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Point Mutations: Types, Processes & Effects

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

You might not think about your DNA that often, but you would probably notice if something was mutated on your DNA! This lesson will discuss what happens when your DNA gets mutated, focusing specifically on point mutations and their impacts.

What Is a Point Mutation?

What do cystic fibrosis, a form of dementia similar to Alzheimer's disease, and sickle-cell anemia have in common? They are all the result of a point mutation, which is when bases are changed in a person's DNA. But in order to understand that definition, we'll need to talk a little more about genes, bases and mutations. So let's get started!

Your DNA, or deoxyribonucleic acid, is the blueprint for how to make you. It gets nicely packaged into chromosomes, which can be found in your cells. There are sections of your chromosomes called genes, which code for specific amino acids. When you get a bunch of amino acids together you have a protein and proteins serve many, many functions in your body, from making muscle to producing insulin.

Overview of a chromosome and gene
chromosome

If you were to zoom in on your DNA, you'd see the following bases, or parts of the nucleotides that make up your DNA:

  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)

These bases pair up: adenine and thymine pair together, and guanine and cytosine pair together. Often you'll see the bases simply written as letters. As you can probably guess, Adenine is 'A', Thymine is 'T', Guanine is 'G', and Cytosine is 'C'.

Take a moment to review the base-pairs on DNA
bases

Okay, now you're ready to look at the definition of a point mutation again. When one of the bases gets changed, you have a point mutation. A point mutation can result in the wrong amino acid getting produced, which alters the protein (remember, a protein is made when a bunch of amino acids are linked together). And if the protein gets altered, you can have all sorts of problems! Let's take a look at the different types of point mutations so you can get a better idea of what can happen when your DNA gets mutated.

Types of Point Mutations

We are going to focus on the following point mutations: frameshift, silent, nonsense and missense. Let's start with a frameshift mutation, where there is an insertion or deletion of one or more bases. In other words, bases gets added or removed. Why would that matter, you ask?

Well, it takes three bases to code for an amino acid, and this is called a codon.

The DNA gives the codon and you can see which amino acid goes with which codon
codon

For example, if you have cytosine followed by two adenines, you get the amino acid valine (abbreviated val). When one base is added or deleted due to a mutation, it shifts the bases and causes a frameshift mutation. Let's take a look at a section of DNA to illustrate this. Say you have the following bases:

GAT CTC AAA

Before the mutation, you had GAT coding for the amino acid aspartic acid, CTC coding for leucine and AAA coding for lysine. So, pretend you have a deletion of the adenine (A) out of GAT. Everything is going to shift. Now you have:

GTC TCA AA

Instead of aspartic acid, you will get valine, instead of leucine you get serine and instead of lysine you may not get any amino acid since AA is not a complete codon. You can see how a frameshift mutation can change all of the amino acids in the protein, thus changing the protein itself. Cystic fibrosis is an example of a frameshift mutation. It's a genetic disease that causes the production of excessive mucus that damages the lungs and digestive system. It is caused by a deletion that results in an amino acid not getting produced.

Next let's check out a silent mutation. This is when a point mutation occurs, but it doesn't change the organism. It may be a mutation that doesn't occur on a gene so the codons aren't affected, or it may code for a similar amino acid that still functions correctly.

In a nonsense mutation, a codon is altered in such a way that it leads to a premature stop before the protein is done. There are a few codons that indicate the amino acids can stop being added and the protein is done. Let's say you had the codon TAC, which codes for the amino acid tyrosine. If you had a nonsense mutation, the cytosine (C) would get replaced by guanine (G) and you'd have TAG, which means 'stop'. The result is an incomplete protein, which may be totally ineffective. There is a rare disease called familial British dementia where the protein that should be produced is stopped short due to a nonsense mutation. The disease usually appears when the person is around 60 and results in dementia similar to Alzheimer's disease.

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