Damage to DNA

Damage to DNA (Christoph Burgstedt, iStockphoto)

Radiation Effects on Cells & DNA

Let's Talk Science

Summary

This backgrounder explains the effects of radiation on cells and DNA.

What is ionizing radiation?

Ionizing radiation is radiation that can remove electrons from an atom. Losing an electron charges, or ionizes, the atom. Sometimes, ionizing radiation takes the form of a wave, like gamma rays or X-rays. But it can also take the form of a particle, like neutrons or alpha and beta particles.

Natural sources of radiation
Natural sources of radiation (© 2019 Let’s Talk Science).

You are surrounded by ionizing radiation. The Earth has always been home to radioactive materials. Here are some natural sources of ionizing radiation:

Did you know?

The soil, water and air naturally contain more than 60 radioactive materials.

Human technology also produces ionizing radiation. For example, it is used in medical X-rays and computed tomography (CT) scans.

How does ionizing radiation affect cells?

When ionizing radiation interacts with a cell, several things can happen: 

  1. The radiation could pass through the cell without damaging the DNA.
  2. The radiation could damage the cell’s DNA, but the DNA repairs itself.
  3. The radiation could prevent the DNA from replicating correctly.
  4. The radiation could damage the DNA so badly that the cell dies. This is called apoptosis. One dead cell is not a big problem. After all, millions of your cells die every day. But if too many cells die at once, the organism could also die.

What is DNA?

Every one of your cells contains deoxyribonucleic acid (DNA). This important molecule is like your body’s instruction manual. It constantly tells your cells what to do and how to do it. Every living organism has DNA in all of its cells. 

A DNA molecule is built like a twisted ladder. The long rails are made of sugar and phosphate molecules. These are called the “backbone” of a DNA molecule. Each rung is a combination of four nucleotide bases. These are adenine, guanine, cytosine, and thymine. Each nucleotide has a letter that represents it. “A” stands for adenine, “G” for guanine, “C” for cytosine and “T” for thymine.

DNA Structure
DNA structure showing the sugar-phosphate backbone and nucleotide bases (Let’s Talk Science using an image by Dosto [CC BY-SA 3.0] via Wikimedia Commons).

 

Ionizing radiation can affect DNA through either direct action or indirect action.

How does direct action affect DNA?

Ionizing radiation can interact directly with a DNA molecule’s atoms. This prevents cells from reproducing. Direct action can also damage critical cellular systems. Sometimes, it can even lead to cancer.

Alpha particles, beta particles and X-rays can directly affect a DNA molecule in one of three ways:

  1. Changing the chemical structure of the bases;
  2. Breaking the sugar-phosphate backbone; or
  3. Breaking the hydrogen bonds connecting the base pairs.
The direct effects of ionizing radiation
The direct effects of ionizing radiation can include: 1) Changing the chemistry of a nucleotide 2) Breaking the sugar-phosphate backbone and 3) Breaking hydrogen bonds between bases (Let’s Talk Science using an image by Dosto [CC BY-SA 3.0] via Wikimedia Commons).

Direct action can lead to either DNA damage or DNA mutations. Keep in mind that these are two different things. 

DNA lesions involve physical damage. This includes changes to the chemical structure of the DNA molecule (numbers 2 and 3 in the list and diagram). 

DNA mutations involve changes to the sequence of base pairs (number 1 on the list and diagram). If DNA damage is not repaired, it can lead to mutations. Mutations can prevent genes from making correct proteins. This can be very harmful to an organism.

How does indirect action affect DNA?

Ionizing radiation can also affect important molecules other than DNA. For example, it can break the bonds holding water molecules together. This creates hydrogen (H+) and hydroxyls (OH-) ions. These are called free radicals

Ball and stick diagrams of water, hydrogen ions and hydroxyl ions
Ball and stick diagrams of water, hydrogen ions and hydroxyl ions (© 2019 Let’s Talk Science).

Free radicals are highly reactive. This means that they easily combine with other ions inside cells. For example, hydroxyl ions (OH-) can react with hydrogen atoms inside a DNA molecule to form hydrogen peroxide (H2O2). This can cause the types of DNA damage we talked about earlier. Over time, damage from free radicals can build up. Scientists think that free radical damage contributes to aging and diseases like cancer, Alzheimer’s and Parkinson’s.

How sensitive are cells to radiation?

Some cells, like blood and reproductive cells, divide more often than others. These types of cells are much more sensitive to radiation. For example, embryos contain a lot of rapidly dividing cells. As a result, they are very sensitive to radiation. That is why pregnant women should limit their exposure to radiation. Fast-growing tumour cells are also very sensitive to radiation. That is why cancer therapy uses radiation to kill cancer cells. 

You are surrounded by ionizing radiation. It can affect cells through direct and indirect action, causing DNA damage as well as mutations. This can be especially harmful to cells that divide very quickly. But sometimes this can be a good thing, like when doctors use radiation to fight cancer.

 

Learn More

What to know about radiation sickness (2017) Christian Nordqvist -- Article discussing the risks of radiation exposure and some of the symptoms of radiation sickness

DNA Replication: Copying the Molecule of Life (2016) Professor Dave Explains (6:15) A video that explains DNA replication; includes background information and diagrams to depict the processes.

References

Advisory Committee on Human Radioactive Experiments. (n.d.). How does radiation affect humans? Georgetown University.

Canadian Nuclear Safety Commission. (2015). Types and sources of radiation.

National Human Genome Research Institute. (2015). Deoxyribonucleic acid (DNA) fact sheet.

Non-Destructive Testing Resource Center. (n.d.). Cell radiosensitivity.