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Missense Mutation: Definition & Example

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  • 0:01 Definition of Missense…
  • 0:36 DNA to RNA to Protein
  • 2:13 Missense Mutation Example
  • 4:11 Lesson Summary
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Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

While people with sickle-cell disease show many symptoms, such as pain and fatigue, this disease is caused by only a single change in a DNA sequence. You can learn more about this kind of change, called a missense mutation, in this lesson.

Definition of Missense Mutation

A mutation is a permanent change in a DNA sequence. DNA is the unit of heredity of all organisms, so this means that mutations can often be passed on to offspring. You probably associate the word mutation with something that is bad, but mutations don't have to be bad. Some are beneficial, and some can have no effect.

There are many different categories of mutations, but in this lesson we will focus on missense mutations. A missense mutation occurs when one DNA nucleotide is changed so that a different amino acid is inserted into a protein.

DNA to RNA to Protein

Let's back up a little and talk about what DNA is and what it does. Nucleotides are the repeating units of a DNA sequence. There are four nucleotides, each with different nitrogenous bases: thymine (T), adenine (A), guanine (G), and cytosine (C).

The coding region of a DNA molecule is made up of genes that code for proteins. Proteins are strings of amino acids. It is the order of nitrogenous bases that determines the sequence of amino acids in a protein. There are two processes involved in using DNA to make proteins:

  1. Transcription uses a DNA sequence to make an RNA molecule. RNA is made up of nucleotides and nitrogenous bases just like DNA, except that thymine is replaced with uracil or U.
  2. Translation uses an RNA sequence to string amino acids together and build a protein. Amino acids are specified by a three-letter sequence in RNA called a codon.

The whole process of transcription and translation that involves going from DNA to RNA to protein is called the central dogma of molecular biology.

The set of rules that indicates which codons specify which amino acids during translation is called the genetic code. There are a total of 64 possible codons and 20 amino acids. This means some amino acids are specified by multiple codons. The genetic code is shown in this chart.

The Genetic Code: All possible codons and which amino acids they specify.
The Genetic Code

Missense Mutation Example

Remember, a missense mutation is a mutation in which one DNA nucleotide is switched out with another one in a way that changes the amino acid specified. When a missense mutation occurs in a DNA molecule, it changes one of the RNA codon sequences made during transcription. This different codon will then cause a different amino acid to be inserted into a protein during translation. Proteins made when missense mutations occur may look very different and may not even be functional.

A good way to think about missense mutations is to imagine you have a sentence in which one letter was changed and the entire meaning was also changed. For example, 'I have a cat' is like your normal DNA sequence. If for some reason the 'c' was changed to an 'r', now you have 'I have a rat'. This new sentence has a very different meaning than the original. The same is true of missense mutations because the new DNA sequence will code for a different amino acid.

Let's look at a real life example. Humans have genes that make the protein betaglobin, which is an important sub-unit of the molecule hemoglobin. Hemoglobin is important for transporting oxygen in the blood. Normally, one betaglobin gene has an A as the second letter in the sixth codon (GAG). This codon normally specifies the amino acid glutamic acid (Glu). If this one A is changed to a T, then the new codon (GTG) now specifies valine (Val).

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