A newborn is found to have cystic fibrosis during routine newborn screening. The parents, both biochemists, are curious about the biochemical basis of their newborn's condition. The pediatrician explains that the mutation causing cystic fibrosis affects the CFTR gene which codes for the CFTR channel. Which of the following correctly describes the pathogenesis of the most common CFTR mutation?
Q12
A group of researchers is studying molecules and DNA segments that are critical for important cellular processes in eukaryotic cells. They have identified a region that is located about 28 bases upstream of the 5’ coding region. This region promotes the initiation of transcription by binding with transcription factors. Which of the following regions have these researchers most likely identified?
Q13
In translation, the wobble phenomenon is best illustrated by the fact that:
Q14
An investigator studying protein synthesis in human stem cells isolates tRNA molecules bound to mRNA molecules. The isolated tRNA molecules have inosine in the 5' position of the anticodon; of these, some are bound to adenine, some to cytosine, and some to uracil at the 3' position of the mRNA codon. Which of the following properties of the genetic code is best illustrated by this finding?
Q15
A 4-year-old boy is brought to the physician because of frequent respiratory tract infections and chronic diarrhea. His stools are bulky and greasy, and he has around 8 bowel movements daily. He is at the 10th percentile for height and 25th percentile for weight. Chest examination shows intercostal retractions along with diffuse wheezing and expiratory rhonchi. Which of the following is the most likely cause of his condition?
Q16
An investigator is studying the function of the endoplasmic reticulum in genetically modified lymphocytes. A gene is removed that facilitates the binding of ribosomes to the endoplasmic reticulum. Which of the following processes is most likely to be impaired as a result of this genetic modification?
Q17
Collagen is a very critical structural protein in many of our connective tissues. Defects in collagen produce diseases such as Ehlers-Danlos syndrome, where there is a defective lysyl hydroxylase gene, or osteogenesis imperfecta, where there is a defect in the production of type I collagen. Which of the following represents the basic repeating tripeptide of collagen?
Q18
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?
Q19
A 25-year-old female comes to the clinic complaining of fatigue and palpitations. She has been undergoing immense stress from her thesis defense and has been extremely tired. The patient denies any weight loss, diarrhea, cold/heat intolerance. TSH was within normal limits. She reports a family history of "blood disease" and was later confirmed positive for β-thalassemia minor. It is believed that abnormal splicing of the beta globin gene results in β-thalassemia. What is removed during this process that allows RNA to be significantly shorter than DNA?
Q20
An 11-month-old boy is brought to a pediatrician by his parents with a recurrent cough, which he has had since the age of 2 months. He has required 3 hospitalizations for severe wheezing episodes. His mother also mentions that he often has diarrhea. The boy’s detailed history reveals that he required hospitalization for meconium ileus during the neonatal period. Upon physical examination, his temperature is 37.0°C (98.6ºF), pulse rate is 104/min, respiratory rate is 40/min, and blood pressure is 55/33 mm Hg. An examination of the boy’s respiratory system reveals the presence of bilateral wheezing and scattered crepitations. An examination of his cardiovascular system does not reveal any abnormality. His length is 67.3 cm (26.5 in) and weight is 15 kg (33 lbs). His sweat chloride level is 74 mmol/L. His genetic evaluation confirms that he has an autosomal recessive disorder resulting in a dysfunctional membrane-bound protein. Which of the following best describes the mechanism associated with the most common mutation that causes this disorder?
Transcription/translation US Medical PG Practice Questions and MCQs
Question 11: A newborn is found to have cystic fibrosis during routine newborn screening. The parents, both biochemists, are curious about the biochemical basis of their newborn's condition. The pediatrician explains that the mutation causing cystic fibrosis affects the CFTR gene which codes for the CFTR channel. Which of the following correctly describes the pathogenesis of the most common CFTR mutation?
A. Insufficient CFTR channel production (Correct Answer)
B. Excess CFTR channel production
C. Defective post-translational hydroxylation of the CFTR channel
D. Defective post-translational phosphorylation of the CFTR channel
E. Defective post-translational glycosylation of the CFTR channel
Explanation: ***Insufficient CFTR channel production***
- The most common CFTR gene mutation, **delta F508**, leads to the production of a misfolded CFTR protein that is retained in the endoplasmic reticulum and subsequently **degraded**, significantly reducing the number of functional channels reaching the cell surface.
- This **protein misfolding** and degradation results in insufficient delivery of CFTR channels to the apical membrane of epithelial cells, causing the characteristic electrolyte and fluid transport defects.
*Excess CFTR channel production*
- Cystic fibrosis is characterized by a *deficiency* in functional CFTR channels, not an excess.
- An overproduction of functional CFTR channels would lead to excessive chloride transport, which is the opposite of what is observed in CF.
*Defective post-translational hydroxylation of the CFTR channel*
- **Hydroxylation** is a common post-translational modification, but it is not the primary defect responsible for the pathogenesis of the most common CFTR mutation.
- Defects in hydroxylation are more typically associated with conditions like **scurvy** (collagen hydroxylation) or issues with protein stability mediated by hydroxylases.
*Defective post-translational phosphorylation of the CFTR channel*
- While CFTR channel activity is regulated by **phosphorylation** by protein kinase A, the primary defect in **delta F508 CFTR** is not a failure of phosphorylation itself.
- The problem is that the misfolded protein never reaches the cell surface to be properly phosphorylated and activated, making phosphorylation a secondary issue rather than the root cause of the channel deficiency.
*Defective post-translational glycosylation of the CFTR channel*
- **Glycosylation** is an important aspect of CFTR protein maturation, occurring in the endoplasmic reticulum and Golgi.
- In the case of the **delta F508 mutation**, the misfolded protein is largely *prevented* from reaching the Golgi, where it would undergo complex glycosylation, so the defect is more fundamental (misfolding and degradation) rather than an error in the glycosylation process itself.
Question 12: A group of researchers is studying molecules and DNA segments that are critical for important cellular processes in eukaryotic cells. They have identified a region that is located about 28 bases upstream of the 5’ coding region. This region promotes the initiation of transcription by binding with transcription factors. Which of the following regions have these researchers most likely identified?
A. TATA Box (Correct Answer)
B. RNA polymerase II
C. Small nuclear ribonucleoprotein (SnRNPs)
D. DNA methyltransferase
E. CAAT Box
Explanation: ***TATA Box***
- The **TATA box** is a core promoter element found in eukaryotic genes, typically located **25-35 base pairs upstream** of the transcription start site.
- It plays a crucial role in initiating transcription by serving as a binding site for **transcription factors**, which in turn recruit **RNA polymerase II**.
*RNA polymerase II*
- **RNA polymerase II** is the enzyme responsible for transcribing protein-coding genes into mRNA.
- While essential for transcription, it is an enzyme that binds to the promoter region (which includes the TATA box), rather than a regulatory DNA sequence itself.
*Small nuclear ribonucleoprotein (SnRNPs)*
- **SnRNPs** are components of the spliceosome, involved in the **splicing of pre-mRNA** to remove introns.
- They are involved in post-transcriptional modification, not in the initiation of transcription.
*DNA methyltransferase*
- **DNA methyltransferase** is an enzyme involved in **DNA methylation**, a process that typically represses gene expression.
- This enzyme modifies DNA, but it is not a DNA region that promotes transcription initiation.
*CAAT Box*
- The **CAAT box** is another common promoter element in eukaryotes, usually located further **upstream (70-80 base pairs)** from the transcription start site.
- While it also binds transcription factors and influences transcription initiation, its location is generally *more distant* than the 28 bases upstream described, making the TATA box a more accurate fit for the given distance.
Question 13: 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 14: An investigator studying protein synthesis in human stem cells isolates tRNA molecules bound to mRNA molecules. The isolated tRNA molecules have inosine in the 5' position of the anticodon; of these, some are bound to adenine, some to cytosine, and some to uracil at the 3' position of the mRNA codon. Which of the following properties of the genetic code is best illustrated by this finding?
A. Unambiguity
B. Non-overlapping
C. Degeneracy (Correct Answer)
D. Specificity of the start codon
E. Specificity of stop codons
Explanation: ***Degeneracy***
- The finding that a single tRNA anticodon (with **inosine** at the 5' position) can bind to multiple different mRNA codons (ending in **adenine, cytosine, or uracil**) illustrates the concept of **degeneracy** in the genetic code.
- This **wobble hypothesis** allows fewer tRNAs to recognize more than one codon for a given amino acid, meaning multiple codons can code for the same amino acid.
*Unambiguity*
- The genetic code is unambiguous, meaning that each codon specifies **only one specific amino acid** (or a stop signal) and never two different amino acids.
- This finding, however, shows one tRNA recognizing multiple codons, not one codon coding for multiple amino acids.
*Non-overlapping*
- The **non-overlapping** nature of the genetic code means that each nucleotide in an mRNA sequence is read only once as part of a single codon, without sharing nucleotides between adjacent codons.
- This concept describes how codons are read sequentially, not the flexibility of codon-anticodon pairing.
*Specificity of the start codon*
- The **start codon (AUG)** specifically initiates translation, coding for methionine, and signals the beginning of a polypeptide chain.
- This finding relates to the wobble pairing at the 3' end of the codon, not the initiation of translation.
*Specificity of stop codons*
- **Stop codons (UAA, UAG, UGA)** specifically signal the termination of translation without coding for any amino acid.
- This finding describes the flexibility of codon-anticodon pairing, not the distinct function of termination codons.
Question 15: A 4-year-old boy is brought to the physician because of frequent respiratory tract infections and chronic diarrhea. His stools are bulky and greasy, and he has around 8 bowel movements daily. He is at the 10th percentile for height and 25th percentile for weight. Chest examination shows intercostal retractions along with diffuse wheezing and expiratory rhonchi. Which of the following is the most likely cause of his condition?
A. Increased frequency of trinucleotide repeats
B. Altered configuration of a protease inhibitor
C. Intracellular retention of misfolded proteins (Correct Answer)
D. Defective ciliary protein function
E. Frameshift mutation of muscle-anchoring proteins
Explanation: ***Intracellular retention of misfolded proteins***
- The combination of **recurrent respiratory infections**, **chronic diarrhea with greasy stools**, and **failure to thrive (low height/weight percentiles)** in a young child strongly suggests **cystic fibrosis (CF)**.
- CF is caused by mutations in the **CFTR gene**, leading to the production of a **misfolded CFTR protein** that is retained in the endoplasmic reticulum and subsequently degraded, preventing it from reaching the cell membrane. This results in defective chloride transport.
*Increased frequency of trinucleotide repeats*
- This is characteristic of disorders like **Fragile X syndrome**, **Huntington's disease**, and **Friedreich's ataxia**.
- These conditions present with neurological symptoms and intellectual disability, not the respiratory and gastrointestinal manifestations seen in this patient.
*Altered configuration of a protease inhibitor*
- This describes **alpha-1 antitrypsin deficiency**, which primarily causes **emphysema** and **liver disease (cirrhosis)**.
- While it can present with respiratory symptoms, chronic greasy stools and failure to thrive are not typical features.
*Defective ciliary protein function*
- This is the underlying cause of **primary ciliary dyskinesia (PCD)**, also known as **Kartagener syndrome** when associated with situs inversus.
- PCD causes recurrent respiratory infections and bronchiole dilatation, but **chronic diarrhea with bulky, greasy stools** and **failure to thrive** are not characteristic.
*Frameshift mutation of muscle-anchoring proteins*
- This type of mutation is associated with various **muscular dystrophies**, such as **Duchenne muscular dystrophy**, affecting muscle function.
- These disorders typically manifest with progressive muscle weakness and do not explain the respiratory and gastrointestinal issues described.
Question 16: An investigator is studying the function of the endoplasmic reticulum in genetically modified lymphocytes. A gene is removed that facilitates the binding of ribosomes to the endoplasmic reticulum. Which of the following processes is most likely to be impaired as a result of this genetic modification?
A. Production of secretory proteins (Correct Answer)
B. Neutralization of toxins
C. Ubiquitination of proteins
D. α-Oxidation of fatty acids
E. Synthesis of ketone bodies
Explanation: ***Production of secretory proteins***
- Ribosomes bound to the **rough endoplasmic reticulum (RER)** are responsible for synthesizing proteins destined for secretion, insertion into membranes, or delivery to organelles like lysosomes.
- If ribosomes cannot bind to the ER, these proteins will be synthesized in the **cytosol** and lack the proper signals and processing for their intended destination and function.
*Neutralization of toxins*
- The **smooth endoplasmic reticulum (SER)**, not the RER, is primarily involved in **detoxification** processes, particularly drug metabolism and neutralization of toxins.
- This function relies on enzymes embedded within the SER membrane and is largely independent of ribosome binding.
*Ubiquitination of proteins*
- **Ubiquitination** is a post-translational modification that tags proteins for degradation by the **proteasome** or for trafficking to specific cellular compartments.
- This process occurs primarily in the **cytosol** and does not directly rely on ribosome binding to the ER for protein synthesis.
*α-Oxidation of fatty acids*
- **α-oxidation of fatty acids** is a metabolic pathway that occurs primarily in the **peroxisomes**.
- It is distinct from protein synthesis on the ER and would not be directly impacted by the inability of ribosomes to bind to the ER.
*Synthesis of ketone bodies*
- The **synthesis of ketone bodies** (ketogenesis) primarily occurs in the **mitochondria** of liver cells.
- This metabolic pathway is not directly dependent on ribosome binding to the endoplasmic reticulum for its function.
Question 17: Collagen is a very critical structural protein in many of our connective tissues. Defects in collagen produce diseases such as Ehlers-Danlos syndrome, where there is a defective lysyl hydroxylase gene, or osteogenesis imperfecta, where there is a defect in the production of type I collagen. Which of the following represents the basic repeating tripeptide of collagen?
A. Gly-X-Y (Correct Answer)
B. Asp-X-Y
C. Met-X-Y
D. Ser-X-Y
E. Glu-X-Y
Explanation: ***Gly-X-Y***
- The **basic repeating tripeptide unit of collagen** is **Glycine-X-Y**, where X and Y are often **proline** and **hydroxyproline**, respectively.
- **Glycine** is essential at every third position because its small side chain allows for the tight packing of the **collagen triple helix**.
*Asp-X-Y*
- **Aspartate (Asp)** is an **acidic amino acid** and is not typically found at the first position of the repeating tripeptide unit of collagen.
- Its bulky side chain would hinder the tight coiling of the **collagen helix**.
*Met-X-Y*
- **Methionine (Met)** is a **hydrophobic amino acid** and, while important in other proteins, it does not occupy the critical first position in the repeating collagen tripeptide.
- The unique structural requirements of collagen favor **glycine** at this position for optimal packing.
*Ser-X-Y*
- **Serine (Ser)** is a **polar, uncharged amino acid** and, like aspartate and methionine, is not the primary amino acid found at the first position of the repeating collagen tripeptide.
- The small size of **glycine** is crucial for collagen's characteristic triple helix.
*Glu-X-Y*
- **Glutamate (Glu)** is another **acidic amino acid** that is not typically found at the first position of the repeating tripeptide in collagen.
- Large or charged amino acids at this position would destabilize the **collagen secondary structure**.
Question 18: 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 19: A 25-year-old female comes to the clinic complaining of fatigue and palpitations. She has been undergoing immense stress from her thesis defense and has been extremely tired. The patient denies any weight loss, diarrhea, cold/heat intolerance. TSH was within normal limits. She reports a family history of "blood disease" and was later confirmed positive for β-thalassemia minor. It is believed that abnormal splicing of the beta globin gene results in β-thalassemia. What is removed during this process that allows RNA to be significantly shorter than DNA?
A. 3'-poly(A) tail
B. Exons
C. Introns (Correct Answer)
D. microRNAs
E. snRNPs
Explanation: **Introns**
- **Introns** are non-coding regions within a gene that are removed from the pre-mRNA transcript during **splicing**.
- This removal and the subsequent ligation of exons lead to a mature mRNA molecule that is significantly shorter than the initial DNA template.
*3'-poly(A) tail*
- The **3'-poly(A) tail** is an addition to the 3' end of the mRNA molecule, not a removed segment during splicing, and it provides stability and aids in translation.
- While it contributes to mRNA processing, its addition does not involve removing existing sequences to shorten the transcript.
*Exons*
- **Exons** are the coding regions of a gene that are retained and ligated together to form the mature mRNA, which is then translated into protein.
- If exons were removed, the resulting protein would be truncated or non-functional, and the mRNA would not contain the necessary genetic information.
*microRNAs*
- **MicroRNAs (miRNAs)** are small non-coding RNA molecules that regulate gene expression by inhibiting translation or promoting mRNA degradation.
- They are not part of the pre-mRNA transcript that is processed into mRNA; rather, they are distinct regulatory molecules.
*snRNPs*
- **Small nuclear ribonucleoproteins (snRNPs)** are components of the spliceosome, the molecular machine responsible for carrying out splicing.
- They are involved in the process of intron removal but are not themselves removed from the RNA; they are catalytic machinery.
Question 20: An 11-month-old boy is brought to a pediatrician by his parents with a recurrent cough, which he has had since the age of 2 months. He has required 3 hospitalizations for severe wheezing episodes. His mother also mentions that he often has diarrhea. The boy’s detailed history reveals that he required hospitalization for meconium ileus during the neonatal period. Upon physical examination, his temperature is 37.0°C (98.6ºF), pulse rate is 104/min, respiratory rate is 40/min, and blood pressure is 55/33 mm Hg. An examination of the boy’s respiratory system reveals the presence of bilateral wheezing and scattered crepitations. An examination of his cardiovascular system does not reveal any abnormality. His length is 67.3 cm (26.5 in) and weight is 15 kg (33 lbs). His sweat chloride level is 74 mmol/L. His genetic evaluation confirms that he has an autosomal recessive disorder resulting in a dysfunctional membrane-bound protein. Which of the following best describes the mechanism associated with the most common mutation that causes this disorder?
A. Decreased chloride transport through the protein
B. Disordered regulation of the protein
C. Decreased transcription of the protein due to splicing defect
D. Complete absence of the protein
E. Defective maturation and early degradation of the protein (Correct Answer)
Explanation: ***Defective maturation and early degradation of the protein***
- The clinical picture (recurrent cough, wheezing, diarrhea, meconium ileus, elevated sweat chloride, autosomal recessive inheritance) strongly points to **cystic fibrosis (CF)**. The most common mutation in CF is **F508del**, which leads to misfolding of the **CFTR protein**, causing retention in the endoplasmic reticulum and subsequent degradation before reaching the cell membrane.
- This **defective processing and early degradation** result in a significant reduction or absence of functional CFTR protein at the cell surface, leading to impaired chloride transport.
*Decreased chloride transport through the protein*
- While **decreased chloride transport** is the ultimate functional consequence of cystic fibrosis, it is not the direct mechanism associated with the **F508del mutation's impact** on the CFTR protein itself.
- This option describes the **physiological result** of the protein defect, not the cellular/molecular mechanism of the most common mutation.
*Disordered regulation of the protein*
- **Disordered regulation** could be a potential mechanism for some CFTR mutations (Class IV mutations), affecting how the channel opens and closes or responds to signaling.
- However, for the **F508del mutation** (Class II mutation), the primary issue is the **lack of properly localized protein** due to misfolding and degradation, rather than a problem with the regulation or gating of the protein once it reaches the membrane.
*Decreased transcription of the protein due to splicing defect*
- **Decreased transcription** or **splicing defects** (Class I and V mutations) would result in reduced mRNA levels or incorrectly formed mRNA, leading to less protein synthesis.
- The **F508del mutation** involves a deletion of three nucleotides in exon 10, leading to a missing phenylalanine at position 508. Importantly, **transcription and splicing occur normally**; the mRNA is produced correctly. The problem arises at the **post-translational level** with protein folding, not at the transcriptional or splicing level.
*Complete absence of the protein*
- While functional CFTR protein is largely absent at the cell surface in F508del, the protein is **initially synthesized** in the endoplasmic reticulum.
- The problem is its **misfolding and rapid degradation**, preventing it from reaching the membrane, rather than a complete failure of protein synthesis from the outset (which would be seen in nonsense or frameshift mutations causing Class I defects).
