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mutants unable to synthesize an essential metabolite that is synthesized by wild-type or phototrophic of the same species

temperature-sensitive mutants

grow at one temperature not another; most are heat sensitive, some cold sensitive

suppressor-sensitive mutants

viable when a second genetic factor (suppressor) is present, but they're non-viable in the absence of the suppressor


mutation resulting from tautomeric shifts in the bases of DNA involve the replacement of a purine in one strand of DNA with the other purine and the replacement of a pyrimidine in the complementary strand with the other pyrimidine


base-pair subs involving the replacement of a purine with a pyrimidine and vice versa

alkylating agent

chemical that donates alkyl groups to other molecules

hydroxylating agent

hydroxyl amine (NH2OH) induces G:C -> A:T transitions

ionizing radiation

x rays, gamma rays, cosmic rays

nonionizing radiation

high energy rays collide with atoms and cause the release of electrons = positively charged free radicals or ions collide with other molecules and release additional electrons

uv radiation

readily absorbed by many organic molecules such as purines and pyrimidines

base excision repair

system removes abnormal or chemically modified bases from DNA

nucleotide excision repair

pathways remove larger defects like thymine dimers

constitutive genes

genes that specify products of this type are continually being expressed constitutively


process of turning on the expression of genes in response to a substance in the environment

inducible genes

genes whose expression is regulated via induction

positive control mechanism

product of the regulator gene is required to turn on the expression of one or more structural genes (specifying the amino acid sequences of enzymes or structural proteins)

negative control mechanism

product of the regulator gene is necessary to shut off the expression of structural genes

catabolite repression

assures that glucose is metabolized when present, in preference to other less efficient energy sources

SOS response

synthesis of a whole set of DNA repair, recombination, and replication proteins in bacteria containing severely damaged DNA

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