Mutagens: Non-Ionizing Radiation

Non-ionizing radiation has enough energy to move atoms in a molecule around or cause them to vibrate, but not enough to remove electrons from atoms.

Non-ionizing radiation This is radiation from:

1. Radio waves

2. Microwave

3. Heat

4. Visible lights

Of the non-ionizing radiation, UV (ultraviolet) radiation from the Sun has the greatest effect. UV radiation has low-penetrating power (about a few mm) so in humans it can cause changes only in the surface skin cells. Most of this radiation is absorbed by the ozone layer. Occurrence of damaged zones in the ozone envelope ie. ozone holes, due to excessive pollution leads to DNA damage in skin cells which causes cancer. It affects the structure of DNA chain by deforming the double helix and thus disrupting replication. This radiation has a cumulative effect - so that small doses received continuously throughout life are added up and can adversely affect a cell or an organism.

Temperature

DNA consists of two helix chains in which 4 nucleotides alternate (adenine, thymine, guanine, cytosine). Each base in a chain is paired has only one correct base pairing on the opposite chain. Such pairing is called complementary pairing. Purine is paired with a pyrimidine though a hydrogen bond, so Adenine pairs only with Thymine (two hydrogen bonds) and Citosine only with Guanine (with three hydrogen bonds). Since the bonds are not covalent, they are easily broken and easily re-formed. What happens if we break the bonds though mechanical force (eg during replication or high temperature)?

In a laboratory environment, this can be caused in one of two ways:

1. Increasing or decreasing the pH value in the DNA molecule. This ionizes the bases and breaks down the hydroxyl bonds that hold these chains together

2. By increasing the temperature of these molecules, thus increasing the energy of the bases which disrupts and breaks the hydroxyl bonds among them. In this process, known as DNA melting, we separate the double helix of DNA and obtain two separate strands. Melting occurs suddenly and at a specific temperature - DNA melting point. The next question that arises is how can we measure how much of the DNA has separated. By measuring the amount of UV light absorbed by this molecule. The line of reasoning behind this method is the following: in normal molecules the fused bases are too busy forming and maintaining these hydroxyl bonds that they cannot absorb the same amount of Uv radiation that a broken-off (free) molecule could.

With the help of this we can conclude, though experiments, that there is a sudden jump in the absorption of UV rays at a temperature of 72 degrees. Accordingly, on temperatures higher than 72 degrees covalent hydroxyl bonds which connect the bases are broken. If these temperatures are lowered to e.g. 60 degrees these connections will begin to re-form, a process known as DNA annealing. This process cannot happen in our body because of the dominance of homeostasis, ie. a self-regulatory process that returns the organism to its normal state. The separation of DNA strands in our cells happens during the replication process, but it does not happen by changing the pH or temperature, but through the use of ATP powered enzymes.

Risks connected to non-ionizing radiation

Dependent on the energy and exposure time, non-ionising radiation can cause localised heating, or photochemical reactions which can lead to possible permanent harm under special circumstances. Therefore, it is advised that exposure to such sources of emition be minimised.

Risks with ultraviolet light (UV) (100 – 400 nm)

Ultraviolet light is emitted by hot light sources such as the sun, filament lamps, halogen lamps, sun beds, welding arcs (electrical welding) and gas discharge lamps (e.g. mercury lamps).

Excessive exposure mainly poses a risk for the eyes and skin. Acute effects are sunburn and cornea inflammation (welder’s flash). Long-term effects are skin cancer, skin thickening, premature aging of the skin and clouding vision (cataract).

Risks with radio wave and microwave fields (10 MHz – 300 GHz) Sources of radio wave and microwave radiation are high-frequency generators (gyrotrons, klystrons etc.) and transmitters, microwave ovens, industrial RF heaters, radio and TV antennae, radar installations and mobile phones. Exposure to electromagnetic fields in this frequency range can warm up exposed tissues through absorbing the radio wave and microwave energy and converting it into heat. Depth of penetration into the body depends on the frequency level of the source. This warming up is the most dangerous for the brain, eyes, genitals, stomach, liver and kidneys.