Mutations

Mutations are changes to the base sequence of DNA Any change to the base (nucleotide) sequence of DNA is called a mutation. Mutations can be caused by errors during DNA replication. The rate of mutation can be increased by mutagenic agents. The types of mutations that can occur include: Substitution - one or more bases are swapped for another, e.g. ATGCCT becomes ATTCCT Deletion - one or more bases are removed, e.g. ATGCCT becomes ATCCT Addition - one or more bases are added, e.g. ATGCCT becomes ATGACCT Duplication - one or more bases are repeated, e.g. ATGCCT becomes ATGCCCCT Inversion - a sequence of bases is reversed, e.g. ATGCCT becomes ACCGTT Translocation - a sequences of bases is moved from one location in the genome to another. This could be movement within the same chromosome or movement to a different chromosome. The order of DNA bases in a gene determines the sequence of amino acids in a particular polypeptide. If a mutation occurs in a gene, the sequence of amino acids in the polypeptides that it codes for could be changed. Polypeptides make up proteins. A change in the amino acid sequence of a polypeptide may change the final 3D shape of the protein, which could mean that it doesn't work properly. Some mutations can cause genetic disorders - inherited disorders caused by abnormal genes or chromosomes. Some mutations can increase the likelihood of developing certain cancers. If a gamete containing a mutation for a genetic disorder or a type of cancer is fertilised, the mutation will be present in the new fetus formed - these are hereditary mutations.

Not all mutations affect the order of amino acids...The degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet. This means not all types of mutation will always result in a change to the amino acid sequence of the polypeptide.

Some mutations do... Additions, deletions and duplications within a gene will almost always change the amino acid sequence of a polypeptide. That's because these mutations all change the number of bases in the DNA code. This cases a frameshift in the base triplets that follow, so that the triplet code is read in a different way.

Mutagenic agents increase the rate of mutationsMutations occur spontaneously, e.g. when DNA is misread during replication. But some things can increase the rate of mutations - these are called mutagenic agents. Ultraviolet radiation, ionising radiation, some chemicals and some viruses are mutagenic agents. They increase the rate of mutations by: Acting as a base - chemicals called base analogs can substitute for a base during DNA replication. E.g. 5-bromouracil is a base analoge that can substitute for thymine. It can pair with guanine (instead of adenine), causing a substitution mutation in the new DNA. Altering bases - some chemicals can delete or alter bases. E.g. alkylating agents can add an alkyl group to guanine, which changes the structure so that it pairs with thymine, instead of cytosine. Changing the structure of DNA - some types of radiation can change the structure of DNA, which causes problems during DNA replication. E.g. UV radiation can cause adjacent thymine bases to pair up together