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    • Section 2 Cellular Response to Radiation Answers and Feedback

    • Section 2  Cellular Response to Radiation Answers and Feedback 

      1. ANSWER: D
      FEEDBACK: X-rays are absorbed in biologic mate­rial to produce a recoil electron. This electron may interact directly with DNA to cause a strand break, but this is relatively unlikely because DNA constitutes a small proportion of the cell. The cell is mostly water, and the electron may ionize a water molecule close to the DNA. This leads to the chemical production of a free hydroxyl radical (OH•), which diffuses to the DNA and causes a strand break. This accounts for about two-thirds of the biologic damage produced by x-rays. The correct option is D. 60%. 

      2. ANSWER: B
      FEEDBACK: Dicentrics and rings are “exchange type aberrations”. In other words, they are formed by the illicit rejoining of two separate chromosome breaks. A dicentric is formed as a consequence of rejoining of breaks in two different chromosomes; a ring forms from the rejoining of breaks in the two arms of the same chromosome. The correct option is B. Interaction of two separate chromosomal breaks  

      3. ANSWER: C
      FEEDBACK: Damage to bases may represent a mutation, but it does not kill the cell. Thymine dimers are produced by ultraviolet radiation, not by ionizing radiation. Double-strand breaks are the most important lesions caused by x-rays. Some are quickly and correctly repaired. Those that remain unrepaired or that rejoin illicitly to form an aberra­tion may cause cell death. The correct option is C. The final number of DNA double-strand breaks 

      FEEDBACK: If a break occurs in two pre-replication chromosomes, and pieces of chromatin without centromeres are exchanged, this is a symmetric translocation and is compatible with life (option D). If rejoining occurs so that the two pieces with cen­tromeres join, this represents a dicentric (option E). If the two pieces without centromeres join, this forms an acentric fragment (option A). A small piece of chromatin with a centromere is a centric fragment. What usually happens is that a break occurs on each side of the centromere, and the two “sticky” ends join to form a ring chromosome. In other words, the centric fragment is often a ring (option B). When two breaks occur in the same piece of chromatin, and the piece isolated reverses and rejoins, this is called an inversion (option C). All options are True 

      5. ANSWER: A
      FEEDBACK: In most mammalian cells, mitosis itself (M) lasts about half an hour to an hour. The DNA syn­thetic phase (S) has a duration of 6–8 hours in rodent cells and about 12 hours in human cells. G2 has a duration of a few hours. G1 is the phase that is most variable in length. It may last only an hour in cells that are dividing rapidly to repair damaged tissue, or it may be 10 days to 2 weeks in, for example, the stem cells of resting skin. The correct option is A. G1 

      6. ANSWER: D
      FEEDBACK: Cells in mitosis (M) or just before, in the G2 phase, are the most radiosensitive, while cells in late S are the most radioresistant. G1 cells are intermediate in radiosensitivity. When an asynchro­nous population of cells is exposed to x-rays, most surviving cells will be from the most radioresistant moiety of the population, S. The correct option is D. Most survivors are in the late S phase. 

      7. ANSWER: C
      FEEDBACK: The shoulder of the survival curve (the β component in the α-β formalism) is most pronounced for low linear energy transfer (LET) radiations and minimal for high-LET radiations. For a given particle, the LET goes down as the energy goes up. For a given energy, the LET increases with the mass of the particle. Electrons are by far the lightest of the particles listed; pi mesons are the next lightest, then neutrons, with alpha particles the heaviest. The correct option is C. 5-MeV electrons. 

      8. ANSWER: E
      FEEDBACK: The number of free radicals produced is a function of dose and does not depend on dose rate; option A cannot be correct. Ion pairs recombine in fractions of a millisecond, so that is not a factor here; option D is incorrect. The mitotic cycle is length­ened, not shortened, by radiation; option B is incorrect. This leaves options C and E. If the dose rate is very low, cell survival would be dominated by cell division occurring during a prolonged exposure, and option C would be correct. However, for any practical dose rates, the dominant factor in the dose-rate effect is the repair of sublethal damage during a prolonged exposure. The correct option is E. Repair of sublethal damage occurs during exposure. 

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