity for the given amino acid
C. A tRNA with the UUU anticodon can bind to either AAA or AAG codons (Correct Answer)
D. There are more amino acids than possible codons
E. The genetic code is preserved without mutations

Explanation: ***A tRNA with the UUU anticodon can bind to either AAA or AAG codons*** - The **wobble phenomenon** allows for non-standard base pairing between the **first nucleotide (5' position) of the tRNA anticodon** and the **third nucleotide (3' position) of the mRNA codon**. - In this example, a tRNA with anticodon **3'-UUU-5'** can bind to either **5'-AAA-3'** or **5'-AAG-3'** codons (both encoding lysine) due to the relaxed base-pairing rules at the wobble position. - This flexibility means fewer tRNAs are needed to recognize all 61 sense codons, illustrating the **degeneracy of the genetic code**. - According to Crick's wobble hypothesis, **U at the 5' position of the anticodon** can pair with either **A or G at the 3' position of the codon**. *Charged tRNA contains energy needed for peptide bonds to form* - While **charged tRNA** (aminoacyl-tRNA) does carry an amino acid activated for peptide bond formation, this statement describes the energy source for translation, not the wobble phenomenon. - The energy for peptide bond formation comes from the **high-energy ester bond** linking the amino acid to the tRNA, not from the base pairing itself. *The last nucleotide provides specificity for the given amino acid* - The **last nucleotide** (3' position) of the mRNA codon is where **wobble pairing** occurs, meaning it does *not* always provide strict specificity for the amino acid due to the relaxed base-pairing rules. - It is often the *first two nucleotides* of the codon that are most critical in determining the specific amino acid incorporated. *There are more amino acids than possible codons* - This statement is incorrect; there are **20 standard amino acids** and **61 sense codons** (three are stop codons), meaning there are more codons than amino acids, leading to **code degeneracy**. - The concept of wobble base pairing helps explain how this degeneracy is managed efficiently, but the premise of this option is false. *The genetic code is preserved without mutations* - This statement refers to the **fidelity of DNA replication and repair** or the evolutionary conservation of the genetic code, not the mechanism of translation or wobble base pairing. - The genetic code being largely universal and degenerate does not mean that mutations never occur, but rather that it is robust.

Question 4: An investigator is studying the modification of newly formed polypeptides in cultured eukaryotic cells. After the polypeptides are released from the ribosome, a chemically-tagged protein attaches covalently to lysine residues on the polypeptide chain, forming a modified polypeptide. When a barrel-shaped complex is added to the cytoplasm, the modified polypeptide lyses, resulting in individual amino acids and the chemically-tagged proteins. Which of the following post-translational modifications has most likely occurred?

A. Glycosylation
B. Acylation
C. Carboxylation
D. Phosphorylation
E. Ubiquitination (Correct Answer)

Explanation: ***Ubiquitination*** - The description of a **chemically-tagged protein** attaching to **lysine residues** on a newly formed polypeptide strongly suggests **ubiquitin**, a small protein that marks other proteins for degradation. - The subsequent lysis by a **barrel-shaped complex** (the **proteasome**) into amino acids and the chemically-tagged proteins is the hallmark of the **ubiquitin-proteasome pathway**, a major mechanism for targeted protein degradation. *Glycosylation* - Involves the **covalent attachment of carbohydrate moieties** to proteins, typically at asparagine, serine, or threonine residues. - While it is a common post-translational modification, it does not involve a "chemically-tagged protein" marking for proteasomal degradation. *Acylation* - Refers to the addition of an **acyl group** (e.g., fatty acids like myristate or palmitate) to a protein, often impacting membrane association. - This process is distinct from the described mechanism of protein tagging and subsequent degradation by a barrel-shaped complex. *Carboxylation* - Involves the **addition of a carboxyl group** to a protein, most notably to glutamate residues in clotting factors, requiring vitamin K. - This modification is not involved in marking proteins for degradation and does not utilize a specific "chemically-tagged protein" for this purpose. *Phosphorylation* - Refers to the **addition of a phosphate group** to a protein, typically at serine, threonine, or tyrosine residues, to regulate protein activity, signaling, and interactions. - While it is a common regulatory mechanism, it does not involve a "chemically-tagged protein" targeting the protein for complete degradation into amino acids by a proteasome.

Question 5: A 12-year-old male presents to the emergency department following several days of facial edema. A urinalysis confirms proteinuria and hematuria. Once admitted, a kidney biopsy is viewed under an electron microscope to confirm the diagnosis of minimal change disease. In the following electron micrograph, what process occurs in the structure marked with an arrow?

A. Podocyte foot process effacement (Correct Answer)
B. Normal podocyte foot process interdigitation
C. Mesangial cell proliferation
D. Endothelial cell fenestration
E. Glomerular basement membrane thickening

Explanation: **Podocyte foot process effacement** - Minimal change disease is characterized by the **effacement (flattening and fusion)** of podocyte foot processes, which are the structures indicated by the arrow in the image. - This effacement leads to the loss of the **slit diaphragm barrier**, causing massive proteinuria. *Normal podocyte foot process interdigitation* - Normal podocyte foot processes exhibit a distinct, highly organized **interdigitating pattern**, which is clearly not observed in the image due to the flattening. - This normal interdigitation is crucial for maintaining the **glomerular filtration barrier** and preventing protein leakage. *Mesangial cell proliferation* - **Mesangial cell proliferation** is characteristic of conditions like IgA nephropathy or mesangioproliferative glomerulonephritis, and is not the primary feature of minimal change disease. - The image illustrates changes in the podocytes, not the mesangial cells (labeled 'M'). *Endothelial cell fenestration* - **Endothelial cell fenestrations** are normal pores in the glomerular endothelial cells that allow for filtration, and are not directly affected or effaced in minimal change disease. - The arrow in the image points to the podocyte layer, not the endothelial cells. *Glomerular basement membrane thickening* - **Glomerular basement membrane (GBM) thickening** is seen in conditions like diabetic nephropathy or membranous nephropathy, but not typically in minimal change disease. - In minimal change disease, the GBM typically appears normal under electron microscopy.

Question 6: An investigator is studying the genotypes of wingless fruit flies using full exome sequencing. Compared to wild-type winged fruit flies, the wingless fruit flies are found to have a point mutation in the gene encoding wing bud formation during embryogenesis. The point mutation in the gene causes the mRNA transcript to have a 'UUG' segment instead of an 'AUG' segment. Which of the following processes is most likely affected by this mutation?

A. Cleavage of 5' intron
B. Binding of met-tRNA to 40S complex (Correct Answer)
C. Catalyzation of peptide bond formation
D. Dissociation of mRNA from ribosome complex
E. Shift of peptidyl-tRNA from A to P site

Explanation: ***Binding of met-tRNA to 40S complex*** - The **start codon AUG** is essential for the initiation of translation, as it signals where the ribosome should begin synthesizing the polypeptide chain and recruits the initiator tRNA carrying **methionine (met-tRNA)** to the 40S ribosomal subunit. - A mutation from **AUG to UUG** means the ribosome will not recognize the correct start site, preventing the initial binding of met-tRNA and the formation of the **initiation complex**. *Cleavage of 5' intron* - This process is part of **RNA splicing**, which occurs after transcription in the nucleus, where introns are removed from the **pre-mRNA**. - The described mutation affects a **codon sequence** in the mRNA, which is a post-splicing event related to translation, not intron cleavage. *Catalyzation of peptide bond formation* - This occurs during the **elongation phase of translation**, where the peptidyl transferase activity of the ribosome forms peptide bonds between amino acids. - The mutation prevents the **initiation of translation** altogether, meaning elongation and peptide bond formation will not even begin. *Dissociation of mRNA from ribosome complex* - This event happens at the **termination phase of translation**, when a stop codon is reached, and release factors cause the ribosome to dissociate from the mRNA and the newly synthesized polypeptide. - The mutation prevents the **start of translation**, so the ribosome will not reach the stage where it would dissociate from the mRNA. *Shift of peptidyl-tRNA from A to P site* - This is a step in the **elongation phase of translation**, specifically the **translocation process**, where the ribosome moves along the mRNA, shifting the peptidyl-tRNA from the A (aminoacyl) site to the P (peptidyl) site. - Since the **initiation of translation** is blocked by the mutated start codon, the ribosome cannot begin polypeptide synthesis, and thus, elongation steps like translocation cannot occur.

Question 7: Given the mRNA sequence shown below, if translation were to start at the first base, what would the tRNA anticodon be for the last amino acid translated in the chain? 5'----GCACCGGCCUGACUAUAA---3'

A. 3' GCG 5'
B. 3' CGC 5'
C. 5' CGG 3'
D. 3' CGG 5' (Correct Answer)
E. 3' GAU 5'

Explanation: ***3' CGG 5'*** - The mRNA sequence is 5'-GCACCGGCCUGACUAUAA-3'. We need to identify the **open reading frame** starting from the first base and translate codons until a stop codon is reached. - The codons are **GCA** (Ala), **CCG** (Pro), **GCC** (Ala), **UGA** (Stop). The **last amino acid** translated is Alanine, corresponding to the mRNA codon **GCC**. The tRNA anticodon for GCC is **3'-CGG-5'** because base pairing rules dictate C pairs with G, and G pairs with C, in an antiparallel orientation. *3' GCG 5'* - This anticodon would pair with an mRNA codon of 5'-CGC-3', which codes for Arginine, not the alanine derived from the last amino acid in the given sequence. - It does not correctly reflect the antiparallel binding and base pairing required for the mRNA codon GCC. *5' CGG 3'* - While it contains the correct bases for pairing with GCC, the **orientation is incorrect**. tRNA anticodons are written 3' to 5'. - A 5'-CGG-3' anticodon would pair with an mRNA codon of 3'-GCC-5', which is not consistent with the standard 5' to 3' mRNA codon reading. *3' GAU 5'* - This anticodon would pair with an mRNA codon of 5'-CUA-3', which codes for Leucine. - Leucine is not the last amino acid translated from the given mRNA sequence before a stop codon. *3' CGC 5'* - This anticodon would pair with an mRNA codon of 5'-GCG-3', which codes for Alanine. - However, the last amino acid translated is encoded by 5'-GCC-3', not 5'-GCG-3'.

Question 8: Expression of an mRNA encoding for a soluble form of the Fas protein prevents a cell from undergoing programmed cell death. However, after inclusion of a certain exon, this same Fas pre-mRNA eventually leads to the translation of a protein that is membrane bound, subsequently promoting the cell to undergo apoptosis. Which of the following best explains this finding?

A. Histone deacetylation
B. DNA missense mutation
C. Alternative splicing (Correct Answer)
D. Base excision repair
E. Post-translational modifications

Explanation: ***Alternative splicing*** - The scenario describes a single **pre-mRNA** producing two different protein forms (soluble vs. membrane-bound Fas) with distinct functions, depending on the inclusion or exclusion of a specific **exon**. This is the hallmark of alternative splicing. - **Alternative splicing** allows a single gene to encode multiple protein isoforms, leading to diverse cellular functions and regulation. *Histone deacetylation* - **Histone deacetylation** is a mechanism of gene regulation that typically represses gene expression by making DNA less accessible for transcription, not by altering the protein product of an already transcribed gene. - It affects whether a gene is turned "on" or "off," but doesn't explain how the same pre-mRNA produces different protein versions. *DNA missense mutation* - A **DNA missense mutation** would alter a single base pair in the DNA, potentially changing one amino acid in the resulting protein. - While it can lead to functional changes in a protein, it would not explain the complete inclusion or exclusion of an entire exon, which profoundly changes the protein's overall structure and membrane association in this manner. *Base excision repair* - **Base excision repair** is a DNA repair pathway that corrects small, non-bulky DNA lesions, such as damaged or modified bases. - This process is involved in maintaining genomic integrity and does not explain the differential processing of an mRNA transcript to produce two distinct protein isoforms. *Post-translational modifications* - **Post-translational modifications** (PTMs) occur after protein translation and involve chemical changes to the protein (e.g., phosphorylation, glycosylation). - While PTMs can alter protein function or localization, they do not explain how an entire exon's inclusion or exclusion leads to fundamentally different protein structures (soluble vs. membrane-bound).

Question 9: An investigator is studying the rate of wound healing by secondary intention. He performs a biopsy of a surgically debrided wound 1 day and 5 days after the initial surgical procedure. The second biopsy shows wound contraction, endothelial cell proliferation, and accumulation of macrophages. The cells responsible for wound contraction also secrete a protein that assembles in supercoiled triple helices. In which of the following structures does this protein type play an important structural role?

A. Basal lamina
B. Dentin
C. Corneal stroma
D. Reticular fibers (Correct Answer)
E. Nucleus pulposus

Explanation: ***Reticular fibers*** - Wound contraction is mediated by **myofibroblasts**, which secrete collagen (primarily **Type I** and **Type III collagen**). - **Type III collagen** forms **reticular fibers**, which provide structural support in early wound healing and are prominent in tissues such as lymphoid organs, liver, bone marrow, and blood vessel walls. - Reticular fibers create a delicate meshwork framework that supports cellular elements in these organs. *Basal lamina* - The basal lamina is primarily composed of **Type IV collagen**, laminin, and proteoglycans. - It functions as a selective barrier and structural support for epithelial cells, not the site for Type III collagen/reticular fibers. *Dentin* - **Dentin** is mainly composed of **Type I collagen** and hydroxyapatite crystals. - While collagen is a major structural component, it is primarily **Type I**, not **Type III collagen** that forms reticular fibers. *Corneal stroma* - The corneal stroma contains primarily **Type I collagen** arranged in highly organized lamellae to maintain transparency. - It also contains **Type V and Type VI collagen** but not **Type III collagen** as the primary structural component. *Nucleus pulposus* - The **nucleus pulposus** is mainly composed of water, proteoglycans, and **Type II collagen**. - Its function is to resist compressive forces in the intervertebral disc, not related to Type III collagen or reticular fiber formation.

Question 10: During protein translation, the triplet code of mRNA is read by a ribosome and assisted by elongation and translation factors until it reaches a stop codon (UAA, UAG, or UGA). When a stop codon is reached, a release factor binds, removing the peptide from the active ribosome and completing translation. What will happen if a mutation causes the recruitment of a release factor prior to the completion of a full peptide?

A. Single nucleotide polymorphism
B. Frameshift mutation
C. Nonsense mutation (Correct Answer)
D. Basepair wobble
E. Missense mutation

Explanation: ***Nonsense mutation*** - A nonsense mutation introduces a **premature stop codon** into the mRNA sequence. - This results in the **premature termination of translation**, leading to a truncated and often non-functional protein. *Single nucleotide polymorphism* - A SNP is a **variation at a single nucleotide position** in the DNA sequence. - While a SNP can cause a nonsense mutation, the term itself only describes the **type of sequence variation**, not its functional consequence as premature termination. *Frameshift mutation* - A frameshift mutation occurs due to the **insertion or deletion of nucleotides** not in multiples of three. - This alters the **reading frame** and typically leads to a completely different amino acid sequence downstream and often a premature stop codon, but the direct cause of premature termination here is a specific stop codon, not a shift in frame. *Basepair wobble* - **Wobble pairing** refers to the flexibility in base pairing between the third nucleotide of a codon and the first nucleotide of an anticodon. - This allows a single tRNA to recognize **multiple codons** for the same amino acid and is a normal part of translation, not a mutation causing premature termination. *Missense mutation* - A missense mutation results in a **single nucleotide change** that codes for a different amino acid. - This leads to a **substitution of one amino acid for another** in the protein, but does not typically cause premature termination of translation.

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