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?
Q2
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?
Q3
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?
Q4
A researcher is trying to decipher how mRNA codons contain information about proteins. He first constructs a sequence of all cytosine nucleotides and sees that a string of prolines is synthesized. He knows from previous research that information is encoded in groups of 3 so generates the following sequences: ACCACCACC, CACCACCAC, and CCACCACCA. Surprisingly, he sees that new amino acids are produced with the first two sequences but that the third sequence is still a string of prolines. Which of the following biochemical principles explains why this phenomenon was observed?
Q5
A 3-year-old boy is diagnosed with an alpha-gal allergy, also known as mammalian meat allergy (MMA). This condition is mediated by a reaction to the carbohydrate, galactose-alpha-1,3-galactose. An experimental treatment has been developed to halt the N-linked oligosaccharide addition that occurs in the synthesis of this compound. Which of the following cellular structures is most likely targeted by this experimental drug?
Q6
A scientist wants to determine if a specific fragment is contained within genome X. She uses a restriction enzyme to digest the genome into smaller fragments to run on an agarose gel, with the goal of separating the resulting fragments. A nitrocellulose blotting paper is then used to transfer the fragments from the agarose gel. A radiolabeled probe containing a complementary sequence to the fragment she is searching for is incubated with the blotting paper. Which of the following is the RNA equivalent of this technique?
Q7
A 21-year-old man comes to the physician because of a 3-week history of yellow discoloration of his skin, right upper abdominal pain, and fatigue. Two years ago, he underwent right-sided pleurodesis for recurrent spontaneous pneumothorax. Pulmonary examination shows mild bibasilar crackles and expiratory wheezing. Laboratory studies show an elevation of serum transaminases. Histopathological examination of a tissue specimen obtained on liver biopsy shows PAS-positive globules within periportal hepatocytes. Genetic analysis shows substitution of lysine for glutamic acid at position 342 of a gene located on chromosome 14 that encodes for a protease inhibitor (Pi). This patient most likely has which of the following Pi genotypes?
Q8
A researcher is tracing the fate of C-peptide, a product of preproinsulin cleavage. Which of the following is a true statement regarding the fate of C-peptide?
Q9
A 21-year-old woman comes to the physician for an annual health maintenance examination. She has no particular health concerns. Laboratory studies show:
Hemoglobin 11.2 g/dL
Mean corpuscular volume 74 μm3
Mean corpuscular hemoglobin concentration 30% Hb/cell
Red cell distribution width 14% (N=13–15)
Genetic analysis shows a point mutation in intron 1 of a gene on the short arm of chromosome 11. A process involving which of the following components is most likely affected in this patient?
Q10
An investigator is studying the activity of N-terminal peptidase in eukaryotes. Sulfur-containing amino acids are radiolabeled and isolated using 35S. During translation of a non-mitochondrial human genome, some of the radiolabeled amino acids bind to the aminoacyl, peptidyl, and exit sites of a eukaryotic ribosome but others bind only to the peptidyl and exit sites. Only the radiolabeled amino acids that do not bind to the ribosomal aminoacyl-site can be excised by the N-terminal peptidase. Which of the following best describes the anticodon sequence of the transfer RNA charged by the amino acid target of the N-terminal peptidase?
Transcription/translation US Medical PG Practice Questions and MCQs
Question 1: 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 2: 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 3: 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.
Question 4: A researcher is trying to decipher how mRNA codons contain information about proteins. He first constructs a sequence of all cytosine nucleotides and sees that a string of prolines is synthesized. He knows from previous research that information is encoded in groups of 3 so generates the following sequences: ACCACCACC, CACCACCAC, and CCACCACCA. Surprisingly, he sees that new amino acids are produced with the first two sequences but that the third sequence is still a string of prolines. Which of the following biochemical principles explains why this phenomenon was observed?
A. mRNA splicing
B. Reading frame shifts (Correct Answer)
C. Trimming of proteins
D. Covalent alterations
E. Translational proofreading
Explanation: ***Reading frame shifts***
- The different sequences (ACCACCACC, CACCACCAC, CCACCACCA) represent different **reading frames** for the original poly-C mRNA. Shifting the starting point by one or two nucleotides changes which codons are read.
- The first sequence (ACCACCACC) is read as ACC-ACC-ACC, which codes for **threonine**.
- The second sequence (CACCACCAC) is read as CAC-CAC-CAC, which codes for **histidine**.
- The third sequence (CCACCACCA) is read as CCA-CCA-CCA, which still codes for **proline** (both CCC and CCA are proline codons), explaining why the same amino acid string is produced.
- This demonstrates how the **reading frame** determines which amino acids are synthesized from the same nucleotide sequence.
*mRNA splicing*
- **mRNA splicing** is the process by which introns are removed from pre-mRNA, and exons are joined together. This occurs before translation.
- While it affects the final mRNA sequence, it does not explain why different amino acids appear from different **starting positions** of essentially the same base sequence during translation.
*Trimming of proteins*
- **Protein trimming** is a post-translational modification where a part of the protein is cleaved to form the active protein.
- This process occurs after the protein has been synthesized and does not influence which amino acids are incorporated based on the mRNA sequence.
*Covalent alterations*
- **Covalent alterations** refer to post-translational modifications of proteins, such as phosphorylation, glycosylation, or acetylation, which change the protein's chemical properties.
- These modifications happen after translation is complete and do not explain the initial amino acid sequence determined by the mRNA codons.
*Translational proofreading*
- **Translational proofreading** mechanisms ensure the accuracy of protein synthesis by correcting errors during amino acid incorporation.
- While important for fidelity, proofreading does not explain how different amino acids are produced from a single nucleotide sequence depending on the **starting position of translation**.
Question 5: A 3-year-old boy is diagnosed with an alpha-gal allergy, also known as mammalian meat allergy (MMA). This condition is mediated by a reaction to the carbohydrate, galactose-alpha-1,3-galactose. An experimental treatment has been developed to halt the N-linked oligosaccharide addition that occurs in the synthesis of this compound. Which of the following cellular structures is most likely targeted by this experimental drug?
A. Rough endoplasmic reticulum
B. Tumor suppressor p53
C. Sodium-potassium pump
D. Proteasome
E. Golgi apparatus (Correct Answer)
Explanation: ***Golgi apparatus***
- The **alpha-gal epitope (galactose-alpha-1,3-galactose)** is synthesized by **alpha-1,3-galactosyltransferase (GGTA1)** enzyme in the **Golgi apparatus**.
- This is a **terminal glycosylation modification** that occurs during the late stages of glycoprotein and glycolipid processing in the Golgi.
- While N-linked glycosylation begins in the RER, the specific **alpha-gal moiety is added in the Golgi** during final carbohydrate processing.
- Targeting the Golgi apparatus would effectively halt the synthesis of the alpha-gal epitope responsible for mammalian meat allergy.
*Rough endoplasmic reticulum*
- The **RER** is where **initial N-linked glycosylation** occurs, with the addition of a core oligosaccharide to asparagine residues.
- However, the alpha-gal epitope is a **terminal modification** added later in the Golgi, not during the initial RER glycosylation steps.
- Blocking N-linked glycosylation in the RER would affect many essential proteins broadly, rather than specifically targeting alpha-gal synthesis.
*Tumor suppressor p53*
- **Tumor suppressor p53** is a protein involved in regulating the cell cycle and apoptosis, acting as a crucial defense against cancer.
- It has no direct role in the synthesis or modification of carbohydrates or the specific mechanism of alpha-gal allergy.
*Sodium-potassium pump*
- The **sodium-potassium pump** is a transmembrane protein responsible for maintaining ion gradients across cell membranes, essential for nerve impulses and cellular volume regulation.
- This pump is unrelated to carbohydrate synthesis pathways or the alpha-gal epitope.
*Proteasome*
- The **proteasome** is a protein complex responsible for degrading cellular proteins that are damaged or no longer needed.
- It is involved in protein turnover and quality control and plays no role in the synthesis of carbohydrate moieties.
Question 6: A scientist wants to determine if a specific fragment is contained within genome X. She uses a restriction enzyme to digest the genome into smaller fragments to run on an agarose gel, with the goal of separating the resulting fragments. A nitrocellulose blotting paper is then used to transfer the fragments from the agarose gel. A radiolabeled probe containing a complementary sequence to the fragment she is searching for is incubated with the blotting paper. Which of the following is the RNA equivalent of this technique?
A. RT-PCR
B. Western blot
C. qPCR
D. Northern blot (Correct Answer)
E. Southern blot
Explanation: **Northern blot**
- The technique described in the question, involving **restriction enzyme digestion**, **agarose gel electrophoresis**, **blotting onto a membrane**, and **hybridization with a labeled probe**, is characteristic of a **Southern blot** for DNA
- The **Northern blot** is the analogous technique used to detect and quantify **RNA** sequences, following the same principles of separation by size and detection by hybridization with a complementary probe
- Both Southern and Northern blots use the same workflow: separate nucleic acids by size on gel → transfer to membrane → detect with complementary probe
*RT-PCR*
- **Reverse transcriptase polymerase chain reaction (RT-PCR)** is used to amplify specific **RNA** sequences by first converting **RNA** into **complementary DNA (cDNA)** using reverse transcriptase, followed by standard PCR
- Unlike Northern blot, it is an **amplification technique** rather than a direct visualization method via blotting
*Western blot*
- **Western blot** is a technique used to detect and identify specific **proteins**, not nucleic acids
- It involves **gel electrophoresis** to separate proteins by size, followed by transfer to a membrane and detection using **antibodies** rather than nucleic acid probes
*qPCR*
- **Quantitative polymerase chain reaction (qPCR)**, also known as real-time PCR, is a technique used to **quantify DNA or RNA** (after reverse transcription) in real-time
- It measures the accumulation of fluorescent signal during the PCR reaction, allowing for real-time monitoring and quantification, which is fundamentally different from a blotting technique
*Southern blot*
- The description in the question *is* a **Southern blot**, which is used for **DNA** detection, not RNA
- Since the question asks for the **RNA equivalent** of the described technique, and Southern blot detects DNA, Northern blot is the correct answer
Question 7: A 21-year-old man comes to the physician because of a 3-week history of yellow discoloration of his skin, right upper abdominal pain, and fatigue. Two years ago, he underwent right-sided pleurodesis for recurrent spontaneous pneumothorax. Pulmonary examination shows mild bibasilar crackles and expiratory wheezing. Laboratory studies show an elevation of serum transaminases. Histopathological examination of a tissue specimen obtained on liver biopsy shows PAS-positive globules within periportal hepatocytes. Genetic analysis shows substitution of lysine for glutamic acid at position 342 of a gene located on chromosome 14 that encodes for a protease inhibitor (Pi). This patient most likely has which of the following Pi genotypes?
A. PiSS
B. PiMZ
C. PiZZ (Correct Answer)
D. PiMS
E. PiSZ
Explanation: ***PiZZ***
- The patient's presentation with **yellow discoloration (jaundice)**, **right upper abdominal pain**, **elevated transaminases**, and **PAS-positive globules in periportal hepatocytes** is classic for **alpha-1 antitrypsin deficiency** (AATD) causing **liver disease**.
- The genetic analysis confirming a **lysine for glutamic acid substitution at position 342** in the **protease inhibitor (Pi) gene** (SERPINA1) specifically describes the **Z allele**. The combination of two Z alleles (**PiZZ**) leads to severe deficiency and is the most common genotype associated with significant liver and lung disease.
*PiSS*
- The **S allele** results from a glutamic acid to valine substitution, leading to a moderately reduced level of AAT.
- While it can manifest as AATD, particularly in combination with the Z allele (PiSZ), **PiSS genotype** generally confers only a mild deficiency, and liver disease is less common or severe than with PiZZ.
*PiMZ*
- This genotype involves one **M allele** (normal) and one **Z allele** (deficient).
- It is a **heterozygous carrier state** and typically results in intermediate AAT levels. While there is an increased risk for emphysema, severe liver disease is unlikely.
*PiMS*
- This genotype involves one **M allele** (normal) and one **S allele** (mildly deficient).
- This combination is usually associated with **normal or near-normal AAT levels** and individuals are typically asymptomatic, not presenting with severe liver or lung disease.
*PiSZ*
- This genotype involves one **S allele** (moderately deficient) and one **Z allele** (severely deficient).
- Individuals with this genotype have **significantly reduced AAT levels** and are at risk for both liver disease and emphysema, but the severity is generally less profound than with PiZZ. The specific genetic mutation described in the stem (lysine for glutamic acid at position 342) points directly to the Z allele, not the S allele.
Question 8: A researcher is tracing the fate of C-peptide, a product of preproinsulin cleavage. Which of the following is a true statement regarding the fate of C-peptide?
A. C-peptide exits the cells via a protein channel
B. C-peptide is further cleaved into insulin
C. C-peptide is packaged with insulin in secretory vesicles (Correct Answer)
D. C-peptide is immediately degraded by the proteasome
E. C-peptide activates an intracellular signaling cascade
Explanation: ***C-peptide is packaged with insulin in secretory vesicles***
- Preproinsulin is cleaved in the **endoplasmic reticulum** to proinsulin (signal peptide removal), which is then transported to the **Golgi apparatus**.
- In the Golgi, proinsulin is cleaved by **prohormone convertases** into **insulin** and **C-peptide**, and both are stored together in **secretory vesicles** within the pancreatic beta cells.
- Upon stimulation, both insulin and C-peptide are **co-secreted** via exocytosis in equimolar amounts, making C-peptide a useful marker of endogenous insulin secretion.
*C-peptide exits the cells via a protein channel*
- C-peptide exits the beta cells via **exocytosis** of secretory granules, not through specific protein channels.
- It is **co-secreted with insulin** when secretory vesicles fuse with the plasma membrane.
- Its presence in the bloodstream in equimolar amounts with insulin makes it an indirect measure of **insulin secretion**.
*C-peptide is further cleaved into insulin*
- **C-peptide** is a product of proinsulin cleavage, alongside insulin; it is not further processed into insulin.
- Insulin itself is composed of two **peptide chains (A and B)** linked by disulfide bonds, formed after C-peptide is removed from proinsulin.
*C-peptide is immediately degraded by the proteasome*
- C-peptide is not immediately degraded by the **proteasome** upon synthesis.
- After secretion, it circulates in the blood with a **longer half-life** than insulin (approximately 30 minutes versus 4-6 minutes), allowing it to be a useful marker of endogenous insulin production.
- Its degradation occurs primarily in the **kidney**.
*C-peptide activates an intracellular signaling cascade*
- While there is some research suggesting C-peptide may have independent **biological activity** and activate certain signaling pathways extracellularly, its primary role in the context of the insulin synthesis pathway is as a **byproduct** of proinsulin processing.
- Its clinical utility is primarily as a **biomarker** of endogenous insulin secretion, particularly useful in distinguishing between endogenous and exogenous insulin in diabetic patients.
Question 9: A 21-year-old woman comes to the physician for an annual health maintenance examination. She has no particular health concerns. Laboratory studies show:
Hemoglobin 11.2 g/dL
Mean corpuscular volume 74 μm3
Mean corpuscular hemoglobin concentration 30% Hb/cell
Red cell distribution width 14% (N=13–15)
Genetic analysis shows a point mutation in intron 1 of a gene on the short arm of chromosome 11. A process involving which of the following components is most likely affected in this patient?
A. TATA-rich nucleotide sequence
B. Transfer RNA
C. Heat shock protein 60
D. Small nuclear ribonucleoprotein (Correct Answer)
E. MicroRNA
Explanation: ***Small nuclear ribonucleoprotein***
- The patient's lab results (low 11.2 g/dL **hemoglobin**, low 74 µm3 **MCV**, and low 30% **MCHC**) indicate **microcytic, hypochromic anemia**, consistent with **thalassemia**.
- A point mutation in **intron 1** of a gene suggests a problem with **RNA splicing**, which is mediated by **small nuclear ribonucleoproteins (snRNPs)** as part of the spliceosome.
*TATA-rich nucleotide sequence*
- The **TATA box** is located in the **promoter region** of genes and is involved in the initiation of **transcription**, not splicing.
- A mutation in the TATA box would affect the **rate of transcription** or gene expression, not the processing of mRNA after transcription.
*Transfer RNA*
- **tRNA** molecules are essential for **protein translation** by carrying specific amino acids to the ribosome.
- A problem with tRNA would affect the **synthesis of proteins**, not the processing of pre-mRNA.
*Heat shock protein 60*
- **Heat shock proteins** are molecular **chaperones** involved in the proper **folding of proteins** and preventing protein aggregation.
- A defect in HSP60 would lead to misfolded proteins, not impaired mRNA splicing.
*MicroRNA*
- **MicroRNAs (miRNAs)** are small non-coding RNA molecules that regulate gene expression by **silencing mRNA** or inhibiting **translation**.
- While miRNAs regulate gene expression, they are not directly involved in the **splicing of introns** from pre-mRNA.
Question 10: An investigator is studying the activity of N-terminal peptidase in eukaryotes. Sulfur-containing amino acids are radiolabeled and isolated using 35S. During translation of a non-mitochondrial human genome, some of the radiolabeled amino acids bind to the aminoacyl, peptidyl, and exit sites of a eukaryotic ribosome but others bind only to the peptidyl and exit sites. Only the radiolabeled amino acids that do not bind to the ribosomal aminoacyl-site can be excised by the N-terminal peptidase. Which of the following best describes the anticodon sequence of the transfer RNA charged by the amino acid target of the N-terminal peptidase?
A. 5'-ACU-3'
B. 5'-UCA-3'
C. 5'-UAC-3'
D. 5'-ACA-3'
E. 5'-CAU-3' (Correct Answer)
Explanation: ***5'-CAU-3'***
- The N-terminal peptidase excises amino acids that do not bind to the ribosomal **aminoacyl-site (A-site)**, indicating they are **initiator methionines**. During translation initiation in eukaryotes, the **initiator tRNA** (Met-tRNAi carrying methionine) directly binds to the **peptidyl-site (P-site)**, bypassing the A-site.
- The **start codon** for protein synthesis is **5'-AUG-3'**. The anticodon must be **complementary and antiparallel** to this codon. When AUG is read 3'-GUA-5' in the antiparallel direction, the complementary anticodon sequence is **5'-CAU-3'**.
- This is the standard anticodon for **initiator methionine tRNA (Met-tRNAi)** in eukaryotes. After translation begins, the N-terminal methionine is often cleaved by methionine aminopeptidase (N-terminal peptidase), especially when the second amino acid is small and uncharged.
*5'-ACA-3'*
- This anticodon would pair with the codon **5'-UGU-3'** or **5'-UGC-3'**, both of which code for **cysteine**, not methionine.
- While cysteine is a sulfur-containing amino acid that was radiolabeled in this experiment, it undergoes normal translation through the A-site and is not the initiator amino acid that bypasses the A-site.
- Cysteine residues are not typically removed by N-terminal peptidase.
*5'-UCA-3'*
- This anticodon would pair with the codon **5'-AGU-3'** or **5'-AGA-3'**, which code for **serine** or **arginine**.
- Neither of these is the initiator amino acid, and they do not contain sulfur, so they would not be radiolabeled with 35S in this experiment.
*5'-UAC-3'*
- This sequence represents the **mRNA codon** (5'-UAC-3' codes for tyrosine), not an anticodon.
- The anticodon should be complementary to AUG, not identical to a tyrosine codon. This is an incorrect representation.
*5'-ACU-3'*
- This anticodon would pair with the codon **5'-AGU-3'** or **5'-AGC-3'**, which code for **serine**.
- Serine does not contain sulfur and would not be radiolabeled with 35S. It is also not the initiator amino acid that bypasses the A-site.
