Effects of Mutations

Test your knowledge with free interactive questions on Seneca — used by over 10 million students.

Effects of Mutation

The sequence of bases in DNA encodes the amino acids that are used in protein synthesis. If this sequence is altered by mutation, this could influence the proteins an organism produces.

Encoding amino acids

There are four different bases in DNA (A, T, C, G).
Three bases in a sequence encode an amino acid. This is called the triplet code or a codon.
Every codon encodes a different amino acid.
Amino acids are joined together to form proteins.

Protein 3D structure

Every amino acid has a side chain (called the R group) that differs slightly.
The bonds that can form between the R groups of different amino acids within a protein can give rise to a complex 3D structure.
The 3D structure of proteins is very important for their function (e.g. the shape of an enzyme active site is determine by its amino acids).

Mutations

Mutations give rise to a change in the base sequence in DNA.
This could alter the amino acids that the DNA encodes.
- E.g. CTG encodes leucine. If this was inverted to GTC, it would encode valine.
If the amino acids in a protein are changed, the 3D structure of the protein may break down.

Effect of mutation

If a protein's structure is affected by a mutation, this could have harmful effects.
Some genetic disorders are caused by mutations in this way.
- E.g. Cystic fibrosis is caused by a mutation that affects proteins in the epithelial cells of the respiratory system.

The Degenerate Code

Some mutations have no effect on protein structure because of the degenerate nature of DNA. This is because some amino acids are encoded by more than one base triplet code.

Triplet codes

Some amino acids are encoded by more than one codon (e.g. tyrosine is encoded by TAT and TAC).
The fact that some amino acids correspond to multiple codons means that DNA is degenerate.

Mutations

The fact that DNA is degenerate means that a mutation in the base sequence does not necessarily affect the protein.
If one base is substituted for another base, (e.g. CAA → CAG), the mutated codon may still encode the same amino acid (e.g. both encode glutamine).

Frameshift

Sometimes a mutation can have a very large impact on the protein structure if it impacts all the base triplets downstream from the mutation. This is called a frameshift.

Causes of a frameshift

A frameshift is caused when a base (or multiple bases) is inserted, removed or duplicated into the genome.
This type of mutation has altered the number of bases in the DNA sequence.
This means that all the triplet codes downstream from the mutation will experience a shift.

Effects of a frameshift

A frameshift causes all amino acids that follow the mutation to be affected.
If only one amino acid in a protein is changed in a mutation, the protein may be able to function as normal.
A frameshift impacts many amino acids so it is much less likely that the protein will function as normal.

E.g. Crohn's disease

Crohn's disease is an inflammatory bowel disease that is caused by a frameshift mutation.
When Cytosine is inserted at position 3020 of the NOD2 gene, the subsequent codons are shifted.
This causes the protein to be shortened and it cannot function properly.