Effects of Ionizing Radiation on Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid (DNA) is the molecule that usually encodes genetic blueprint in an organism i.e. it contains all information that is required to build as well as maintain an organism. DNA was discovered in 1824 by a twenty-four Swiss Physician called Miescher after he isolated a compound that was neither a protein nor a carbohydrate nor lipid from nuclei of a white blood cell (Scitable, N.D.). It was thus identified as a unique type of a biological molecule and Miescher named nuclei because he had taken it from a nucleus of a cell. However, today the molecule is called DNA and nearly all the cells in an identified organism consist of exactly the same DNA.

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The molecular shape of DNA is double helix and composes of four nucleotide bases: adenine (A, C5H5N5), cytosine (C, C4H5H3O), guanine (G, C5H5N5O), and thymine (T, C5H5N2O2) whose orders is called DNA sequence (Griffiths, et al. 1999). The specific segments of the DNA that contain the genetic information are called genes. Usually, the genes are inheritable by an offspring from their parents at reproduction. A double-stranded DNA has two opposite linear strands that twist together to form a double helix. The chemical foundations of the helix are a composition of a phosphate and two sugar molecules (deoxyribose) connected by chemical bonds (Alberts, 2017). Rungs of the helix consist of a base pair of units either between C and G or A and T (Scitable, N.D.). The base pairs connect two sugar-phosphates through hydrogen bonds. At the nucleus, the DNA usually form a complex called Chromatin with the proteins. The complex occurs as a result of wraps between the DNA and the nuclear proteins so that the volume becomes smaller. The DNA chromosomes are recognized as X shaped and are is acquired due to DNA replication during cell division (Griffiths, et al. 1999).

Ionizing radiation result from unstable radionuclides/ radioisotopes that usually emit particles of high energies that are capable of displacing unstable electrons in an atom, a process known as ionization. This displacement facilitates an electron injection. The major forms of ionizing radiations are beta particles, alpha particles, X-rays and Gamma Rays. In comparing the three, alpha particles have a mass of approximately four times of a neutron or proton. It also about 8000 times heavier than the beta particle (Kudr & Heger, 2015). Because of its large mass, it has the highest ionizing power and the greatest ability to damage tissues. However, due to their large size, the particles have a less ability of penetrating matter. Usually, when trying to penetrate the matter, they quickly collide with molecules, acquire two electrons to become a harmless helium atom (Bewick et al. 2017). Alpha particles are less penetrative and can easily be stopped a layer of clothes, outer dead skin layer or a thick sheet of paper. The threat of alpha particles is only reduced from external sources but once they are inhaled through food, water or radiation emitters, one has no protection at all.

Beta particles are smaller than alpha ones and thus have a less ionizing power i.e. the ability to damage tissues. Their smaller size makes them very penetrative. Beta particles can be stopped a quarter inch thickness of a sheet of aluminum. The danger is especially dangerous once the particles are inhaled. Gama rays because of their high ionizing power, they are not the particle but rather an electromagnetic radiation. They are more powerful that X-ray and usually, do not have charge or mass. Their penetrative power is tremendous and usually require several inches of a dense material such as lead. Gamma rays can go through the human body even without striking anything. They have the least ionizing power (Bewick, et al. 2017).

When the DNA is exposed to Ionizing radiation, there can be damage through both direct and indirect action. The direct action occurs whenever beta, alpha or X-ray emit ions that cause a physical breakage the base pair of the DNA or cause breakage of one or the two sugar-phosphate backbone. Heavy particles such as alpha particles possess a greater probability of causing direct damage that the lighter particles such as X-ray and beta that damage by indirect effects. However, when such breakage occurs, the cell repairs itself through a continuous process known as excision that entrails three steps; endonucleases removes damaged DNA, resynthesis of the DNA by polymerase and litigation in which repair of sugar-phosphate backbone happens (Genetic Science Learning Center, 2014). However, if a DNA is wrongly repaired, and a wrong nucleotide instead, there is likely to be a mutation or cell death. Also, ionizing radiations can damage or impair cells indirectly by generating free radicals. Free radicals are molecules that are usually highly reactive because of the presence of unpaired electrons on the molecule. These free radicals can form compounds e.g. hydrogen peroxide that consequently triggers harmful chemical reactions in the cell causing it to undergo structural changes. The structural changes can alter the function of a cell or cause its death (Genetic Science Learning Center, 2014).

In causing cancer, the ionizing radiation causes several transformations on the DNA through mutation of the nature substitution or frameshift mutation (Eric & Amato, 2006). There are two types of strand breakage that can be caused by ionizing radiation; single strand break that usually occurs when a sugar-phosphate backbone breaks. These are basically repaired with the opposite strand as a template with possibilities of frameshift mutation or base pair substitution. Double strand breaks are detrimental lesions produced by the ionizing radiation on the chromosome (Stephen, 2002). Since these breaks are difficult to repair, they cause mutation, the cell death, loss of cell fragments that result to joining of a non-homologous chromosome during repair. This leads to either amplification or loss of chromosomal material. All these events can lead to tumorigenesis i.e. tumor creation. For instance, a deleted chromosome encrypts a tumor suppressor of if an amplified region encrypts a protein with a cancer potential (oncogenic potential) (Stephen, 2002). In instances of genetic code damage and the cell does not undergo apoptosis, the mutation is inherited during cell division, perhaps causing cancer or other mutation.

References

Alberts, B. (2017). Molecular Biology of the Cell.

Bewik, S., Parsons, R., Forsythe, T., Robinson, S., Dupon, J. (2017). Types of Radioactivity: Alpha, Beta, and Gamma Decay. Libre Tests.

Eric J. H & Amato J. G. (2006). Radiobiology for the Radiologist. Lippincott Williams & Wilkins.

Genetic Science Learning Center (June 10, 2014). Learn Genetics. Retrieved on 19th November, fromhttp://learn.genetics.utah.edu/Griffiths A.J.F, Gelbart W.M. Miller J.H. (1999). Modern Genetic Analysis. New York: W. H. Freeman.

Kudr, J., & Heger, Z. (2015). Effects of ionizing radiation on nucleic acids and transcription factors. J. Metallomics Nanotechnologies, 4, 22-29.

Scitable. (N.D). Deoxyribonucleic acid/DNA. https://www.nature.com/scitable/definition/deoxyribonucleic-acid-dna-107Stephen P. J. (2002). Sensing and repairing DNA double-strand breaks: Carcinogenesis, 23(5) 68-696.