Chapters 1-2

Question 1

What are the 4 R’s of radiobiology?

Answer 1

• Re-oxygenation
• Re-assortment of cells within cell cycle
• Repair of sublethal damage between fractions
• Repopulation

Question 2

What are the 4 cycles of a cell?

Answer 2

• M- Mitosis
• G1 - Gap 1
• S - Synthesis
• G2 - Gap 2

Question 3

What are the 3 types of localized energy deposition called and what are some characteristics?

Answer 3

• Spurs - 95% of energy deposition events, on average 3 ion pairs, 4nm, <100eV
• Blobs - 100-500 eV, 12 ion pairs on average, diameter ~7nm
• Tracks - 500-5000 eV energy

Question 4

What is direct action of radiation damage?

Answer 4

Direct action is when radiation interacts directly with critical targets in a cell, which causes ionization. This leads to a chain of events that causes biologic change.

This is predominantly caused by high LET radiations such as neutrons and alpha particles.

Question 5

What is indirect action of radiation damage?

Answer 5

Indirect action occurs when radiation interacts not with the critical target but with other atoms in the cell, producing free radicals that then damage the target.

This is the predominant process for low LET radiation, such as x-rays.

Question 6

How are free radicals created?

Answer 6

Free radicals are created when photons hit a water molecule and ionize it. The new ion H₂O⁺ interacts with other water molecules, usually in the form:

H₂O⁺ + H₂O -> H₃O⁺ + OH

The OH can diffuse to DNA and interact with it to damage the DNA.

A radical is an atom or molecule that has an unpaired electron in its outermost shell.

Question 7

Which bases pair with each other in DNA strands?

Answer 7

• Adenine and Thymine
• Guanine with Cytosine

Question 8

Why are single strand breaks (SSBs) not of large consequance? When can they be a problem?

Answer 8

SSBs are repaired readily using the oppositive strands as a template. This only causes a problem if the repair is incorrect and causes a mutation, which can result in cancer.

Question 9

What is a DSB?

Answer 9

A double strand break occurs when two single strand breaks occur on the same DNA. This becomes a problem when the two breaks are close together or opposite each other. Then the chromatin snaps into 2 pieces, each of which has a “sticky” end.

Question 10

What is the most important lesion produced in chromosomes by radiation?

Answer 10

Double-strand breaks are the most important lesions. The interaction of two double-strand breaks may result in cell killing, carcinogenesis, or mutation.

Question 11

What is the diameter of a DNA double helix?

Answer 11

The diameter of a DNA double helix is 2 nm.

Question 12

What are the possible effects of a double strand DNA break?

Answer 12

Possible effects of DSB. Results in pieces of DNA with sticky ends that can only attach to other “sticky” ends:

• May reattach in their original configuration (no effect)
• May fail to reattach. Results in loss of chromosomal material (a deletion) at the next cell division.
• May reattach to other “sticky” broken ends to give rise to very abnormal looking chromosomes when seen at the next mitosis

Question 13

What are the two possible categories of DNA recombination?

Answer 13

• Homologous recombination
  
  • A sister chromatid is available as a template because the break occured after the replication of the DNA. An accurate repair can be made.
  • This is rare in mammalian cells
• Non-homolgous (illegitimate) recombination
  
  • No identical sister chromatid is available as a template. Often gap filling is done but it is frequently incorrect.

Homologous: (of chromosomes) pairing at meiosis and having the same structural features and pattern of genes.

Question 14

How many DNA lesions per cell of each type are detected immediately after a dose of D₀ (usually 1-2 Gy)?

Answer 14

After a dose of D₀, there are usually lesions of

• base damage: > 1,000
• Single-strand breaks (SSBs): 1,000
• Double-strand breaks: 40

Question 15

What type of radiation produces a higher proportion of blobs?

Answer 15

High LET radiation, such as neutrons and alpha particles produce a higher proportion of blobs.

Question 16

Describe the pass and energy deposition pattern of a low-energy (less than 1 MeV) electron.

Answer 16

The range of the electron in tissue is only a few millimeters.

• During the early part of the track, the electron will move quickly and deposit little energy. This results in a relatively straight track in which ionizations are ~0.1mm apart on average.
• As the electron slows down, it interactsmore with orbital electrons in the medium
  
  • The rate of energy loss increases, the track becomes more tortorous and clustering of ionization events (few nanometeres wide) take place near the ends of the tracks.
  • Creates clusters of hotspots, which can be severely damagings if they overalp with a DNA molecule.

Question 17

What occurs during the G1 cell phase?

Answer 17

During G1

• cell growth and metabolism not related to cell replication

Question 18

What occurs during the S phase of the cell cycle?

Answer 18

During the Synthesis phase:

• Chromosome replicates to have 2 sistr chromatids attached together via a centromere
• Damage to DNA is detected and fixed during S-phase.

Question 19

What occurs during the G2 phase?

Answer 19

During the G2 phase, cells make and seemble molecules required for cell division.

Question 20

What are the phases of mitosis?

Answer 20

Phases of Mitosis:

1. Prophase:
   
   • “Condensation of chromosomes” - Chromatin thickens
   • Centromere forms
   • Nuclear membrane and nucleoli disappear
2. Metaphase:
   
   • Chromosmes move to centre of cell
   • Formation of spindle, which runs from the centromeres to the poles of the cell
   • Centromeres divide
3. Anaphase
   
   • Chromosomes pulled via fibers attached to centromeres to poles of cell
4. Telophase
   
   • Uncoiling chromosomes at poles of cells
   • Formation of nuclear membrane and nucleoli
5. Cytokinesis
   
   • Cytoplasm splits to give two daughter cells

Question 21

What are the two categories of DNA aberrations?

Answer 21

2 types of aberrations (badly joined DNA after DSB) observed in metaphase:

• Chromosomal aberrations: irradiation before replication (G1). Chromosome has only one strand of chromatin when irradiated. When this chromosome is copied during S phase, the copy will have the same error in it.
• Ghromatid aberrations: DNA irradiated after replication (G2). Chromosome has 2 strands of chromatin when irradiated. One strand damaged but not the other one.

Question 22

Name the types of lethal aberrations.

Answer 22

If a DNA has one of these lethal aberrations, the cell dies when it tries to undergo mitosis:

• Ring
• Dicentric
• Anaphase bridge

Question 23

Name the types of non-lethal DNA aberrations.

Answer 23

If DNA has one of these non-lethal aberrations, it may lead ot other problems like carcinogenesis or mutation:

• Translocations
• Small deletions

Question 24

Describe a Dicentric Chromosome aberration.

Answer 24

In dicentric chromosome aberration,

• Two chromosomes are damaged
• Their sticky ends join together incorrectly so that one has 2 centromeres and the other has none
• This aberration occurs before it is copied and is then included in the copy during S phase.
• During mitosis, the dicentric chromosome is stuck in a tug of war between two poles
• The acentric fragments are lost

Question 25

What does homologous mean?

Answer 25

Homologous means pairing at meiosis and having the same structural features and pattern of genes. Pertaining to (a member of) a pair of chromosomes, as in homologous chromosomes. A pair of chromosomes having the same gene sequences, each derived from one parent.

Question 26

What is a centromere?

Answer 26

A centromere is the constricted region joining the two sister chromatids that make up an X-shaped chromosome.

Question 27

What are all the ways an aberration can be formed? Picture the diagram with the 5 ways.

Question 28

Describe a ring chromosome aberration.

Answer 28

A ring chromosome aberration occurs when there are breaks in both arms of a chromsome.

• The sticky ends rejoin incorrectly so that on piece has a centromere in the middle of a ring and the other piece has no centromere
• During S phase: forms an overlapping ring formation + acentric fragment
• In mitosis: ring chromsome change be divided
• Acentric fragments lost

This is a lethal aberration.

Question 29

Describe an anaphase bridge aberration.

Answer 29

An anaphase bridge aberration is a chromatid aberration, it occurs after the chromatid is copied.

• Breaks occur in both chromatids of the same chromosome
• The sticky ends join together incorrectly
  
  • The sticky ends of the two sister chromatids with the centromeres join together
  • Other piece has no centromere
• In mitosis: when the centromere divides in metaphase, the dicentric chromatid gets caught in a tug of war in the anaphase (when the chromosomes are pulled via fibers attached to centromeres to the poles of the cell).
  
  • acentric fragments lost

Looks like an upward facing “alpha”.

Question 30

What is a translocation aberration?

Answer 30

A translocation aberration is a chromatid aberration, it occurs after the chromatid is copied.

• Breaks in 2 copies of the same chromosome at the same point in the DNA (if balanced translocation)
• Sticky ends rejoin incorrectly, exchange with each other
• In mitosis: no effect
• Associated with several human malignancies eg. Burkitt’s lymphoma
  

Question 31

Describe how a small deletion occurs in DNA due to radiation damage?

Answer 31

Small deletions occur when

• a chromosome experiences 2 DSBs in the same arm
• the sticky ends join incorrectly, leaving out the middle piece
  
  • an acentric fragment is lost
• Associated with several human malignancies eg. retinoblastoma

Question 32

What are lymphocytes?

Answer 32

A lymphocyte is any of three subtypes of white blood cell in a vertebrate’s immune system. They are the main type of cell found in lymph, which prompted the name lymphocyte.

Question 33

How can lymphocytes be used to detect the amount of total body radiation received recently?

Answer 33

To measure dose received, assess blood samples

• Amount of asymmetrical aberrations in the lymphocytes (dicentrics and ring aberrations) is proportional to the radiation dose received
• Compare these amounts to known doses given to in vitro cultures exposed to known doses
• Can detect TBI doses as low as 10-20 cGy
• As time from exposure increases, fewer abnormal lymphocytes will be found in blood.

Question 34

What mathematical model best describes the relationship between the number of aberrations per lymphocyte as a function of absorbed dose?

Answer 34

The number of aberrations per lymphocyte as a function of absorbed dose is best described by a linear-quadratic model.

• The linear component results from a single charged particle causing 2 breaks
• The quadratic component results from 2 different charged particles causing 2 breaks

Question 35

What features of stem cells can be used to estimate dose received to a person?

Answer 35

Translocations in stem cells can be used to estimate dose received, even more than 40 years later. Dicentrics and rings can’t be used because those cells die when they attempt mitosis.