Nonsense Mutation: Definition & Example

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  • 0:00 Nonsense Mutation Definition
  • 0:57 Genetic Code Review
  • 1:36 Point Mutations
  • 2:15 Example of Nonsense Mutation
  • 3:04 Consequences of…
  • 4:07 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Mutations are changes in a cell's DNA sequence. When a mutation happens in the coding sequence of a gene, the resulting protein is changed. In this lesson, discover what a nonsense mutation is and what effects it has on the protein that is made from a gene.

Nonsense Mutation Definition

When you think about a mutant, you might think about sci-fi movies where mutated creatures become powerful and evil and then attempt to destroy the world. But what are mutations, really? Mutations are changes to a cell's DNA sequence, and there are several different types.

A nonsense mutation is a point mutation that introduces a premature stop codon into the part of the gene that encodes a protein. A stop codon is like a period at the end of a sentence. It instructs the ribosome to stop making the protein. So, if a mutation leads to an early stop codon, only part of the protein will be made. Half-baked proteins that result from nonsense mutations are often nonfunctional or defective. Now let's learn more about how nonsense mutations work and their consequences.

Genetic Code Review

The DNA sequence of the coding region of a gene is transcribed into a messenger RNA molecule, whose sequence is in turn read and decoded by the ribosome and transfers RNAs during translation.

The sequence of a messenger RNA is read in groups of three nucleotides called codons. Each possible combination of the three RNA nucleotides (A, G, C and U) codes for an amino acid, translation start or translation stop. What you're looking at on screen is a table that shows what each codon means to the ribosome and transfer RNAs.

This table shows all of the possible codons and their meanings.
A table showing all of the possible codons.

Point Mutations

Point mutations are changes in one base pair of a cell's DNA sequence. For example, if an A in the DNA code is changed to a C, that is a point mutation. Point mutations in the coding region of a gene can have different effects depending on the resulting changes to the codons in the messenger RNA.

There are a few major kinds of point mutations: missense mutations, nonsense mutations, silent mutations and readthrough mutations.

Here, we're going to focus on nonsense mutations, which are mutations that introduce a premature stop codon into the coding sequence of a gene.

Example of a Nonsense Mutation

This diagram shows an example of a nonsense mutation.

A diagram showing different types of point mutations.

Under 'No mutation,' you can see that the normal DNA sequence at this locus is TTC, which is transcribed to the sequence AAG in the messenger RNA. AAG normally codes for the amino acid lysine at the protein level.

Now let's imagine that the cell undergoes some radiation or other DNA damage and a point mutation occurs that changes the TTC to ATC. This is a nonsense mutation, and here's why: now, the DNA sequence will be transcribed to UAG in the messenger RNA. UAG is a stop codon, which tells the ribosome to stop translating and let the protein go out into the world of the cell to begin doing its job.

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