Gene Mutation Alters the DNA Sequence of a Gene
Mutations can cause two basic types of changes to a gene:
- The base sequence within a gene can be changed
- One or more nucleotides can be added to or removed from the gene.
A point mutation affects only a single base pair within the DNA.
For Example, the DNA sequence shown here has been altered by a base substitution:
A point mutation could also involve the addition or deletion of a single base pair to a DNA sequence.
For example, in the following sequence, a single base pair has been added to the DNA:
Gene Mutations May Affect the Amino Acid Sequence of a Polypeptide
Silent Mutations do not alter the amino acid sequence of the polypeptide, even though the nucleotide sequence has changed.
Because the genetic code is degenerate, silent mutations can occur in the third base within most codons without changing the type of amino acid it encodes.
Silent Mutations are considered neutral mutations, because they don’t affect the function of the encoded protein.
Missense Mutation is a base substitution that changes a single amino acid in a polypeptide sequence.
Missense mutations may not alter protein function because they change only a single amino acid within polypeptides that are hundreds of amino acids in length.
IMPORTANT: Alternatively, some missense mutations have a dramatic effect on protein function.
Two Other Types of Point Mutations
Two other types of point mutations cause more dramatic changes to a polypeptide sequence.
Nonsense Mutation: involves a change from a normal codon to a stop or termination codon. This causes translation to be terminated earlier than expected, producing a smaller polypeptide.
A shorter polypeptide is much less likely than a normal polypeptide to function properly.
Frameshift Mutation: involves the addition or deletion of nucleotides that are not in multiples of three nucleotides.
For Example: a frameshift mutation could involve the addition or deletion of one, two, four, or five nucleotides.
Because the codons are read in multiples of three, this shifts the reading frame so that a completely different amino acid sequence occurs downstream from the mutation.
Gene Mutations Can Occur Outside of Coding Sequences and Influence Gene Expression
Effects of Mutations Outside of the Coding Sequence of a Gene
Effect of Mutation
|May increase or decrease the rate of transcription|
Transcriptional Response Element/ Operator Site
|Molecules and macromolecules associate with each other to form larger structures such as membranes. A cell is formed from the association of these larger structures.|
|May alter the ability of pre-mRNA to be properly spliced|
Translational Response Element
|May alter the ability of mRNA to be translationally regulated|
Insertions: add one or more extra nucleotides into the DNA. They are usually caused by transposable elements, or errors during replication of repeating elements.
Deletions: remove one or more nucleotides from the DNA. Like insertions, these mutations can alter the reading frame of the gene.
Chromosomal translocations: interchange of genetic parts from non-homologous chromosomes.