Effects of Ionizing Radiation on DNA

Ionizing radiation affects living things on an atomic level, by ionizing molecules inside the microscopic cells that make up your body. When ionizing radiation comes in contact with a cell any or all of the following may happen:

  1. It may pass directly through the cell without causing any damage.
  2. It may damage the cell but the cell will repair itself.
  3. It may affect the cell’s ability to reproduce itself correctly, possibly causing a mutation.
  4. It may kill the cell. The death of one cell is of no concern but if too many cells in one organ such as the liver die at once, the organism will die.

Inside the nucleus of each cell are microscopic bodies called chromosomes. Chromosomes are organized in pairs and are responsible for the function and reproduction of each cell in an organism’s body. Different species of animals and plants may have a different number of chromosomes. Humans and potatoes have 46 chromosomes, while chickens have 78. Chromosomes are made of two large molecules or strands of deoxyribonucleic acid (DNA). These strands of DNA make up the genetic code, which in many ways acts much like a computer program. DNA is made up of four nucleic acids: adenine, cytosine, guanine and thymine. How these nucleic acids are arranged in the DNA is the genetic code that determines everything from hair colour to how tall you grow and even susceptibility to certain diseases.

When cells divide to reproduce, an exact copy of the cells’ chromosomes are created for the new cell. If the DNA in the chromosome is damaged, the instructions that control the cell’s function and reproduction are also damaged. If the cell reproduces instead of dying, a new mutated cell may be produced. In many cancers, the instructions that turn off cell growth are somehow damaged causing out of control cell reproduction, creating a tumour. Ionizing radiation, along with many other substances such as some chemicals, heavy metals and intense electromagnetic waves, can damage cells in this manner which can lead to diseases such as cancer.

When talking about biological effects from ionizing radiation there are two categories of injury: somatic injury and genetic injury.  Somatic injury is damage that occurs to the organism exposed to high levels of ionizing radiation and does not include reproductive cells.  Effects like sickness, hair loss or internal bleeding are visible shortly after exposure. Other illness such as cancer may take a number of years to appear.

damaged DNA
Damaged DNA.

Genetic injury is damage to the reproductive cells due to exposure to high levels of ionizing radiation and can be passed down to an organism’s offspring, perhaps generations later. Some potential illnesses could include birth abnormalities and cancer. Somatic and genetic injuries are not solely caused by ionizing radiation. Many chemical pollutants found in our environment such as cadmium, lead and mercury also can cause similar injuries.

If a strand of DNA is damaged, the cell may repair the damage, die or kill itself through a process known as apoptosis.

Sometimes the cell survives but incorrectly repairs itself and then passes the genetic abnormality on to other cells during reproduction. Genetic injury due to radiation exposure has not been observed in human populations (e.g., Hiroshima and Nagasaki survivors) exposed to radiation.

Approximately 70% of the ionizing radiation you will be exposed to over your lifetime will come from natural sources found in our environment. Another 27% will come from necessary medical treatments while 3% will come from other man made sources such as smoke detectors, television sets and airplane travel.

Direct and Indirect Action of Ionizing Radiation on DNA

Inside the nucleus of each human cell there are 46 chromosomes organized into two sets of 23 chromosomes. Packaged inside these chromosomes is our DNA, the genetic material we receive from our parents. The DNA within our cells is continually being exposed to DNA-damaging agents. These agents include ultraviolet light, natural and man made mutagenic chemicals and reactive oxygen species generated by ionizing radiation. When cells are exposed to ionizing radiation, radiochemical damage can occur either by direct action or indirect action. Direct action occurs when alpha particles, beta particles or x-rays create ions which physically break one or both of the sugar phosphate backbones or break the base pairs of the DNA. The base pairs adenine, thymine guanine and cytosine are held together by weak hydrogen bonds. Adenine always pairs with thymine (except in RNA where thymine is substituted by uracil) and guanine always pairs with cytosine. The bonding of these base pairs can also be affected by the direct action of ionizing radiation.

direct action
Direct action of ionizing radiation on DNA.

Please note this diagram gives the impression that alpha particle breaks the “backbone” of the DNA, the beta particle breaks hydrogen bonds, and X-rays damage bases when in fact all three types of radiation can cause all three types of direct damage. However, heavy charged particles such as alpha particles have a greater probability of causing direct damage compared to low charged particles such as X-rays which causes most of its damage by indirect effects.

The DNA base pairs form sequences called nucleotides which in turn form genes. Genes tell the cell to make proteins which determine cell type and regulate cell function. When such breaks occur, DNA usually repairs itself through a process called excision. The excision process has three steps:

  1. Endonucleases cut out the damaged DNA
  2. Resynthesis of the original DNA by DNA polymerase
  3. Ligation whereby the sugar phosphate backbone is repaired.

These repair processes are highly efficient since we have evolved as a species in a sea of radiation. DNA repair takes place continuously, involving every cell in our bodies several times per year. Occasionally, however, damage to the base pair can occur when the DNA is incorrectly repaired and the wrong nucleotide is inserted which can lead to cell death or a mutation. Remember your DNA is the code which determines the type and function of the cell. There are two basic types of mutations:

  • Substitutions — this is the replacement of one base by another. For example, if a DNA molecule usually contains guanine at a certain position, but adenine takes the place of the guanine, then a base substitution has occurred. There are two types of base substitutions:
  • transitions — these involve the replacement of one purine with the other purine, (adenine and thymine), or the replacement of one pyrimidine with the other pyrimidine (cytosine and guanine)
  • transversions — these involve the replacement of a purine with a pyrimidine or vice versa
  • Frameshift Mutations — these change the reading frame of a gene (the triplet code). There are two types of frameshift mutations:
  • insertions — as the name implies, these involve the insertion of one or more extra nucleotides into a DNA chain
  • deletions — these result from the loss of one or more nucleotides from a DNA chain

To illustrate the effects of these mutations, consider the following phrase, read as a triplet code (groups of three letters):

The fat cat ate the hot dog.

A base substitution might have an effect like this:

The fat car ate the hot dog.

or perhaps:

The fat cat are the hot hog.

In each case, the phrase still makes sense, but the meaning has been slightly changed.

A mutation, on the other hand, would have a more profound effect:

The fma tca tat eth eho tdo g.

Insertion of a single letter (“m” in this case) causes the phrase to become gibberish, because the reading frame has been changed. A deletion would have the same effect:

The atc ata tet heh otd og.

 

Two types of breaks in the sugar phosphate backbone can also be caused by ionizing radiation. A single strand break occurs when only one of the sugar phosphate backbones is broken. Single strand breaks are readily repaired using the opposite strand as a template. However, base pair substitutions and frameshift mutations can still occur.

Double-strand breaks are believed to be the most detrimental lesions produced in chromosomes by ionizing radiation. Because such breaks are difficult to repair, they can cause mutations and cell death. Unrejoined double strand breaks are cytotoxic (they kill cells). Double strand breaks can also result in the loss of DNA fragments which, during the repair process, can cause the joining of non-homologous chromosomes (chromosomes not of the same pair) leading to the loss or amplification of chromosomal material.

direct action
Single strand and double strand breaks.

These events can lead to tumorigenesis (creation of tumour cells) if, for example, the deleted chromosomal region encodes a tumour suppressor or if an amplified region encodes a protein with oncogenic potential (cancer potential). If the genetic code is damaged and the cell does not undergo apoptosis (cell suicide), the mutation may be passed on during cell division, perhaps leading to a cancer or other mutation. In some cases a mutation may remain dormant for years and perhaps forever.

Ionizing radiation can also impair or damage cells indirectly by creating free radicals. Free radicals are molecules that are highly reactive due to the presence of unpaired electrons on the molecule. Free radicals may form compounds, such as hydrogen peroxide, which could initiate harmful chemical reactions within the cells. As a result of these chemical changes, cells may undergo a variety of structural changes which lead to altered function or cell death.

indirect action
Indirect action of ionizing radiation on DNA.

Sources:

Genetic Science Learning Center (June 10, 2014) Learn Genetics. Learn.Genetics. http://learn.genetics.utah.edu/
Stephen P. Jackson, “Sensing and repairing DNA double-strand breaks,” Carcinogenesis, Vol. 23, No. 5, 687-696, May 2002, Oxford University Press.
DNA Replication, Repair and Recombination  http://www.garlandscience.com/res/pdf/ecb4_ch6_draft.pdf.
Eric J. Hall, Amato J. Giaccia, Radiobiology for the Radiologist, Lippincott Williams & Wilkins, 2006, p. 16.
Stephen P. Jackson, “Sensing and repairing DNA double-strand breaks,” Carcinogenesis, Vol. 23, No. 5, 687-696, May 2002, Oxford University Press.
“Biological Effects of Ionizing Radiation at Molecular and Cellular Levels. Module VIII,” an IAEA and World Health Organization presentation, http://www-pub.iaea.org/MTCD/publications/publications.asp.
Kum Kum Khanna & Stephen P. Jackson, “DNA double-strand breaks: signalling, repair and the cancer connection,” Nature Genetics 27, 247-254, 2001, doi: 10.1038/85798.
NDT Resource Center, www.ndt-ed.org/EducationResources/CommunityCollege/RadiationSafety/theory/ionization.htm.