What Is a Mutation?

A mutation is simply a change in the set of instructions, the DNA, of living things including people. And mutations are really common: they aren’t as exciting or rare as science fiction like the X-Men might have you believe.

In fact, every living thing has changes in its DNA that it did not have when it was born. These mutations are ultimately the reason for all of the wonderful variety we see in nature.

What Exactly Are DNA Mutations?

A DNA mutation is a change in the set of instructions for building and running a living thing. These instructions are written in the DNA using a set of four simple molecules called nucleotides or nitrogenous bases.

The four bases in DNA are thymine (T), adenine (A), guanine (G), and cytosine (C). So a mutation is a change somewhere in the long string of As, Gs, Cs and Ts that make up a living thing's DNA.

Just like there are lots of ways to change the instructions in a book, there are lots of ways to change them in our DNA too. These can be big changes that affect a large amount of DNA or tiny changes that only alter the DNA at a single position. In humans, that would translate to a change that affects around one in six billion bases.

Here are some of the kinds of mutations:

• Insertions: An insertion adds extra bases to a DNA sequence. For example, the DNA sequence CAGTAG becomes CATGTAG.
• Deletions: A deletion takes out individual bases or an entire segment of DNA. For example, the DNA sequence CAGTAG becomes CATAG.
• Substitutions: A substitution exchanges one base for another. For example, the DNA sequence CAGTAG becomes CAATAG.
• Inversions: An inversion happens when a sequence of DNA is reversed. For example, the DNA sequence CAGTAG becomes GATGAC.
• Repeat expansions: A repeat expansion copies the same DNA sequence one or more times. For example, the DNA sequence CGGCGGCGG becomes CGGCGGCGGCGGCGGCGG .

What Causes Mutations?

Mutations can happen in lots of ways. One way is when the cell makes a mistake when it copies its DNA. Another way is when something from the outside world, like UV light from the sun, damages the DNA. When the cell repairs the damaged DNA, it will sometimes make a mistake resulting in a mutation.

If the mutation happens in a sperm or egg cell, it can be passed on to the next generation. This is called a hereditary mutation and ends up in every cell in the offspring in the next generation. If the mutation happens in any other cell, it is called an acquired mutation. It will not be passed on to the next generation.

What is the Impact of Mutations in DNA?

Most mutations are neutral, meaning they don't affect things one way or the other. A few can cause problems, and a few others can even be helpful. It all depends on where in the DNA the mutation happens to fall.

One area in which mutations can have a big effect is if they happen in large segments of DNA called genes. Mutations within these genes can sometimes affect things in a big way. For example, skin cancer usually happens because the UV light from the sun damages the DNA within a gene, causing a mutation. But not all mutations are so severe. The genes in our skin cells have many UV-induced mutations that have no effect.

Mutations that happen in the 98 percent of your DNA that isn't genes are less likely to have an impact, but they still can. For example, a few thousand years ago someone ended up with a mutation outside of the OCA2 gene. Ultimately this led to blue eyes becoming common in Europe.

When Does DNA Polymerase Check for Mutations?

Cells use DNA polymerases to copy their DNA. While the DNA polymerase is copying the DNA, it is also checking to see if it made a mistake. If it does add the wrong base, it cuts it out and replaces it. This process is called proofreading.

How Does DNA Polymerase Prevent Mutations?

Sometimes the proofreading done by the DNA polymerase doesn't catch every mistake, so a new DNA strand may have the wrong bases. Other enzymes can recognize these errors and bind to the DNA strand to cut out the incorrect bases. Then, DNA polymerase can replace the bases.

This process can happen immediately after replication or later on, such as after DNA is damaged by radiation. In this way, DNA polymerase is still able to prevent mutations, even if it doesn't catch them during replication.