- 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 mutation which can occur include :
substitution - one or more bases are swapped for another.
deletion - one or more bases are removed.
addition - one or more bases are added.
duplication - one or more bases are repeated.
inversion - a sequence of bases is reversed.
translocation - a sequence 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.
what are mutations?
- 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 polypeptide 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 : a mutation in a polypeptide that makes up an enzyme may change the shape of the enzyme's active site. this may stop substrates from being able to bind to the active site, leaving the enzyme unable to catalyse the reaction.
what are mutations?
- some mutations can increase the likelihood of developing certain cancers, e.g. mutations of the gene brca1 can increase the chances of developing breast cancer. - some mutations can cause genetic disorders - inherited disorders caused by abnormal genes or chromosomes, e.g. cystic fibrosis. - if a gamete (sex cell) containing a mutation for a type of cancer or a genetic disorder is fertilised, the mutation will be present in the new fetus formed; these are called hereditary mutations because they are passed on to the offspring.
mutations and proteins
- not all mutations affect the order of amino acids in a protein. the degenerate nature of the genetic code means that some amino acids are coded for by more than one dna triplet. this means that not all types of mutation will always result in a change to the amino acid sequence of the polypeptide.
- some mutations have a huge effect on the base sequence of a gene. - additions, duplications and deletions 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 causes a shift (called a frameshift) in the base triplets that follow, so that the triplet code is read in a different way.