Any agent capable of altering a cell's genetic makeup by changing the structure of the hereditary material, DNA. Many forms of electromagnetic radiation (e.g., cosmic rays, X rays, ultraviolet light) are mutagenic, as are various chemical compounds. The effects of some mutagens are increased or suppressed in some organisms by the presence of certain other, nonmutagenic substances; oxygen, for example, makes cells more sensitive to the mutagenic effects of X rays.
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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 many of these mutations are lethal, or cause serious disease, some have minor effects, as the changes they cause in the sequence of encoded proteins are not significant. Many mutations cause no visible effects at all, either because they occur in introns or because they do not change the amino-acid sequence, due to redundancy of codons.
The change in a population’s genetic material due to the accumulation of random chance is called drift, and serves as a molecular clock. In general, the more nucleotide differences between two organisms, the more time has elapsed since their last common ancestor. Though it is difficult to determine in many organisms, estimates for mutation rates have been made for both E. coli and eukaryotes. It was estimated that, in these organisms, about one nucleotide in every 1010 is changed, and continues through reproduction to future generations of cells.
In the 1920s, Hermann Muller discovered that x-rays caused mutations in fruit flies. He went on to use x-rays to create Drosophila mutants that he used in his studies of genetics. He also discovered that x-rays not only mutate genes in fruit flies but also have effects on the genetic makeup of humans.
The Ames test is one method to determine how mutagenic an agent is.