Types of DNA damage — MCQs

10 questions

A 23-year-old woman is brought to the emergency department 30 minutes after stepping on a piece of broken glass. Physical examination shows a 3-cm, ragged laceration on the plantar aspect of the left foot. The physician uses hydrogen peroxide to clean the wound. Which of the following is the most likely mechanism of action of this disinfectant?

A 17-year-old patient presents to the emergency department with left wrist pain after falling off of his bike and landing on his left hand. On physical exam the thenar eminence is red, swollen, and tender to palpation, so a radiograph is ordered. The patient is worried because he learned in biology class that radiography can cause cancer through damaging DNA but the physician reassures him that radiographs give a very minor dose of radiation. What is the most common mechanism by which ionizing radiation damages DNA?

A 3-year-old male child is found to have a disease involving DNA repair. Specifically, he is found to have a defect in the endonucleases involved in the nucleotide excision repair of pyrimidine dimers. Which of the following is a unique late-stage complication of this child's disease?

While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?

A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?

A 5-year-old girl is brought to the physician by her mother because of a 1-month history of a painful ulcer on her face. She has developed painful sunburns in the past with minimal UV exposure. Examination of the skin shows a 2-cm ulcerated nodule on the left cheek. There are scaly, hyperpigmented papules and plaques over the skin of the entire body. Ophthalmologic examination shows decreased visual acuity, clouded corneas, and limbal injection. Examination of a biopsy specimen from the facial lesion shows poorly-differentiated squamous cell carcinoma. Impairment of which of the following proteins is the most likely cause of this patient's condition?

An 84-year-old man comes to the emergency department because of lower back pain and lower extremity weakness for 3 weeks. Over the past week, he has also found it increasingly difficult to urinate. He has a history of prostate cancer, for which he underwent radical prostatectomy 8 years ago. His prostate-specific antigen (PSA) level was undetectable until a routine follow-up visit last year, when it began to increase from 0.8 ng/mL to its present value of 64.3 ng/mL (N < 4). An MRI of the spine shows infiltrative vertebral lesions with a collapse of the L5 vertebral body, resulting in cord compression at L4–L5. The patient receives one dose of intravenous dexamethasone and subsequently undergoes external beam radiation. Which of the following cellular changes is most likely to occur as a result of this treatment?

DNA replication is a highly complex process where replication occurs on both strands of DNA. On the leading strand of DNA, replication occurs uninterrupted, but on the lagging strand, replication is interrupted and occurs in fragments called Okazaki fragments. These fragments need to be joined, which of the following enzymes is involved in the penultimate step before ligation can occur?

A mutant stem cell was created by using an inducible RNAi system, such that when doxycycline is added, the siRNA targeting DNA helicase is expressed, effectively knocking down the gene for DNA helicase. Which of the following will occur during DNA replication?

Q10

A 38-year-old woman applies a PABA sunscreen to her skin before going to the beach. Which type(s) of ultraviolet light will it protect her against?

Types of DNA damage US Medical PG Practice Questions and MCQs

Question 1: A 23-year-old woman is brought to the emergency department 30 minutes after stepping on a piece of broken glass. Physical examination shows a 3-cm, ragged laceration on the plantar aspect of the left foot. The physician uses hydrogen peroxide to clean the wound. Which of the following is the most likely mechanism of action of this disinfectant?

A. Formation of free radicals (Correct Answer)
B. Intercalation of DNA
C. Crosslinking of proteins
D. Halogenation of nucleic acids
E. Congealing of cytoplasm

Explanation: ***Formation of free radicals*** - **Hydrogen peroxide** acts as an **oxidizing agent**, generating highly reactive **oxygen-free radicals** (e.g., superoxide, hydroxyl radicals) that damage microbial cellular components. - This **oxidative damage** disrupts proteins, lipids, and nucleic acids, leading to bacterial and viral cell death. *Intercalation of DNA* - This mechanism is characteristic of certain **chemotherapeutic agents** (e.g., doxorubicin, ethidium bromide) and some **antimicrobials**, which insert themselves between DNA base pairs, disrupting replication and transcription. - Hydrogen peroxide does not typically target DNA in this manner for its disinfectant action. *Crosslinking of proteins* - This mechanism is characteristic of **aldehydes** like **formaldehyde** and **glutaraldehyde**, which form covalent bonds between amino groups of proteins, denaturing them and disrupting cellular function. - While hydrogen peroxide can modify proteins, its primary disinfectant action is not through widespread protein crosslinking. *Halogenation of nucleic acids* - This mechanism is primarily associated with **halogens** such as **chlorine** and **iodine**, which react with nucleic acids to form halogenated compounds, thereby inactivating them. - Hydrogen peroxide, while an oxidizer, does not lead to halogenation as its primary mode of action. *Congealing of cytoplasm* - This mechanism, which refers to the coagulation or solidification of cellular contents, is typical of **alcohols** (e.g., ethanol, isopropanol) and some **heavy metal salts** that denature proteins and lipids, leading to cell lysis. - Hydrogen peroxide's action is more specific to oxidative damage rather than general cytoplasmic congealing.

Question 2: A 17-year-old patient presents to the emergency department with left wrist pain after falling off of his bike and landing on his left hand. On physical exam the thenar eminence is red, swollen, and tender to palpation, so a radiograph is ordered. The patient is worried because he learned in biology class that radiography can cause cancer through damaging DNA but the physician reassures him that radiographs give a very minor dose of radiation. What is the most common mechanism by which ionizing radiation damages DNA?

A. Strand breakage (Correct Answer)
B. Thymidine dimer formation
C. Microsatellite instability
D. Cyclobutane pyrimidine dimer formation
E. Cytosine deamination

Explanation: ***Strand breakage*** - Ionizing radiation, such as X-rays, directly or indirectly causes **breaks in the phosphodiester backbone** of DNA, resulting in single or double-strand breaks. - **Double-strand breaks** are particularly dangerous as they are difficult to repair and can lead to chromosomal rearrangements and cell death or malignant transformation. *Thymidine dimer formation* - This is primarily caused by **ultraviolet (UV) radiation**, not ionizing radiation like X-rays. - **UV radiation** causes covalent bonds between adjacent pyrimidine bases, particularly thymine, leading to the formation of thymine dimers. *Microsatellite instability* - This is a hallmark of defects in **DNA mismatch repair pathways**, often associated with hereditary disorders like Lynch syndrome or certain sporadic cancers. - It involves changes in the length of **microsatellites** (short, repetitive DNA sequences) due to insertion or deletion errors, not direct radiation damage. *Cyclobutane pyrimidine dimer formation* - Similar to thymidine dimers, **cyclobutane pyrimidine dimers (CPDs)** are the most common photoproducts formed in DNA after exposure to **UV radiation**. - These dimers distort the DNA helix and interfere with replication and transcription, but are not characteristic of ionizing radiation damage. *Cytosine deamination* - This is a spontaneous chemical reaction where a **cytosine base (C)** loses its amino group and is converted to **uracil (U)**. - It is a common endogenous DNA lesion that can lead to C-to-T transition mutations if not repaired, but it is not directly induced by ionizing radiation.

Question 3: A 3-year-old male child is found to have a disease involving DNA repair. Specifically, he is found to have a defect in the endonucleases involved in the nucleotide excision repair of pyrimidine dimers. Which of the following is a unique late-stage complication of this child's disease?

A. Telangiectasia
B. Colorectal cancer
C. Malignant melanoma (Correct Answer)
D. Lymphomas
E. Endometrial cancer

Explanation: **Malignant melanoma** - The described condition is **xeroderma pigmentosum**, an autosomal recessive disorder characterized by a defect in **nucleotide excision repair (NER)**, specifically the inability to remove **pyrimidine dimers** caused by **UV radiation**. - This severely impaired DNA repair leads to an extreme predisposition to **UV-induced skin cancers**, including basal cell carcinomas, squamous cell carcinomas, and, most aggressively, **malignant melanoma**, which is a unique and life-threatening late-stage complication. *Telangiectasia* - **Telangiectasias** are dilated small blood vessels that appear on the skin or mucous membranes and can be associated with various conditions. - While skin abnormalities are prevalent in xeroderma pigmentosum due to sun damage, **melanoma** is a more specific and severe late-stage complication directly resulting from the DNA repair defect. *Colorectal cancer* - **Colorectal cancer** is typically associated with other DNA repair defects, such as those in the **mismatch repair system**, as seen in conditions like **Lynch syndrome**. - It is not a primary or most significant late-stage complication of xeroderma pigmentosum, which is primarily characterized by skin cancers. *Lymphomas* - **Lymphomas** are cancers of the lymphatic system, often linked to immune deficiencies or specific genetic translocations. - While individuals with genetic syndromes can have increased cancer risks, **lymphoma** is not the hallmark late-stage complication of xeroderma pigmentosum; skin cancers are the predominant concern. *Endometrial cancer* - **Endometrial cancer** is a gynecological cancer often associated with hormonal factors or genetic predispositions like Lynch syndrome, which involves mismatch repair defects. - This type of cancer is not a characteristic or unique late-stage complication of xeroderma pigmentosum, whose pathology is centered on **UV-induced DNA damage** and subsequent skin malignancies.

Question 4: While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?

A. Nucleotide excision repair
B. Nonhomologous end joining repair
C. Homologous recombination
D. Mismatch repair
E. Base excision repair (Correct Answer)

Explanation: **Base excision repair** - This mechanism is specifically involved in correcting **single-base DNA damage** or **modified bases**, such as **apurinic/apyrimidinic (AP) sites**. - It involves removing the damaged base by a **DNA glycosylase**, creating an AP site, which is then processed by an **AP endonuclease** to cleave the phosphodiester backbone, followed by DNA polymerase and ligase. *Nucleotide excision repair* - Primarily repairs **bulky DNA lesions**, such as **thymine dimers** caused by UV radiation, or damage from chemical adducts that distort the DNA helix. - It involves excising a larger oligonucleotide containing the damage, not just a single base. *Nonhomologous end joining repair* - This pathway is used to repair **double-strand DNA breaks**, where both strands of the DNA molecule are broken. - It is a "quick-and-dirty" repair mechanism that ligates the broken ends together, often leading to small insertions or deletions. *Homologous recombination* - A repair mechanism for **double-strand DNA breaks** that uses a homologous DNA template (e.g., sister chromatid) to accurately repair the break. - This process is highly accurate but occurs only when a homologous template is available, typically during the S and G2 phases of the cell cycle. *Mismatch repair* - Corrects **base-pair mismatches** and **small insertions/deletions** that occur during DNA replication, which were not corrected by DNA polymerase proofreading. - It targets newly synthesized DNA strands based on methylation patterns in the parental strand.

Question 5: A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?

A. Homology-directed repair
B. Base excision repair
C. Nonhomologous end joining repair (Correct Answer)
D. DNA mismatch repair
E. Nucleotide excision repair

Explanation: ***Nonhomologous end joining repair*** - This pathway is crucial for repairing **double-strand DNA breaks**, which are a major form of damage caused by **ionizing radiation**. - It directly ligates the broken DNA ends without requiring a homologous template, making it an efficient but potentially error-prone repair mechanism. *Homology-directed repair* - This pathway is also used to repair **double-strand DNA breaks** but requires a **homologous DNA template** (usually a sister chromatid) for accurate repair. - While highly accurate, it is typically active during the S and G2 phases of the cell cycle and is generally slower and less dominant than NHEJ for immediate radiation-induced damage in non-dividing cells like neurons. *Base excision repair* - This mechanism primarily corrects damage to individual DNA bases, such as **oxidative damage**, alkylation, or deamination. - It is not the primary mechanism for repairing the **double-strand breaks** induced by ionizing radiation. *DNA mismatch repair* - This pathway corrects errors that arise during **DNA replication**, specifically mismatched base pairs or small insertions/deletions. - It is not involved in repairing radiation-induced DNA damage like **double-strand breaks**. *Nucleotide excision repair* - This pathway repairs bulky DNA lesions, such as those caused by **UV radiation** (e.g., pyrimidine dimers) or chemical mutagens. - It removes a segment of DNA containing the damage but is not the primary repair mechanism for **double-strand breaks** caused by ionizing radiation.

Question 6: A 5-year-old girl is brought to the physician by her mother because of a 1-month history of a painful ulcer on her face. She has developed painful sunburns in the past with minimal UV exposure. Examination of the skin shows a 2-cm ulcerated nodule on the left cheek. There are scaly, hyperpigmented papules and plaques over the skin of the entire body. Ophthalmologic examination shows decreased visual acuity, clouded corneas, and limbal injection. Examination of a biopsy specimen from the facial lesion shows poorly-differentiated squamous cell carcinoma. Impairment of which of the following proteins is the most likely cause of this patient's condition?

A. Rb nuclear protein
B. Base-specific glycosylase
C. Excision endonuclease (Correct Answer)
D. ATM serine/threonine kinase
E. DNA helicase

Explanation: ***Excision endonuclease*** - This patient's presentation with **painful sunburns**, **early-onset squamous cell carcinoma** on the face, and **ocular abnormalities (clouded corneas, decreased visual acuity)** is highly suggestive of **xeroderma pigmentosum (XP)**. - XP is an autosomal recessive disorder caused by a defect in **nucleotide excision repair (NER)**, which is responsible for removing DNA damage primarily induced by **UV radiation**. **Excision endonucleases** are key enzymes in the initiation phase of NER, recognizing and excising the damaged DNA segment. *Rb nuclear protein* - The **Rb nuclear protein** is a tumor suppressor involved in cell cycle regulation (G1/S checkpoint). - Impairment of Rb is associated with **retinoblastoma** and several other cancers, but not typically with this specific constellation of light sensitivity, skin cancer, and ocular damage seen in XP. *Base-specific glycosylase* - **Base-specific glycosylases** are involved in **base excision repair (BER)**, which primarily corrects small, non-helix-distorting base lesions (e.g., deaminated or alkylated bases). - While important for DNA repair, defects in BER would not explain the extreme UV sensitivity and subsequent skin cancers characteristic of xeroderma pigmentosum, as these are primarily linked to UV-induced pyrimidine dimers. *ATM serine/threonine kinase* - **ATM (ataxia-telangiectasia mutated) kinase** is a critical protein involved in initiating the cellular response to **DNA double-strand breaks**. - Defects in ATM cause **ataxia-telangiectasia**, characterized by cerebellar ataxia, immunodeficiency, and a predisposition to lymphoid malignancies, but not the specific skin and eye findings of XP. *DNA helicase* - **DNA helicases** are enzymes that unwind DNA and are involved in various DNA processes, including replication, recombination, and repair. - While critical for many functions, a general defect in **DNA helicase** would lead to a broader range of severe developmental and cellular defects, and is not specifically linked to the clinical phenotype of xeroderma pigmentosum which results from specific NER pathway defects.

Question 7: An 84-year-old man comes to the emergency department because of lower back pain and lower extremity weakness for 3 weeks. Over the past week, he has also found it increasingly difficult to urinate. He has a history of prostate cancer, for which he underwent radical prostatectomy 8 years ago. His prostate-specific antigen (PSA) level was undetectable until a routine follow-up visit last year, when it began to increase from 0.8 ng/mL to its present value of 64.3 ng/mL (N < 4). An MRI of the spine shows infiltrative vertebral lesions with a collapse of the L5 vertebral body, resulting in cord compression at L4–L5. The patient receives one dose of intravenous dexamethasone and subsequently undergoes external beam radiation. Which of the following cellular changes is most likely to occur as a result of this treatment?

A. Intercalation of neighbouring DNA base pairs
B. Disruption of microtubule assembly
C. Formation of DNA crosslinks
D. Generation of hydroxyl radicals (Correct Answer)
E. Formation of pyrimidine dimers

Explanation: ***Generation of hydroxyl radicals*** - **External beam radiation** primarily causes cellular damage through the **ionization of water molecules**, leading to the formation of highly reactive **hydroxyl radicals**. - These radicals directly damage **DNA**, proteins, and cell membranes, leading to **cell death or apoptosis**, especially in rapidly dividing cells like cancer cells. *Intercalation of neighbouring DNA base pairs* - This mechanism is characteristic of certain **chemotherapeutic agents** (e.g., **doxorubicin**, **daunorubicin**) that insert themselves between stacked DNA base pairs. - This process distorts the DNA helix, interfering with replication and transcription, but it is **not the primary mechanism of radiation therapy**. *Disruption of microtubule assembly* - **Microtubule inhibitors** (e.g., **vincristine**, **paclitaxel**) disrupt the formation or disassembly of microtubules, which are essential for cell division and intracellular transport. - While this is a common mechanism of action for some **chemotherapeutic drugs**, it is **not how radiation therapy works**. *Formation of DNA crosslinks* - **Alkylating agents** (e.g., **cyclophosphamide**, **cisplatin**) form covalent bonds within or between DNA strands, creating crosslinks that prevent DNA replication and transcription. - Though highly damaging to DNA, this is a distinct mechanism of action typically associated with **chemotherapy**, not direct radiation. *Formation of pyrimidine dimers* - **Ultraviolet (UV) radiation** causes the formation of **pyrimidine dimers** (e.g., thymine dimers) in DNA. - This type of DNA damage is characteristic of UV light exposure and is **not the primary mechanism of action for external beam radiation therapy**, which uses higher-energy ionizing radiation.

Question 8: DNA replication is a highly complex process where replication occurs on both strands of DNA. On the leading strand of DNA, replication occurs uninterrupted, but on the lagging strand, replication is interrupted and occurs in fragments called Okazaki fragments. These fragments need to be joined, which of the following enzymes is involved in the penultimate step before ligation can occur?

A. DNA gyrase
B. DNA ligase
C. DNA helicase
D. DNA polymerase I (Correct Answer)
E. DNA polymerase III

Explanation: **DNA polymerase I** - **DNA polymerase I** plays a crucial role in removing the **RNA primers** from the Okazaki fragments on the lagging strand. - After primer removal, it fills the resulting gaps with **deoxyribonucleotides** before DNA ligase seals the nicks. *DNA gyrase* - **DNA gyrase** (a type of **topoisomerase**) is involved in relieving **supercoiling** ahead of the replication fork. - It does not directly participate in the joining of Okazaki fragments, but rather in maintaining DNA topology during replication. *DNA ligase* - **DNA ligase** is responsible for the **final sealing** of the nicks between adjacent Okazaki fragments. - It forms a **phosphodiester bond** between the 3'-hydroxyl end of one fragment and the 5'-phosphate end of the next, following primer removal and gap filling. *DNA helicase* - **DNA helicase** unwinds the double-stranded DNA helix, separating the two strands at the **replication fork**. - This enzyme is essential for initiating replication but does not participate in processing Okazaki fragments. *DNA polymerase III* - **DNA polymerase III** is the primary enzyme responsible for the **elongation of new DNA strands** in both leading and lagging strand synthesis. - It synthesizes the actual Okazaki fragments but does not directly remove primers or fill the gaps.

Question 9: A mutant stem cell was created by using an inducible RNAi system, such that when doxycycline is added, the siRNA targeting DNA helicase is expressed, effectively knocking down the gene for DNA helicase. Which of the following will occur during DNA replication?

A. The RNA primer is not created
B. DNA is not unwound (Correct Answer)
C. The two melted DNA strands reanneal
D. DNA supercoiling is not relieved
E. Newly synthesized DNA fragments are not ligated

Explanation: ***DNA is not unwound*** - **DNA helicase** is essential for unwinding the **double-stranded DNA** helix, separating it into two single strands. This process creates the **replication fork**. - Without functional DNA helicase due to **gene knockdown**, the DNA helix cannot be unwound, thus halting DNA replication. *The RNA primer is not created* - **RNA primers** are synthesized by **primase**, an enzyme distinct from DNA helicase. - While unwinding is necessary for primer synthesis, the *creation* of the primer itself is a function of primase. *The two melted DNA strands reanneal* - **Reannealing** of DNA strands is prevented by **single-strand binding proteins (SSBs)**, which bind to the separated single strands. - While helicase unwinds, SSBs specifically keep the strands apart to allow DNA polymerase access. *DNA supercoiling is not relieved* - **DNA supercoiling** is relieved by **topoisomerases**, enzymes that cut, unwind, and religate DNA strands to reduce torsional stress. - This is a distinct function from DNA helicase, which focuses on breaking hydrogen bonds between strands. *Newly synthesized DNA fragments are not ligated* - **Ligation** of newly synthesized **Okazaki fragments** on the lagging strand is performed by **DNA ligase**. - This process occurs downstream from the unwinding step facilitated by DNA helicase.

Question 10: A 38-year-old woman applies a PABA sunscreen to her skin before going to the beach. Which type(s) of ultraviolet light will it protect her against?

A. UVA and UVB
B. UVB and UVC
C. UVB (Correct Answer)
D. UVA
E. UVC

Explanation: ***UVB*** - **Para-aminobenzoic acid (PABA)** and its derivatives primarily absorb **UVB radiation** (290-320 nm), which is responsible for sunburn and erythema. - PABA-containing sunscreens are effective at preventing the acute effects of sun exposure like sunburn caused by UVB. - Traditional PABA sunscreens are primarily effective against **UVB only**, not broad-spectrum. *UVA and UVB* - While some modern sunscreens offer broad-spectrum protection against both UVA and UVB, traditional PABA sunscreens are primarily effective against **UVB only**. - **UVA filters** (e.g., avobenzone, zinc oxide, titanium dioxide) are needed in addition to PABA to achieve protection against both types of radiation. *UVB and UVC* - PABA sunscreens protect against **UVB**, but **UVC radiation** (100-280 nm) is mostly blocked by the Earth's ozone layer and does not reach the Earth's surface. - Sunscreens are not typically formulated to protect against UVC, as it is not a clinical concern for typical sun exposure. *UVA* - PABA is **not effective** at significantly absorbing or blocking **UVA radiation** (320-400 nm), which is primarily associated with photoaging, deeper skin damage, and tanning. - Protection against UVA requires different chemical filters (avobenzone, ecamsule) or physical blockers (zinc oxide, titanium dioxide). *UVC* - **UVC radiation** does not reach the Earth's surface due to complete absorption by the ozone layer, making protection against it unnecessary for sunscreen formulations. - Sunscreens, including those containing PABA, are not designed to filter UVC as it poses no risk in normal outdoor settings.

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