Although many different kinds of mutations occur spontaneously, the frequency with which mutations occur can be increased as much as 10 fold by treatment of cells with a mutagen. Here are some general categories of mutagens Type of Mutagen Mechanism Examples Type of Mutations Base anal Analog is incorporated into DNA and can pair with more 5- bromural|A·T→G·C.G·C→A·T than one base 2- aminopurine A'T→G·C Base Modifying Chemical or photo damage to Hydroxylamine GC-A.T Agent DNA can be repaired, but repair itself is error prone EMS G·C→A·T, CG or TA A‖ I changes Intercalating Polycyclic compounds can fit Acridine Frameshifts(+ or -) Agent between bases and cause mis copying by polymerase to add or Proflavine delete bases Suppressor mutations A powerful mode of genetic analysis is to investigate the types of mutations that can reverse the phenotypic effects of a starting mutation. Say that you start with a min phage mutant that makes small plaques. After plating a large number of these mutant phage rare revertants can be isolated by looking for phage that have restored the ability to make large plaques. These revertants could have either been mutated such that the starting mutation was reversed or they could have acquired a new mutation that somehow compensates for the starting mutation. The possibilities are 1)back mutation-true wild type 2)intragenic suppressor-compensating mutation in same gene 3)extragenic suppressor- compensating mutation in different geneAlthough many different kinds of mutations occur spontaneously, the frequency with which mutations occur can be increased as much as 103 fold by treatment of cells with a mutagen. Here are some general categories of mutagens Type of Mutagen Mechanism Examples Type of Mutations Base Analog Analog is incorporated into DNA and can pair with more 5-bromouracil A•T → G•C, G•C → A•T than one base 2-aminopurine A•T → G•C Base Modifying Chemical or photo damage to Hydroxylamine G•C → A•T Agent DNA can be repaired, but repair itself is error prone EMS G•C → A•T, C•G or T•A UV All changes Intercalating Agent Polycyclic compounds can fit between bases and cause mis￾copying by polymerase to add or Acridine Proflavine Frameshifts (+ or –) “ delete bases ICR-191 “ Suppressor mutations A powerful mode of genetic analysis is to investigate the types of mutations that can reverse the phenotypic effects of a starting mutation. Say that you start with a mi– λ phage mutant that makes small plaques. After plating a large number of these mutant phage rare revertants can be isolated by looking for phage that have restored the ability to make large plaques. These revertants could have either been mutated such that the starting mutation was reversed or they could have acquired a new mutation that somehow compensates for the starting mutation. The possibilities are: 1) back mutation - true wild type 2) intragenic suppressor - compensating mutation in same gene 3) extragenic suppressor - compensating mutation in different gene