mutations

• mutations are any change to the base (nucleotide) sequence of DNA
• mutations can be caused by errors during DNA replication
• the rate of mutation can be increased by mutagenic agents
• types of mutation that can occur:

  • substitution
  • deletion
  • addition
  • duplication
  • inversion
  • translocation
• 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 - could be movement within the same chromosome or movement to a different chromosome
• the order of 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 - could mean that it doesn't work properly
• e.g. a mutation in a polypeptide that makes up an enzyme may change the shape of the enzyme's active site - may stop substrates from being able to bind to the active site, leaving the enzyme unable to catalyse the reaction
• some mutations can increase the likelihood of developing certain cancers e.g. mutations in the BRCA1 gene can increase the chances of developing breast cancer
• when more than 2 amino acids join together, they form a polypeptide chain. the sequence of amino acids in the chain forms the primary structure of a protein and the final folding of the chain forms the tertiary structure
• some mutations can cause genetic disorders - inherited disorders caused by abnormal genes or chromosomes e.g. CF
• if a gamete (sex cell) containing a mutation for a type of cancer or a genetic order is fertilised, the mutation will be present in the new fetus formed - these are called hereditary mutations bc they are passed on to offspring
• 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 (e.g. tyrosine can be coded for by TAT or TAC in DNA)
  • this means that not all types of mutation will always result in a change to the amino acid sequence of the polypeptide
• some substitutions will still code for the same amino acid
• sometimes, inversion mutations don't cause a change in the amino acid sequence either
• frameshift mutations:

  • 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 is bc these mutations all change the number of bases in the DNA code
  • this causes a shift (frameshift) in the base triplets that follow
  • so the triplet code is read in a different way
• CF is a genetic disorder of the cell membranes

  it results in the body producing a lot of thick sticky mucus in the air passages and the pancreas
• not all hereditary mutations are harmful - beneficial hereditary mutations drive evolution
• if a mutation doesn't cause a change in the amino acid order, it is called a silent mutation
• a frameshift mutation affects more amino acids than a substitution mutation, so it will have a bigger overall effect on the protein's structure
• the base triplets that follow on from the mutation are said to be 'downstream' of the mutation
• the altered base sequence will affect both the amino acid sequence and the protein's structure