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The mutagen 5 bromo-deoxyuridine exists in two forms, one of which pairs with adenine, and the other of which pairs with guanine. Adapted from <http://fig.cox.miami .edu/Faculty/Dana/ baseanalog.jpg>.

carbon, or derived from living organisms; also, a type of agriculture

"Base-altering mutagens" cause chemical changes in bases that are part of the DNA. For example, nitrite preservatives in food convert to the mutagen nitrous acid. Nitrous acid causes deamination, or loss of an -NH2 group, of cytosine. When this occurs, cytosine becomes uracil, a base that is not normally incorporated in DNA but that is very similar to thymine. Unless repaired, this uracil will cause an adenine to enter the opposite strand instead organic composed of of a guanine. Many base-altering mutagens are complex organic molecules.

These are formed in large quantities in smoke, making up the "tar" of cigarette smoke, for example. They act as alkylating agents, combining with stressing soil fertility DNA to form bulky groups that interrupt replication.

thet i c^estic^e^aid "Intercalating agents" are flat molecules that insert themselves between fertilizers adjacent bases in the double helix, distorting the shape at the point of inser tion. Where this occurs, DNA polymerase may add an additional base opposite the intercalating agent. If this occurs in a gene, it induces a frameshift mutation (that is, it alters the reading of the gene transcript, changing which amino acids are added to the encoded protein). Ethidium bromide is one such agent, widely used in DNA research because its dark color allows DNA to be easily visualized. This is useful in gel electrophoresis, for instance, to find the DNA bands that have been separated in a gel.

Other damaging agents include chemicals that create "free radicals" inside a cell. Free radicals are compounds in which an atom, usually an oxygen, has an unbonded electron. Free radicals are highly reactive and can cause several types of damage to DNA.

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