Ionising Radiation and Its Effects on DNA | Genetics

In this article we will discuss about: 1. Introduction to Ionising Radiation 2. Measuring Ionising Radiation 3. Effects.

Introduction to Ionising Radiation:

Due to their shorter wavelength, X-rays and gamma rays penetrate tissues deeper than visible and UV light. They can impart enough localised energy to absorbing tissue to ionize atoms and molecules. When a highly energetic wave moving at high speed is stopped, it releases energy. This energy makes an atom lose an electron and become a charged particle or ion. The process is called ionisation.

The free moving electron causes other atoms to lose electrons and become positively charged ions. The two processes generate pairs of positively and negatively charged ions. A number of ions may be clustered together to form an ion track.

Ions undergo chemical reactions to neutralize their charge to reach a more stable configuration. While doing so they (ions) produce breaks in chromosomes (DNA) thereby inducing mutations. The free ions moreover, may combine with oxygen and produce highly reactive chemicals which may also react with DNA and cause mutagenesis.

Some ionising radiation is electromagnetic such as X-rays and gamma rays and some consists of subatomic particles such as electrons, protons, neutrons and alpha particles. X-rays and gamma rays have a low rate of linear energy transfer as they produce ions sparsely along the ion track and penetrate deeply into the tissue. Charged particles have a higher linear energy transfer, they do not penetrate deeply and produce more damage than X-rays and gamma rays.

Measuring Ionising Radiation:

Radiation is measured in terms of an ionisation unit called roentgen or r unit, one r being equal to 1.8 x 10⁹ ion pairs per cubic cm of air. In tissue which is ten times as dense as air, a high energy radiation produces about 1000 times the number of ion pairs per cubic cm as it does in air.

Another unit called rad measures the total amount of radiant energy absorbed by the medium. One rad equals 100 ergs per gram of tissue. Another unit called gray is equivalent to 100 rads.

In the case of X-rays about 90 % of the energy left in the tissue is used to produce ions, the rest produces heat and excitation. Ultraviolet (UV) is a non-ionising type of radiation and is measured in rads instead of r units. When ionisation is caused by subatomic particles, the doses are measured in different units called rem and sievert.

One rem is defined as the amount of any radiation that produces a biological effect equivalent to that resulting from one rad of gamma rays. A sievert is equal to 100 rems. For detecting radiation the Geiger-Muller tube is used. The tube contains a gas which is ionised by radiation. The amount of radiation is gauged from suitable amplifiers and counters.

Effects of Ionising Radiation on DNA:

Zirkle in 1930 showed that in plants the nucleus is more sensitive to ionising radiation than the cytoplasm. It is now known with certainty that many molecules including DNA are affected by ionising radiation. The purines are less sensitive to radiation than pyrimidines.

Out of the pyrimidines, thymine is most sensitive. Large doses of ionising radiation destroy thymine, uracil and cytosine in aqueous solutions. By depolymerizing DNA, ionising radiations prevent DNA replication and stop cell division.

Several mechanisms have been proposed to explain the effects of X-rays and gamma rays. They can break different kinds of chemical linkages and damage genetic material in a variety of ways. Figure 20.2 shows that the effect may be direct or indirect. When a hydrogen atom consisting of one proton and one electron is ionised, the free electron may directly interact with DNA.

Or the electron may interact with a molecule of water to produce OH, a free radical which can cause damage to DNA in the same way as the free electron. The following types of destruction of DNA are possible; hydrogen bonds may break between chains; a base may be changed or deleted; a single or double chain fracture may occur; cross linking might take place within the double helix; a deoxyribose may become oxidised.

If a cell is irradiated in the S phase, DNA replication is inhibited resulting in failure of cell division and cell death. But if the cell is irradiated during mitosis or in G1, in that case DNA replicates normally but mitosis is delayed.

Ionising radiation causes breakage and rearrangements in chromosomes which may interfere with normal segregation of chromosomes during cell division. When breaks in two different chromosomes in a cell occur close together in time and space they can join to produce chromosomal aberrations such as inversions, translocations and deletions.

Micro-organisms are more resistant to ionising radiation than higher organisms. It is found that D₃₇ dose, that is the radiation dose to a cell population with 37% survival is about 2000 to 30000 rads in bacteria. In human cells D₃₇ is about 120 rads.

Some chemicals have a protective effect on the cell in reducing the effect of a radiation dose. Aminothiols which have an – SH and – NH₂ group separated by two carbon atoms are most powerful in reducing the effect. The protective effect is expressed as dose reduction factor (DRF).

DRF is the ratio of LD₅₀₍₃₀₎ for protected animals to LD₅₀₍₃₀₎ for unprotected animals. LD is the lethal dose or the amount of radiation that kills all individuals in a large group of organisms. LD₅₀₍₃₀₎ is the dose which kills 50 % of organisms within 30 days of exposure. LD₅₀ for dog is estimated to be 350 rads, for mouse 550, goldfish 2300.

Whether the natural background radiation, though small in amount is dangerous for human beings or not has been questioned. The background radiation consists mainly of cosmic rays, emissions from radioactive elements in the earth such as uranium, radium and thorium, as well as emissions from radioactive isotopes (carbon 14, potassium 40) occurring naturally in the body.

People living at sea level receive an average dose of about 0.8 millisievert of radiation per year. A study of the coastal area of Kerala in South India, a region having high background radiation, has revealed a high incidence of Down’s syndrome in the population. Radiation-induced genetic and chromosomal anomalies were also observed.