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Introduction and types of mutation

A genetic mutation is a change in the nucleotide sequence of DNA.

Most mutations only affect a single gene. This will affect a single protein.

Most genetic mutations will not be noticeable. However, mutations can produce an organism with a new phenotype. This phenotype may be advantageous or disadvantageous to an individual. As a result, mutations are controlled by natural selection.

A mutant gene that increases survival will spread throughout the population.

Sickle cell anaemia is a blood disorder. It is caused by an inherited mutation in the haemoglobin gene used by red blood cells.

Environmental factors can increase the rate of mutations in cells:

  • Radiation: X-rays and ionising radiation can cause cells to mutate.
  • Carcinogens: chemicals such as the tar in cigarettes can increase the mutation rate.
UV light can cause DNA damage that leads to mutations.
UV light can cause DNA damage that leads to mutations.

Mutations can be divided into two broad categories: small scale and large-scale.

Type of mutation Definition
Small scale These mutations affect only a few nucleotides or a gene.
Point mutation Single base substitution.
Insertion Nucleotides are incorporated into the DNA. These can be more severe than substitutions as they often result in a frameshift.
Deletion Nucleotides are removed from the DNA sequence. May also result in a frameshift..
Large scale These mutations have a significant effect on the entire chromosome.
Gene duplication Whole genes or sets of genes are duplicated.
Gene deletion Whole genes or sets of genes are removed from the chromosome.
Chromosomal translocation Genes from one chromosome are moved into another non-paired chromosome
Chromosomal inversion The sequence of bases in a gene segment is inverted (e.g. ATGC to CGTA).

A point mutation is a single base substitution. In a point mutation, one base is substituted for a different base.

Point mutations can be classed as either transitions or transversions depending on the class of base that is substituted.

  • A transition substitution is the swapping of one purine (adenine or guanine) with another, or one pyrimidine (cytosine or thymine) with another.

    Transitions are more common than transversions as they occur between bases of a similar structure.

  • A transversion substitution involves the exchange of a purine base for a pyrimidine base or vice versa.

There are three possible outcomes of a point mutation.

  • Silent mutation: the substitution results in a new codon that still codes for the same amino acid. There is no effect on the overall protein structure.
  • Missense mutation: the mutation results in the coding of a different amino acid. This results in proteins with different compositions.

    Sickle cell anaemia is the result of a missense mutation.

  • Nonsense mutation: the substitution causes a stop codon to be introduced in a coding sequence. This means that protein translation stops prematurely, resulting in a truncated protein.

    The disease thalassaemia is a result of a nonsense mutation of a protein.

A frameshift mutation is the result of the addition or deletion of nucleotides.

A frameshift mutation is often a more damaging mutation than a point mutation. It can result in changing virtually every amino acid in a protein.

If a mutation results in a cytosine base being inserted into the sequence TAT CCG TCG ACG TAC, the sequence will be read completely differently: TCA TCC GTC GAC GTA C.

Every single amino acid after the point of mutation changes because the codons are altered.

Tay-Sachs disease, which affects the nervous system, is a frameshift mutation which is caused by a four base insertion.

Not all insertion and deletion mutations result in a frameshift. If the insertion or deletion is a multiple of three, there will be no frame shift.

This is because the change still allows the codons after the mutation to be read properly.

Errors and mutations in DNA tend to be more severe than mutations in RNA.

These errors can occur during transcription, and may cause problems in the translation of proteins. The errors may also be due to mutations in the DNA.

RNA tends to be short lived (mRNA in mammalian cells exists for a maximum of a few days) so any mutated strands are quickly replaced.

Mutated RNA is also not replicated during cell division, so mutated strands are not passed to the next generation. Mutated DNA is maintained through cell division.