Epigenetic mechanisms — MCQs

10 questions

A researcher is investigating compounds that modulate the cell cycle as possible chemotherapeutic agents against peripheral T-cell lymphoma. The researcher discovers a group of natural compounds with inhibitory activity against histone deacetylases, a class of enzymes that remove acetyl groups from the lysine residues of histones. A histone deacetylase inhibitor most likely causes which of the following?

Researchers are investigating oncogenes, specifically the KRAS gene that is associated with colon, lung, and pancreatic cancer. They have established that the gain-of-function mutation in this gene increases the chance of cancer development. They are also working to advance the research further to study tumor suppressor genes. Which of the genes below is considered a tumor suppressor gene?

A mother from rural Louisiana brings her 4-year-old son to a pediatrician. Her son is intellectually disabled, and she hopes that genetic testing will help determine the cause of her son's condition. She had previously been opposed to allowing physicians to treat her son, but his impulsive behavior and learning disabilities are making it difficult to manage his care on her own. On exam, the child has a long, thin face with a large jaw, protruding ears, and macroorchidism. The physician also hears a high-pitched holosystolic murmur at the apex of the heart that radiates to the axilla. Which of the following trinucleotide repeats is most likely affected in this individual?

A 4-year-old child presents with developmental delay, ataxia, and inappropriate laughter. The parents undergo genetic testing to determine the cause of their child's symptoms. Results show no mutations in all three family members that would cause this constellation of symptoms. Karyotyping reveals no deletions, insertions, or gene translocations. However, methylation studies demonstrate abnormal imprinting patterns at the 15q11-q13 region. Based on these findings, the child is diagnosed with Angelman syndrome. Which of the following genetic mechanisms best describes the cause of this disorder?

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 2-year-old male is referred to a geneticist for developmental delay and intellectual disability. He was hypotonic at birth and his parents are concerned that he tries to eat everything, including erasers and chalk. Physical exam is remarkable for severe obesity and hypogonadism. Genetic analysis reveals that he has one mutated allele and one normal allele at the gene of interest. Which of the following is the most likely explanation for why this patient is affected despite having a normal allele?

A 40-year-old male with Down syndrome is brought to your clinic by his mother. She reports that over the past few months he has started having difficulty managing his daily routine at his assisted-living facility and no longer seems like himself. She says that last week he wandered away from the facility and was brought back by police. Additionally, he has stopped taking his regular antiepileptic medication, and she is concerned that he might have a seizure. TSH is checked and is normal. Which of the following is most likely to be responsible for this man's current presentation?

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?

Epigenetic mechanisms US Medical PG Practice Questions and MCQs

Question 1: A researcher is investigating compounds that modulate the cell cycle as possible chemotherapeutic agents against peripheral T-cell lymphoma. The researcher discovers a group of natural compounds with inhibitory activity against histone deacetylases, a class of enzymes that remove acetyl groups from the lysine residues of histones. A histone deacetylase inhibitor most likely causes which of the following?

A. Prevention of DNA strand reannealing
B. Increased heterochromatin formation
C. Suppression of gene transcription
D. Relaxation of DNA coiling (Correct Answer)
E. Tighter coiling of DNA

Explanation: ***Relaxation of DNA coiling*** - Histone deacetylase (HDAC) inhibitors block the removal of **acetyl groups** from **histones**, leading to increased histone acetylation. - Increased acetylation **reduces the positive charge** of histones, loosening their grip on the negatively charged DNA and causing **relaxation of DNA coiling**. *Prevention of DNA strand reannealing* - This process is primarily influenced by factors affecting **hydrogen bonding** between DNA strands, such as **temperature** or **DNA denaturing agents**, not directly by histone acetylation. - DNA reannealing is the reformation of a **double helix** from single strands, a different mechanism than chromatin structure. *Increased heterochromatin formation* - **Heterochromatin** is characterized by **tightly coiled DNA** and is associated with **deacetylated histones** and gene silencing. - Increased acetylation, as caused by HDAC inhibitors, would lead to less heterochromatin and more **euchromatin**. *Suppression of gene transcription* - **Relaxation of DNA coiling** makes the DNA more accessible to transcription factors and RNA polymerase, thereby generally **promoting gene transcription**, not suppressing it. - **HDAC inhibitors** primarily promote gene expression by increasing the accessibility of DNA to the transcriptional machinery. *Tighter coiling of DNA* - **Deacetylation of histones** leads to stronger interaction between histones and DNA, resulting in **tighter coiling** and chromatin condensation. - HDAC inhibitors, by preventing deacetylation, promote the opposite effect: **DNA uncoiling** and relaxation.

Question 2: Researchers are investigating oncogenes, specifically the KRAS gene that is associated with colon, lung, and pancreatic cancer. They have established that the gain-of-function mutation in this gene increases the chance of cancer development. They are also working to advance the research further to study tumor suppressor genes. Which of the genes below is considered a tumor suppressor gene?

A. Her2/neu
B. BRAF
C. BCL-2
D. JAK2
E. Rb (Correct Answer)

Explanation: ***Rb*** - The **retinoblastoma (Rb)** gene is a classic example of a **tumor suppressor gene**. Its protein product, Rb, plays a critical role in regulating the **cell cycle** by preventing uncontrolled cell division. - When **Rb is mutated or inactivated**, cells can divide without proper checks, leading to tumor formation, particularly in cases like retinoblastoma. *Her2/neu* - **Her2/neu** (also known as ERBB2) is an **oncogene** that encodes a receptor tyrosine kinase involved in cell growth and differentiation. - Its overexpression or amplification is associated with certain cancers, notably **breast cancer**, but it is not a tumor suppressor. *BRAF* - **BRAF** is an **oncogene** that codes for a serine/threonine kinase involved in the RAS/MAPK signaling pathway, which regulates cell growth. - **Gain-of-function mutations** in BRAF are frequently found in melanoma, thyroid cancer, and colorectal cancer, promoting uncontrolled cell proliferation. *BCL-2* - **BCL-2** is an **anti-apoptotic gene**, meaning it prevents programmed cell death. While its overexpression can contribute to cancer by allowing abnormal cells to survive, it is not classified as a tumor suppressor gene. - Instead, BCL-2 is considered an **oncogene** because mutations or overexpression promote cell survival and inhibit apoptosis. *JAK2* - **JAK2** (Janus Kinase 2) is a **proto-oncogene** encoding a tyrosine kinase involved in cytokine receptor signaling, which regulates hematopoiesis. - **Gain-of-function mutations**, such as JAK2 V617F, are frequently found in **myeloproliferative neoplasms** (e.g., polycythemia vera, essential thrombocythemia, myelofibrosis), leading to uncontrolled blood cell production.

Question 3: A mother from rural Louisiana brings her 4-year-old son to a pediatrician. Her son is intellectually disabled, and she hopes that genetic testing will help determine the cause of her son's condition. She had previously been opposed to allowing physicians to treat her son, but his impulsive behavior and learning disabilities are making it difficult to manage his care on her own. On exam, the child has a long, thin face with a large jaw, protruding ears, and macroorchidism. The physician also hears a high-pitched holosystolic murmur at the apex of the heart that radiates to the axilla. Which of the following trinucleotide repeats is most likely affected in this individual?

A. GAA on chromosome 9
B. CGG on the sex chromosome X (Correct Answer)
C. CTG on chromosome 19
D. CTG on chromosome 8
E. CAG on chromosome 4

Explanation: ***CGG on the sex chromosome X*** - The constellation of **intellectual disability**, a **long, thin face with a large jaw**, **protruding ears**, and **macroorchidism** are classic features of **Fragile X syndrome**. - Fragile X syndrome is caused by an expansion of the **CGG trinucleotide repeat** in the **FMR1 gene** on the **X chromosome**. The **high-pitched holosystolic murmur at the apex radiating to the axilla** suggests **mitral valve prolapse**, which is also frequently associated with Fragile X. *GAA on chromosome 9* - This describes the **GAA trinucleotide repeat expansion** associated with **Friedreich's ataxia**, affecting the **FXN gene** on **chromosome 9**. - Friedreich's ataxia is characterized by **progressive ataxia**, **dysarthria**, and **loss of vibratory/proprioceptive sensation**, not macroorchidism or the specific facial features seen here. *CTG on chromosome 19* - This describes the **CTG trinucleotide repeat expansion** associated with **myotonic dystrophy type 1**, affecting the **DMPK gene** on **chromosome 19**. - Myotonic dystrophy is characterized by **myotonia** (delayed muscle relaxation), **muscle weakness**, and **cataracts**, which are not consistently present in this case. *CTG on chromosome 8* - While **CTG repeats** are involved in some genetic conditions, the specific association with **chromosome 8** as a cause for the described symptoms (intellectual disability, specific facial features, macroorchidism, and mitral valve prolapse) is not a common trinucleotide repeat disorder. - This option does not correspond to a recognized trinucleotide repeat disorder that presents with the given clinical picture. *CAG on chromosome 4* - This describes the **CAG trinucleotide repeat expansion** associated with **Huntington's disease**, affecting the **HTT gene** on **chromosome 4**. - Huntington's disease typically presents with **chorea**, **psychiatric symptoms**, and **dementia** later in life, not with the childhood onset intellectual disability and physical features described.

Question 4: A 4-year-old child presents with developmental delay, ataxia, and inappropriate laughter. The parents undergo genetic testing to determine the cause of their child's symptoms. Results show no mutations in all three family members that would cause this constellation of symptoms. Karyotyping reveals no deletions, insertions, or gene translocations. However, methylation studies demonstrate abnormal imprinting patterns at the 15q11-q13 region. Based on these findings, the child is diagnosed with Angelman syndrome. Which of the following genetic mechanisms best describes the cause of this disorder?

A. Variable expressivity
B. Uniparental disomy
C. Incomplete penetrance
D. Anticipation
E. Imprinting defect (Correct Answer)

Explanation: ***Imprinting defect*** - **Angelman syndrome** in this case results from a **primary imprinting defect** at the 15q11-q13 region, causing abnormal methylation patterns without deletions, mutations, or uniparental disomy. - The key diagnostic findings are: **normal karyotype** (ruling out deletions), **no mutations in family members** (ruling out UBE3A mutations), and **abnormal methylation studies** demonstrating the imprinting center defect. - An **imprinting defect** refers to an error in the establishment or maintenance of methylation patterns at imprinted genes, which in this case leads to silencing of the maternally inherited *UBE3A* gene expression. - This mechanism accounts for approximately **3-5%** of Angelman syndrome cases and is diagnosed specifically through methylation studies when other causes are excluded. *Uniparental disomy* - While **paternal uniparental disomy (UPD)** of chromosome 15 can cause Angelman syndrome, it would typically be detected through genetic testing and represents a different mechanism. - UPD involves inheriting **two copies of a chromosome from one parent** and none from the other, which **leads to** an imprinting defect as a secondary consequence. - The question stem emphasizes that methylation studies were the key finding after ruling out mutations and karyotype abnormalities, suggesting a **primary imprinting center defect** rather than UPD. *Variable expressivity* - This refers to individuals with the **same genotype** exhibiting **different phenotypes** ranging from mild to severe. - While Angelman syndrome can show variable severity, this term describes **phenotypic variation**, not the underlying genetic mechanism causing the disorder. *Incomplete penetrance* - This occurs when only a **portion of individuals** with a particular genotype actually **express the associated phenotype**. - In Angelman syndrome due to imprinting defects, the condition is fully penetrant when the genetic abnormality is present, so this is not the relevant mechanism. *Anticipation* - **Anticipation** describes symptoms becoming **more severe** and/or appearing at **earlier ages** in succeeding generations. - This phenomenon is associated with **trinucleotide repeat expansion disorders** (e.g., Huntington disease, myotonic dystrophy), not imprinting disorders like Angelman syndrome.

Question 5: 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 6: A 2-year-old male is referred to a geneticist for developmental delay and intellectual disability. He was hypotonic at birth and his parents are concerned that he tries to eat everything, including erasers and chalk. Physical exam is remarkable for severe obesity and hypogonadism. Genetic analysis reveals that he has one mutated allele and one normal allele at the gene of interest. Which of the following is the most likely explanation for why this patient is affected despite having a normal allele?

A. Uniparental disomy
B. X-linked inheritance pattern
C. Locus heterogeneity
D. Autosomal dominant inheritance pattern
E. Imprinting (Correct Answer)

Explanation: ***Imprinting*** - The constellation of **hypotonia**, **developmental delay**, **hyperphagia** (eating everything), **obesity**, and **hypogonadism** is characteristic of **Prader-Willi syndrome**. - **Prader-Willi syndrome** is typically caused by the **loss of function of specific paternally inherited genes** on chromosome 15 (15q11-q13), a mechanism known as **genomic imprinting**, where only one parent's copy of a gene is active. - In this case, the patient has one mutated (paternal) allele and one normal but imprinted/silenced (maternal) allele, explaining why disease occurs despite the presence of a structurally normal allele. *Uniparental disomy* - **Maternal uniparental disomy** of chromosome 15 can also cause Prader-Willi syndrome if both copies of the chromosome are inherited from the mother, leading to no functional paternal genes. - However, the question explicitly states "one mutated allele and one normal allele," which describes a **deletion or mutation of the paternal copy** rather than UPD (where both chromosomes would be from the mother and appear structurally normal on standard testing). - The question asks for the *mechanism* explaining why the patient is affected despite having a normal allele - the answer is **imprinting**, which is the underlying mechanism in both deletion and UPD cases. *X-linked inheritance pattern* - **X-linked inheritance** typically affects males more severely with different phenotypic presentations and does not explain the combination of a normal and mutated allele resulting in the specific Prader-Willi phenotype. - The described syndrome (Prader-Willi) is associated with **chromosome 15** abnormalities, not the X chromosome. *Locus heterogeneity* - **Locus heterogeneity** describes a single disease phenotype caused by mutations in different gene loci, which is not relevant to explaining why a single mutated allele results in disease in the context of Prader-Willi syndrome. - The clinical picture strongly points to a specific genetic disorder with a known genetic cause on a single locus. *Autosomal dominant inheritance pattern* - While **autosomal dominant disorders** manifest with one mutated allele, the specific combination of symptoms with obesity, hypogonadism, and hyperphagia in this scenario is not typical for a general autosomal dominant pattern. - The question describes features characteristic of **Prader-Willi syndrome**, which is primarily an **imprinting disorder**, rather than a straightforward autosomal dominant condition where the presence of one normal allele would be expected to provide sufficient function in the absence of imprinting.

Question 7: A 40-year-old male with Down syndrome is brought to your clinic by his mother. She reports that over the past few months he has started having difficulty managing his daily routine at his assisted-living facility and no longer seems like himself. She says that last week he wandered away from the facility and was brought back by police. Additionally, he has stopped taking his regular antiepileptic medication, and she is concerned that he might have a seizure. TSH is checked and is normal. Which of the following is most likely to be responsible for this man's current presentation?

A. Premature degradation of a protein
B. Abnormal protein metabolism (Correct Answer)
C. Hormone deficiency
D. Nutritional deficiency
E. Expansion of trinucleotide repeats

Explanation: ***Abnormal protein metabolism*** - Individuals with **Down syndrome (trisomy 21)** are at a significantly higher risk of developing early-onset **Alzheimer's disease** due to an extra copy of the **APP gene** on chromosome 21. - **Amyloid Precursor Protein (APP)** metabolism is altered, leading to excessive production and aggregation of **beta-amyloid plaques**, which is a hallmark of Alzheimer's pathology and causes the cognitive decline observed. *Premature degradation of a protein* - This option points to a general protein issue but doesn't specifically address the **pathophysiology of Alzheimer's** in Down syndrome, which is primarily about abnormal production and aggregation, not premature degradation. - While protein degradation pathways are involved in neurodegenerative diseases, the core problem in Alzheimer's relates to the **accumulation of misfolded proteins**. *Hormone deficiency* - The patient's **TSH is normal**, ruling out **hypothyroidism**, which can cause cognitive changes and is more common in Down syndrome. - While other hormonal imbalances could affect cognitive function, the significant and progressive decline described is more consistent with a **neurodegenerative process** than a general hormone deficiency. *Nutritional deficiency* - While nutritional deficiencies can certainly impact cognitive function and behavior, the patient's age, underlying condition (**Down syndrome**), and the specific pattern of progressive decline strongly suggest a **neurodegenerative cause** rather than a primary nutritional deficiency. - There is no specific information in the vignette to point toward a nutritional deficiency. *Expansion of trinucleotide repeats* - **Trinucleotide repeat expansion disorders** (e.g., Huntington's disease, Fragile X syndrome) cause a variety of neurological and psychiatric symptoms. - However, the patient's presentation in the context of Down syndrome is characteristic of **early-onset Alzheimer's disease**, which is not caused by trinucleotide repeat expansion.

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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