Basic Biologic Interactions of Radiation

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Basic Biologic Interactions of Radiation

• Radiation Biology
• ME 397P
• Spring, 2000

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Keep in Mind...

• All radiation interactions are probability
  functions
• We can predict overall effects, but cannot
  predict specific events which cell or DNA
  molecule will be affected

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Keep in Mind...

• Time frame
• Energy is transferred from photon/particle very
  quickly (10-17 to 10-15 seconds)
• Chain of events may appear to be simultaneous

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Keep in Mind...

• Radiation interaction is nonselective
• Radiation doesnt seek certain cells, etc.

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Keep in Mind...

• Radiation damage to the cell cannot be
  distinguished from damage caused by other sources
  such as chemicals, heat, or trauma
• This concept makes it difficult to tell what
  caused damage - especially when studying effects
  of low-level radiation

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Keep in Mind...

• Radiation effects exhibit a latent period during
  which no evidence of damage is present
• Latent period may last from few minutes to
  decades (length is dependent on dose and type of
  cells involved)

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Initial Changes

• Begin at subcellular level
• Depend upon where initial interactions occur
• Initial interactions may be caused by
  excitation or ionization of medium or target
  
  

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Target

• Usually a critical biologic macromolecule (e.g.,
  DNA)

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Direct Interaction of Radiation

• Energy of radiation is deposited directly in
  critical target resulting in excitation or
  ionization. Starts a chain reaction which may or
  may not be fatal to cell (e.g., may be repaired)
• More likely to occur with high-LET radiation such
  as particles or neutrons

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Indirect Interaction of Radiation

• Involves the radiolysis of water
• Is the most common effect since cell is primarily
  water
• Example For every one DNA molecule, there are
  1.2X107 water molecules

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Radiolysis of Water

• Involves the production of a free radical (highly
  reactive species)
• Free radicals are designated by a dot and contain
  a single unpaired electron in outer electron
  shell

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Radiolysis of Water

• HOH --gt HOH e
• HOH --gt H OH.
• OH. OH . H2O2
• Hydrogen peroxide is very toxic!

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Direct or Indirect Effect?

• Experiments have been performed with simple
  organisms such as Artemia (brine shrimp) cysts in
  wet and dry conditions. Which tolerated the
  larger dose? Can you think of other ways
  researchers could study these effects?

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LET and RBE

• LET (Linear Energy Transfer) - rate at which
  energy of radiation is transferred to medium in
  units of keV/micron
• RBE (Relative Biological Effectiveness) -
  comparison of dose of test radiation to dose of
  250 keV x-rays to produce same biological effect

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LET

• X-rays and gamma rays are low-LET radiations
• Alpha and fast neutrons are high-LET radiations
• LET directly related to ionization and therefore,
  biological damage

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RBE

• High-LET results in high RBE
• Is similar to Quality Factor (Q)
• RBE dose of 250-keV x-ray to produce same
  effect divided by dose of test radiation to
  produce same effect
• Same biological endpoint (effect) is constant
  not doses

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Radiation and Cellular Targets

• Damage to a critical molecule which results in
  significant impact on cell
• Radiation does not seek out targets
• Most critical site in cell is probably DNA in
  nucleus

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DNA Damage

• Much of damage may be repairable
• Different kinds of damage (base damage,
  single-strand breaks, double-strand breaks, and
  crosslinking)

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Radiation Effects on Chromosomes

• DNA damage may not be visible as chromosome
  damage
• Can have actual visible changes in chromosomes
• Chromosome damage can occur in either sex cells
  or somatic cells

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Structural Changes in Chrom.

• Actual breaks
• Stickiness or clumping of chromosomes

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Chromosome Damage

• Broken pieces may correctly rejoin
• Piece is lost (acentric fragment)
• Rearrangement of broken ends yield rings,
  dicentrics, etc.
• Rearrangements not visible, but genetic material
  has been affected resulting in a mutation

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Ring Chromosome Aberration
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Accident Dosimetry
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Radiation Mutation Effects Summary

• Germ cell changes may produce genetic effects
  (future generations). Somatic effects will cause
  changes in exposed person only.
• Radiation effects are nonspecific (no radiounique
  effects)
• Most mutations are undesirable

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Radiation Effects on Other Organelles

• Nucleus most important
• Cell membrane permeability changes
• Any membrane-bound organelle susceptible to
  radiation-induced changes

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Membrane Leakage
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Glossary

• Please refer to definitions in glossary of
  textbook

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Coming Attractions...

• Bean lab results next week
• Will be starting Drosophila lab
• Research paper assigned
• Exam 1 coming up