“You’re performing the Ames test on two known carcinogens; Benzo(a)pyrene (BaP) and methyl nitrosourea (MNU). If you treat His-Salmonella with MNU, you generate many His+ colonies. In contrast, if you treat His auxotrophic Salmonella with BaP, you generate no His prototrophs. However, if you first incubate the BaP with a liver extract before adding it to the His auxotroph Salmonella, you generate many His prototrophs. How do BaP and MNU differ?”
MNU is a direct-acting carcinogen; BaP is an indirect-acting carcinogen