- Addition occurs when an extra base is added into the sequence. All the bases that follow are also moved along. This is called a frame shift.
Original base sequence AGC TAC GTA CGT
Becomes: AGC ATA CGT ACG T...
The codons that come after the addition will now be affected and most amino acids will be changed into another amino acid.
- Deletion involves the loss of a base, which causes a frame shift in the other direction as all the bases move along to replace the one that has been lost.
There are three consequences of a gene mutation. The change in the code caused by the mutation may mean that a different polypeptide, hence a different protein is produced by the gene. When the mutated DNA replicates, the mutation passes onto other cells.
Mutations may be lethal. The new amino acid caused the protein to be significantly different to the original so that it does not perform the same function in the organism. Mutations that cause frame shifts are more likely to be lethal than mutations that substitute, because they cause more changes in the sequence.
A mutation may have no effect at all. The protein may still function despite the new amino acid because the change does not alter the tertiary structure or the shape of the protein which interacts with other chemicals. Some amino acids are coded by several different codons, so there may not be a change in the amino acid.
The mutation may be beneficial. The new amino acid may alter the protein in a way that is beneficial to the organism.
Examples of Mutations
Mutations are normally completely random occurrences in DNA although the rate of mutation can be greatly increased by mutagenic agents. Mutagens may cause DNA molecules to break, or change a small section of DNA chemically. High energy radiation, including X-rays, gamma rays, and ultraviolet light, are mutagens. X-rays and gamma rays can penetrate deep into the body and may cause mutations in any tissue.
Chemicals in tobacco tar also causes mutations in the lungs, destroying alveoli, making it hard for the person to breath in enough oxygen. Mustard gas also causes mutations and is very harmful to the body.
