<unk> s may be of physical, chemical or biological origin. They may act directly on the DNA, causing direct damage to the DNA, and most often result in replication error. Some however may act on the replication mechanism and chromosomal partition. Many<unk> s are not<unk> ic by themselves, but can form<unk> ic metabolites through cellular processes, for example through the activity of the cytochrome P450 system and other oxygenases such as cyclooxygenase. Such<unk> s are called promutagens. Physical<unk> s * Ionizing radiations such as X-rays, gamma rays and alpha particles cause DNA breakage and other damages. The most common lab sources include cobalt-60 and cesium-137. * Ultraviolet radiations with wavelength above 260 nm are absorbed strongly by bases, producing pyrimidine dimers, which can cause error in replication if left uncorrected. * Radioactive decay, such as 14C in DNA which decays into nitrogen. DNA reactive chemicals benzoapyrene from tobacco smoke. A large number of chemicals may interact directly with DNA. However, many such as PAHs, aromatic amines, benzene are not necessarily<unk> ic by themselves, but through metabolic processes in cells they produce<unk> ic compounds. * Reactive oxygen species (ROS) – These may be superoxide, hydroxyl radicals and hydrogen peroxide, and large number of these highly reactive species are generated by normal cellular processes, for example as a by-products of mitochondrial electron transport, or lipid peroxidation. As an example of the latter, 15-hydroperoxyicosatetraenocic acid, a natural product of cellular cyclooxygenases and lipoxygenases, breaks down to form 4-hydroxy-2(E)-nonenal, 4-hydroperoxy-2(E)-nonenal, 4-oxo-2(E)-nonenal, and cis-4,5-epoxy-2(E)-decanal; these bifunctional electophils are<unk> ic in mammalian cells and may contribute to the development and/or progression of human cancers (see 15-Hydroxyicosatetraenoic acid). A number of<unk> s may also generate these ROS. These ROS may result in the production of many base adducts, as well as DNA strand breaks and crosslinks. * Deaminating agents, for example nitrous acid which can cause transition mutations by converting cytosine to uracil. * Polycyclic aromatic hydrocarbon (PAH), when activated to diol-epoxides can bind to DNA and form adducts. * Alkylating agents such as ethylnitrosourea. The compounds transfer methyl or ethyl group to bases or the backbone phosphate groups. Guanine when alkylated may be mispaired with thymine. Some may cause DNA crosslinking and breakages. Nitrosamines are an important group of<unk> s found in tobacco, and may also be formed in smoked meats and fish via the interaction of amines in food with nitrites added as preservatives. Other alkylating agents include mustard gas and vinyl chloride. * Aromatic amines and amides have been associated with carcinogenesis since 1895 when German physician Ludwig Rehn observed high incidence of bladder cancer among workers in German synthetic aromatic amine dye industry. 2-Acetylaminofluorene, originally used as a pesticide but may also be found in cooked meat, may cause cancer of the bladder, liver, ear, intestine, thyroid and breast. * Alkaloid from plants, such as those from Vinca species, may be converted by metabolic processes into the active<unk>  or carcinogen. * Bromine and some compounds that contain bromine in their chemical structure. * Sodium azide, an azide salt that is a common reagent in organic synthesis and a component in many car airbag systems * Psoralen combined with ultraviolet radiation causes DNA cross-linking and hence chromosome breakage. * Benzene, an industrial solvent and precursor in the production of drugs, plastics, synthetic rubber and dyes. Base analogs * Base analog, which can substitute for DNA bases during replication and cause transition mutations.some examples are 5 bromo uracil and 2 amino purine Intercalating agents * Intercalating agents, such as ethidium bromide and proflavine, are molecules that may insert between bases in DNA, causing frameshift mutation during replication. Some such as daunorubicin may block transcription and replication, making them highly toxic to proliferating cells. Metals Many metals, such as arsenic, cadmium, chromium, nickel and their compounds may be<unk> ic, but they may act, however, via a number of different mechanisms. Arsenic, chromium, iron, and nickel may be associated with the production of ROS, and some of these may also alter the fidelity of DNA replication. Nickel may also be linked to DNA hypermethylation and histone deacetylation, while some metals such as cobalt, arsenic, nickel and cadmium may also affect DNA repair processes such as DNA mismatch repair, and base and nucleotide excision repair. Biological agents * Transposon, a section of DNA that undergoes autonomous fragment relocation/multiplication. Its insertion into chromosomal DNA disrupts functional elements of the genes. * Virus – Virus DNA may be inserted into the genome and disrupts genetic function. Infectious agents have been suggested to cause cancer as early as 1908 by Vilhelm Ellermann and Oluf Bang, and 1911 by Peyton Rous who discovered the Rous sarcoma virus. * Bacteria – some bacteria such as Helicobacter pylori cause inflammation during which oxidative species are produced, causing DNA damage and reducing efficiency of DNA repair systems, thereby increasing mutation.
Mutagen
Antimetabolite
Specifier 
Toxicophore
Mitogen
Saponification
Pollutant
Azo compound