Chemical mutagens directly affect chromosomes, it makes the chromosomes unstable and causes them to rearrange themselves. Chemical mutagens do it by acting as base analogs, whose chemical structure is similar to a DNA base, examples of this type of chemical mutagens are bromouracil and aminopurine. They can also do it by directly altering the structure and pairing properties of the DNA bases, examples of this type of chemical mutagens are nitrous acid, nitrosoguanidine, methyl methanesulfonate, and ethyl methanesulfonate.