<unk> s can cause changes to the DNA and are therefore genotoxic. They can affect the transcription and replication of the DNA, which in severe cases can lead to cell death. The<unk>  produces mutations in the DNA, and deleterious mutation can result in aberrant, impaired or loss of function for a particular gene, and accumulation of mutations may lead to cancer.<unk> s may therefore be also carcinogens. However, some<unk> s exert their<unk> ic effect through their metabolites, and therefore whether such<unk> s actually become carcinogenic may be dependent on the metabolic processes of an organism, and a compound shown to be<unk> ic in one organism may not necessarily be carcinogenic in another. Different<unk> s act on the DNA differently. Powerful<unk> s may result in chromosomal instability, causing chromosomal breakages and rearrangement of the chromosomes such as translocation, deletion, and inversion. Such<unk> s are called clastogens.<unk> s may also modify the DNA sequence; the changes in nucleic acid sequences by mutations include substitution of nucleotide base-pairs and insertions and deletions of one or more nucleotides in DNA sequences. Although some of these mutations are lethal or cause serious disease, many have minor effects as they do not result in residue changes that have significant effect on the structure and function of the proteins. Many mutations are silent mutations, causing no visible effects at all, either because they occur in non-coding or non-functional sequences, or they do not change the amino-acid sequence due to the redundancy of codons. Some<unk> s can cause aneuploidy and change the number of chromosomes in the cell. They are known as aneuploidogens. In Ames test, where the varying concentrations of the chemical are used in the test, the dose response curve obtained is nearly always linear, suggesting that there may be no threshold for<unk> esis. Similar results are also obtained in studies with radiations, indicating that there may be no safe threshold for<unk> s. However, the no-threshold model is disputed with some arguing for a dose rate dependent threshold for<unk> esis. Some have proposed that low level of some<unk> s may stimulate the DNA repair processes and therefore may not necessarily be harmful. More recent approaches with sensitive analytical methods have shown that there may be non-linear or bilinear dose-responses for genotoxic effects, and that the activation of DNA repair pathways can prevent the occurrence of mutation arising from a low dose of<unk> .
Mutagen
Antimetabolite
Specifier 
Toxicophore
Mitogen
Saponification
Pollutant
Azo compound