By now, you know what mutations are and what effects they can cause, but how do mutations happen? In this lesson, learn about one important cause of mutations, irradiation.
This little guy is a mutant.
Mutation is caused by DNA changes
That means that his DNA has been changed in such a way that, well, he's just not normal anymore. But how did this happen?
Mutations, or changes in DNA, can happen in a couple of different ways. Two major causes of mutations are irradiation and chemical mutagens. Irradiation is exposure to radiation and chemical mutagens are chemicals that cause changes to DNA sequences. In this lesson, we'll learn about how radiation exposure can cause mutations and even cancer.
Radioactivity and radiation are scary words - you probably associate them with accidents that can happen at nuclear power plants and a pretty harsh type of cancer treatment. But did you know that even in our normal, everyday lives, we're constantly experiencing radiation?
Radiation comes from many different sources, including the sun, other cosmic rays from outer space, X-rays at the doctor's office and at the airport security screening area, and even cell phones, microwaves and TV sets. You may also have had your home tested for radon, a natural source of radiation that comes from uranium in the soil and is the second leading cause of lung cancer after cigarette smoking.
Whoa! How do we even stay alive in the face of all this radiation? Well, first of all, not all radiation is harmful. And we're usually only exposed to the harmful kinds of radiation in very low doses.
Let's look at the different types of radiation. There's ionizing radiation and non-ionizing radiation. Ionizing radiation, as you might imagine, is a high-energy kind of radiation that causes ions and free radicals to form. The radiation has such high energy that it can knock an atom's electrons out of their normal orbit.
That atom was just sitting around, minding its own business, but now it's an ion or a free radical, and it can go on to cause lots of problems, such as breaking covalent bonds in other molecules. We'll come back to that in a minute. Examples of ionizing radiation are X-rays, gamma rays and alpha radiation, which is the kind of radiation that comes from the decay of radioactive elements.
Non-ionizing radiation, on the other hand, is much less harmful because the rays carry much less energy. Radio waves, light and even heat are examples of non-ionizing radiation. For the most part, these kinds of radiation do not cause any damage. However, ultraviolet light is a kind of non-ionizing radiation that can be harmful - it can cause mutations in DNA.
Mutations by Ionizing Radiation
All right, let's look at how ionizing radiation can cause DNA mutations. Like we just talked about, ionizing radiation causes normal atoms to turn into ions and free radicals, which are charged particles that can go on to break covalent bonds in other molecules. What are covalent bonds again? They are chemical bonds that hold molecules together. Covalent bonds are found all over the place; they hold the Hs and the Os together in water, H2O, for example.
Covalent bonds also hold DNA together. The backbones of the two DNA strands are made of nucleotides linked together by covalent bonds. So, if ionizing radiation comes along and breaks these bonds, we have a big problem: the DNA will be chopped up into tiny little pieces! The cell will try to repair these DNA breaks, but just like Humpty Dumpty, who broke into a million tiny pieces when he fell off a wall, it is very difficult for the cell to correctly put all those DNA pieces back together again.
Inevitably, some mistakes may be made, and these mistakes are mutations because they change the DNA sequence. This means that ionizing radiation can cause mutations in cells that are deep inside of us, not just the cells on the surface of our bodies. These mutations can eventually lead to cancer; ionizing radiation is known to cause leukemia and thyroid cancer, just to name a few.
But ionizing radiation is not only a bad thing! We can sometimes use it to our advantage. For example, you may have heard of treating cancer with radiation therapy. The goal of radiation therapy is to cause so many mutations in cancer cells' DNA that the tumor cells will die because of the damage. Of course, radiation damages healthy cells too, so doctors must take care to limit the exposure of a patient's normal cells to the radiation.
Another use for ionizing radiation is to sterilize foods and medical equipment. Gamma rays and X-rays are commonly used to kill off any microbial contaminants. Of course, we can only use types of radiation that disappear quickly, not longer-lasting forms of radiation that would make our food radioactive!
Mutations by UV Light
Remember when we said that non-ionizing radiation is a lot less harmful than ionizing radiation? That's true, but one type of non-ionizing radiation can still cause mutations, and that's ultraviolet (UV) light. You know that you're supposed to wear sunscreen when you go out in the sun to avoid getting sunburns and skin cancer, right?
Did you know that the reason your skin peels after a sunburn is because all of those skin cells got so many mutations that they died? Whoa. And skin cancer is caused by skin cells getting so many mutations over time that eventually the cells begin to divide out of control. Not good.
Let's look at how UV light causes DNA mutations in the first place. Pyrimidines, which are a type of nucleotide base in DNA, are really good at absorbing UV light. When this happens, two pyrimidines that are sitting next to each other, like two thymines or two cytosines, or even one of each, can form a dimer. That means that the bases change structure so that they are covalently stuck together. This is not normal behavior for these nucleotides.
Pyrimidine dimers prevent DNA bases from pairing normally with each other and create a bulge in the backbone of the DNA. This bulge makes it so that DNA replication cannot occur normally. When the DNA polymerase gets to the bulge, it will just stop and not be able to go on any further. So if it wants to survive and keep dividing, the cell has to do something to get around this block in DNA replication.
Unfortunately, the strategies that cells use to do this cause a lot of errors in the DNA. For example, they may start using a special DNA polymerase that can put in random nucleotides when there isn't even a template strand of DNA to read from. Or the polymerase might just assume that the pyrimidine dimer was a T-T and put an A-A in across from it.
But maybe it wasn't a T-T and the polymerase just caused a mutation. Whoops! This is kind of like someone typing up a handwritten essay but making a lot of typos along the way, so the typed copy has so many mistakes that it is really hard to read. Lucky for us, UV radiation can't penetrate very well into our bodies, so we only really have to worry about our skin cells getting damaged by UV. So wear sunscreen!
Today we learned how irradiation can cause mutations in DNA. We learned that there are two major types of radiation: ionizing radiation and non-ionizing radiation. Ionizing radiation, such as X-rays and gamma rays, is a high-energy kind of radiation that causes ions and free radicals to form. These ions and free radicals can go on to break covalent bonds in other molecules, such as DNA. When a DNA molecule is broken into pieces, the cell's best efforts at repairing the damage may not be good enough, and mutations will most likely occur.
We learned that non-ionizing radiation, such as radio waves, heat and light, is much less harmful because the rays carry much less energy. However, one kind of non-ionizing radiation, ultraviolet (UV) light from the sun, can cause mutations in DNA. It does this by causing pyrimidine dimers to form. Pyrimidine dimers occur when two pyrimidines, thymines or cytosines, that are sitting next to each other become covalently stuck together. This disrupts the structure of the DNA molecule and stops DNA replication. Unfortunately, some of the cell's strategies to circumvent this problem lead to mistakes in DNA replication and thus mutations.
At the end of this lesson you should be able to:
- Compare ionizing and non-ionizing radiation and give examples
- Explain the cellular process that results in mutations
- Give examples of beneficial radiation