During last week’s lab you exposed his- mutant strains of S. typhimurium to
suspected mutagens. Some of your his- bacteria reverted to a his+ phenotype, but how do
you suppose this happened? As you remember, different mutagens affect DNA in
different ways. However, even though a specific mutagen will typically cause one type
of mutation, this is not necessarily the only mutation that particular mutagen can induce.
Furthermore, we know that TA1535 and TA1538 have contain different mutations, so
perhaps different secondary mutations can lead to the his+ phenotype. This week we will
prepare DNA to identify the molecular changes that confer the revertant phenotype.