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Silent Mutation: Definition & Example

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  • 0:00 Definition
  • 0:44 Central Dogma of…
  • 1:32 The Genetic Code
  • 2:01 Point Mutations
  • 2:37 Examples & Consequences
  • 3:51 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, or changes in the DNA sequence of a cell, don't always have negative effects. In fact, the effects are sometimes completely unnoticeable. In this lesson, learn about so-called silent mutations and test your knowledge with a quiz.

Definition

A mutation sounds like a bad thing, right? Well, except in the cases of certain movies and comics, where mutants gain special powers. In any case, you'd think that a mutation would have some effect on an organism, right? Surprisingly, this isn't always true.

Mutations are changes in a DNA sequence, and they can occur due to radiation or other types of DNA damage. If mutations occur in the part of a gene that encodes a protein (the coding region of a gene), they can change the amino acid sequence of that protein.

But not always; a silent mutation is a type of mutation in the coding region of a gene that doesn't actually change the amino acid sequence of the protein that is made.

The Central Dogma of Molecular Biology

Perhaps you already know about the central dogma of molecular biology. It describes how genetic information is carried from the DNA sequence to the amino acid sequence of a protein product.

The amino acid sequence of a protein is very important, because it dictates how the protein will fold into a 3-dimensional structure. Proteins need to have very precise structures in order to perform their functions in the cell. Thus, amino acid sequence changes can lead to nonfunctional proteins.

The central dogma says that DNA is first copied into messenger RNA in a process called transcription. Then, during translation, the messenger RNA sequence is decoded into the amino acid sequence of a protein.

How does this decoding work?

The Genetic Code

Messenger RNAs are 'read' by the ribosome and transfer RNAs based on sets of three nucleotides called codons. Every possible group of three RNA nucleotides (A, G, C and U) has a meaning during translation. Most codons direct the addition of a certain amino acid to the chain, and a few codons tell the ribosome when to start or stop translating.

In this table, you can see what each codon means.

The meaning of each possible codon is shown in this table.
A table showing all of the possible codons.

Point Mutations

A point mutation is a change in one nucleotide or base pair of a DNA sequence. An example would be if a C in the DNA code were changed to a T.

Point mutations can happen anywhere along a DNA sequence, but in this lesson we will stick to mutations that occur in the coding regions of the genes. Mutations in the coding region of DNA can directly affect the amino acid sequence of the protein product.

There are a few types of point mutations, including nonsense mutations, missense mutations, readthrough mutations, and silent mutations.

Examples and Consequences

In this lesson, we will learn about silent mutations, which are mutations that do not change the amino acid sequence of the protein product.

In this diagram, you can see an example of different types of point mutations including a silent mutation.

A diagram showing different types of point mutations.

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