Mutations Caused by changes in the DNA sequence There are many different categories of mutations gametic or somatic phenotypic effect - recessive, dominant, sexlinked, morphological, nutritional, behavioral, regulatory, lethal, conditional, etc. type of change in the DNA or protein DNA - transition, transversion, deletion, insertion protein - silent, neutral, missense, nonsense, frameshift Detection of mutations Assay systems in bacteria and fungi - prototrophs and auxotrophs, resistance fruit flies - ClB and attached-X detection systems plants - amino acid composition and tissue culture auxotrophs human - pedigree analysis Spontaneous Mutations Rates rates are very low (1x10-8 to 1x10-5) rates vary from species to species (lower in prokaryotes) rates vary from gene to gene Molecular Basis of Mutations replication mistakes hey, so I goof once every 10 million bases. Think you could do any better? 3'-5' exonuclease allows editing tautomerization (keto - enol and amino - imino) causes transition mutations slippage leads to small insertions and deletions (frameshifts) caused by repeated sequences Fragile-X 50 repeats of CGG is normal 50 to 200 is a carrier (anticipation) >200 causes mental retardaton also the cause of Huntington's (CAG) and Myotonic dystrophy (CTG) damage to the DNA template apurinic sites (AP sites) caused by loss of the base (the glycosidic bond breaks) deamination of cytosine to uracil (or adenine to hypoxanthine) usually repaired by uracil-DNA-glycosidase however, 5-methylcytosine becomes thymidine and can't be repaired methylation of cytosine occurs at only specific locations (only CG in mammals) leads to mutational hot spots many fewer CG's than would be expected radiation UV - thymine dimers gamma rays and X rays - generate free radicals can not only change bases but can also break chromosomes Chemical mutagens base analogs 5-bromouracil can pair with A or G 2-aminopurine can pair with T or C modifying agents alkylating agents (add alkyl groups ) nitrosoguanidine (NG) attacks N7 of guanine and N3 of adenine ethylmethanesulfonate (EMS) attacks O6 of guanine can cause mispairing unless the damaged base is removed removal of base creates apurinic site (sugar only) -AP polymerase will put a random base opposite AP site Aflatoxin B1 attacks N7 of guanine - AP site Nitrous acid converts NH2 to =O (deamination) intercalating agents insert into the DNA between the stacked bases, this causes insertions and deletions acridine orange proflavin ICR-191 transposons This document is copyright of Jeff Bell Last Update: Thursday, July 13, 2006