Cellular Effects of Radiation — MCQs
What is the repair mechanism associated with CRISPR-cas9?
Which of the following is most susceptible to ionizing radiation?
Cancer patient undergoes radiotherapy, pick the true statement for radiosensitivity of tissues?
Which phase of the cell cycle does not have a fixed duration?
Mutations are due to changes in:
Which of the following is most harmful to an individual cell?
Which compound is not considered as a radiosensitizer?
When is oxygen effective during radiotherapy?
Which of the following is a stochastic effect of radiation?
What is the threshold radiation dose for the hematological syndrome?
Cellular Effects of Radiation Indian Medical PG Practice Questions and MCQs
Question 1: What is the repair mechanism associated with CRISPR-cas9?
- A. Mismatch repair
- B. Non-homologous end joining (Correct Answer)
- C. Nucleotide excision repair
- D. Base excision repair
Explanation: ***Non-homologous end joining*** - **CRISPR-Cas9** creates a **double-strand break (DSB)** in DNA, which is primarily repaired by **non-homologous end joining (NHEJ)**. - **NHEJ** is the **predominant repair pathway** in most cells, accounting for 60-90% of DSB repairs. - **NHEJ** is an error-prone repair mechanism that ligates the broken ends directly, often leading to small **insertions or deletions (indels)** causing gene knockout. - Note: **Homology-directed repair (HDR)** is another CRISPR-associated mechanism used for precise editing when a donor template is provided, but **NHEJ is the primary endogenous repair pathway**. *Mismatch repair* - This mechanism corrects errors that arise during **DNA replication**, such as incorrect base pairing. - It does not repair **double-strand breaks** induced by CRISPR-Cas9. *Nucleotide excision repair* - This pathway removes **bulky DNA adducts** and lesions, such as those caused by UV radiation. - It is not involved in repairing **CRISPR-Cas9 induced double-strand breaks**. *Base excision repair* - This mechanism corrects **damaged or modified bases**, typically single base changes, without affecting the sugar-phosphate backbone significantly. - It handles different types of DNA damage than the **double-strand breaks** generated by CRISPR-Cas9.
Question 2: Which of the following is most susceptible to ionizing radiation?
- A. Nerve cells
- B. Small lymphocytes (Correct Answer)
- C. Epithelial cells
- D. Mature bone
Explanation: ***Small lymphocytes*** - **Lymphocytes** are highly sensitive to **ionizing radiation** due to their rapid proliferation and vulnerability during DNA synthesis [1]. - Exposure to radiation can lead to rapid **apoptosis** (programmed cell death) of lymphocytes, causing **lymphocytopenia** [1]. *Nerve cells* - **Nerve cells** are generally considered **radioresistant** because they are terminally differentiated and do not undergo cell division. - Their primary function is transmission of electrical impulses, which is not immediately disrupted by typical radiation doses. *Epithelial cells* - **Epithelial cells** (e.g., skin, gut lining) are moderately sensitive to radiation, primarily due to their **high turnover rate** [1]. - However, they are generally less sensitive than lymphocytes, and their damage often requires higher doses or prolonged exposure [1]. *Mature bone* - **Mature bone** is highly resistant to radiation because its cells (osteocytes) are largely quiescent and the extracellular matrix is inert [1]. - Significant damage to bone typically occurs at very high doses, affecting the more sensitive **bone marrow** stem cells within it rather than the mature bone itself [1]. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 111-114.
Question 3: Cancer patient undergoes radiotherapy, pick the true statement for radiosensitivity of tissues?
- A. GI mucosa is one of the most radioresistant tissues in the body
- B. Rapidly dividing cells are resistant to radiation
- C. Small blood vessels are radiosensitive tissues (Correct Answer)
- D. The intensity of radiation is inversely proportional to the square of distance from the source
Explanation: ***Small blood vessels are radiosensitive tissues*** - **Endothelial cells** lining small blood vessels are actively dividing and have a relatively short lifespan, making them susceptible to radiation-induced damage. - Damage to these vessels can lead to **fibrosis** and **tissue hypoxia**, contributing to late radiation effects. *GI mucosa is one of the most radioresistant tissues in the body* - The **GI mucosa** is one of the **most radiosensitive** tissues due to its rapid cell turnover and high proliferative capacity, leading to symptoms like mucositis and diarrhea after radiation. - The constant renewal of epithelial cells makes them highly vulnerable to the **cytotoxic effects** of radiation. *Rapidly dividing cells are resistant to radiation* - **Rapidly dividing cells**, such as those in the bone marrow, GI crypts, and hair follicles, are generally **most sensitive** to radiation. - This forms the basis of radiation therapy, as cancer cells often have a higher proliferative rate than many normal tissues, making them a target. *The intensity of radiation is inversely proportional to the square of distance from the source* - This statement describes the **inverse square law** for radiation intensity, which is true, but it is a principle of radiation physics, not a statement about the radiosensitivity of tissues. - The inverse square law dictates how radiation dose **attenuates with distance**, which is critical for dose calculation but not directly about biological radiosensitivity.
Question 4: Which phase of the cell cycle does not have a fixed duration?
- A. S phase (DNA synthesis)
- B. M phase (mitosis)
- C. G1 phase (cell growth) (Correct Answer)
- D. G2 phase (preparation for mitosis)
Explanation: ***G1*** - The **G1 phase** of the cell cycle is variable in length and can differ significantly between cell types and conditions, unlike S, M, and G2 phases [1][2]. - Cells can spend an **indeterminate amount of time** in G1, depending on factors like nutrients and signals for division [2]. *S* - The **S phase** is characterized by a fixed duration where **DNA replication** occurs, and is critical for cell division [1]. - It typically has a well-defined time frame in the cell cycle that is consistent across different cells [1]. *M* - The **M phase** (mitosis) requires a set duration to ensure that the **cell divides** accurately and equally into two daughter cells [2]. - Fluctuations in this phase can result in aberrant cell division and aneuploidy. *G2* - The **G2 phase** also has a consistent timeframe dedicated to preparing the cell for mitosis, focusing on DNA repair and organelle duplication [2]. - The cell ensures readiness for division during this phase, which is critical for genomic integrity. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 78-79. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 37-38.
Question 5: Mutations are due to changes in:
- A. DNA nucleotide sequence (Correct Answer)
- B. RNA nucleotide sequence
- C. Amino acid sequence of ribonuclease
- D. Cell membrane
Explanation: ***DNA nucleotide sequence*** - **Mutations** are defined as changes in the **genetic material**, which is primarily composed of **DNA**. - These changes in the **nucleotide sequence** of DNA can alter the genetic code, leading to changes in **protein structure and function**. *RNA nucleotide sequence* - While RNA can have its nucleotide sequence altered, these changes are generally not considered true **mutations** in the heritable sense for most organisms. - RNA is typically a temporary molecule, and changes to its sequence are usually not passed down to subsequent generations. *Amino acid sequence of ribonuclease* - An altered **amino acid sequence** in a protein like ribonuclease is a consequence of a **mutation in the DNA**, not the mutation itself. - **Ribonucleases** are enzymes that catalyze the degradation of RNA, and their structure is determined by the **DNA sequence**. *Cell membrane* - The cell membrane is a **lipid bilayer** with embedded proteins that regulates cellular transport and communication. - While its components can be affected by genetic mutations, alterations in the cell membrane itself do not constitute the primary definition of a **mutation**.
Question 6: Which of the following is most harmful to an individual cell?
- A. Alpha particles (Correct Answer)
- B. X-rays
- C. Beta particles
- D. Gamma rays
Explanation: ***Alpha particles*** - **Alpha particles** have a **high linear energy transfer (LET)**, meaning they deposit a large amount of energy over a very short distance within a cell. - Due to their size and double positive charge, they cause dense ionization tracks, leading to **complex and irreparable DNA damage**, making them highly harmful to individual cells despite their limited penetrating power. *X-rays* - **X-rays** are a form of **electromagnetic radiation** with lower LET than alpha particles, causing more scattered ionization events. - While X-rays can cause DNA damage, it is often less complex and more amenable to cellular repair mechanisms compared to alpha particle damage. *Beta particles* - **Beta particles** are high-energy electrons with a **lower mass and charge** than alpha particles. - They have an intermediate LET, causing less concentrated ionization than alpha particles, making them less damaging to an individual cell compared to alpha particles. *Gamma rays* - **Gamma rays** are **electromagnetic radiation** with the **lowest LET** among the options, traveling long distances and causing sparse ionization within tissue. - Although highly penetrating, the energy deposition is spread out, resulting in less concentrated damage to individual cells compared to alpha particles.
Question 7: Which compound is not considered as a radiosensitizer?
- A. Hyperbaric oxygen
- B. Misonidazole
- C. Amifostine (Correct Answer)
- D. Idoxuridine
Explanation: ***Amifostine*** - **Amifostine** is a **radioprotector**, meaning it selectively protects healthy cells from the damaging effects of radiation, rather than enhancing radiation's effect on tumor cells. - It works by being dephosphorylated by alkaline phosphatase in normal tissues to an active thiol metabolite that scavenges **free radicals** generated by radiation. *Hyperbaric oxygen* - **Hyperbaric oxygen** increases the amount of dissolved oxygen in tissues, which is a potent **radiosensitizer**. - **Oxygen** enhances the formation of **free radicals** and fixes radiation-induced DNA damage, making tumor cells more susceptible to radiation. *Misonidazole* - **Misonidazole** is a **hypoxic radiosensitizer** that mimics oxygen, becoming relatively toxic under hypoxic conditions (common in tumors). - It forms **free radicals** and binds to cellular macromolecules when reduced by nitroreductases in hypoxic cells, thereby enhancing the effects of radiation. *Idoxuridine* - **Idoxuridine** is a **halogenated pyrimidine** that acts as a radiosensitizer by being incorporated into DNA in place of thymidine. - This incorporation sensitizes the DNA to radiation by making it more susceptible to **strand breaks** and other damage.
Question 8: When is oxygen effective during radiotherapy?
- A. During and within microseconds of starting (Correct Answer)
- B. Just before starting the therapy
- C. After 5 minutes
- D. After 10 minutes
Explanation: ***During and within microseconds of starting*** - Oxygen is effective during radiotherapy primarily due to the **oxygen enhancement ratio (OER)**, which describes the increased radiosensitivity of cells in the presence of oxygen. - This effect is almost instantaneous, as oxygen acts as a **radical sensitizer** by stabilizing DNA damage caused by radiation, making it irreparable by cellular repair mechanisms. *Just before starting the therapy* - While having oxygen present just before therapy is important, the actual sensitization effect requires oxygen to be present **during** the radiation exposure itself. - Simply having oxygen before without its presence during treatment will not maximize the therapeutic benefit. *After 5 minutes* - The critical period for oxygen's radiosensitizing effect is during and immediately after the ionization events caused by radiation, which occur over **microseconds**. - Oxygen administered 5 minutes after radiation exposure would be too late to impact the initial damage fixation process. *After 10 minutes* - Similar to the 5-minute mark, oxygen delivered 10 minutes after radiation would have **no significant impact** on the immediate radiation-induced cellular damage. - The window of opportunity for oxygen to enhance radiosensitivity is extremely short, occurring at the moment of radiation interaction with biological molecules.
Question 9: Which of the following is a stochastic effect of radiation?
- A. Alopecia in the irradiated portal
- B. Local desquamation in the irradiated field
- C. Genetic mutation (Correct Answer)
- D. All of the options
Explanation: ***Genetic mutation*** - **Stochastic effects** are those for which the probability of occurrence, rather than the severity, is a function of radiation dose without a threshold. **Genetic mutations** are a classic example, as any dose carries some probability of inducing a change in DNA that can be passed to offspring. - The severity of a genetic mutation, if it occurs, is independent of the dose. It's the chance of it happening that increases with exposure. *Alopecia in the irradiated portal* - **Alopecia** (hair loss) due to radiation is a **deterministic effect**, meaning there is a threshold dose below which it does not occur, and above that threshold, the severity increases with dose. - It occurs locally in the **irradiated field** because it is a direct tissue reaction to cellular damage. *Local desquamation in the irradiated field* - **Desquamation** (skin peeling) is a **deterministic effect** that results from direct cell death and damage in the skin, a tissue reaction with a dose threshold. - Its occurrence and severity are directly related to the **radiation dose received** in the specific area. *All of the options* - This option is incorrect because **alopecia** and **desquamation** are deterministic effects, not stochastic effects. - Only **genetic mutation** falls under the category of stochastic effects among the choices provided.
Question 10: What is the threshold radiation dose for the hematological syndrome?
- A. 2 Gy (Correct Answer)
- B. 6 Gy
- C. 15 Gy
- D. 50 Gy
Explanation: ### Explanation **Acute Radiation Syndrome (ARS)** occurs after whole-body exposure to high doses of ionizing radiation. It is categorized into three distinct sub-syndromes based on the dose received and the organ system affected. **1. Why Option A (2 Gy) is Correct:** The **Hematological (Bone Marrow) Syndrome** occurs at doses between **2 and 10 Gy**. At this threshold, the radiation destroys the highly mitotic precursor cells in the bone marrow, leading to pancytopenia (depletion of white blood cells, platelets, and red blood cells). Death, if it occurs, is usually due to infection or hemorrhage within 3–6 weeks. **2. Why the Other Options are Incorrect:** * **Option B (6 Gy):** While 6 Gy falls within the range of hematological syndrome, it is above the *threshold* (starting point). At doses above 6–10 Gy, the Gastrointestinal syndrome begins to overlap and dominate. * **Option C (15 Gy):** This dose triggers the **Gastrointestinal (GI) Syndrome** (threshold: **6–10 Gy**). It involves the destruction of intestinal crypt cells, leading to severe diarrhea, dehydration, and electrolyte imbalance. Death typically occurs within 5–10 days. * **Option D (50 Gy):** This dose triggers the **Cerebrovascular (CNS) Syndrome** (threshold: **>20–50 Gy**). It results in immediate neurological symptoms, seizures, and coma, with death occurring within 24–48 hours. **High-Yield Clinical Pearls for NEET-PG:** * **LD 50/60:** The lethal dose required to kill 50% of the population in 60 days is approximately **3–4 Gy** (without medical intervention). * **Prodromal Phase:** The initial stage of ARS characterized by nausea, vomiting, and anorexia (NVA). * **Radiosensitivity:** According to the **Law of Bergonie and Tribondeau**, cells with high mitotic activity and low differentiation (like hematopoietic stem cells) are the most radiosensitive.
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