mutagen

mutation

mutation, in biology, a sudden, random change in a gene, or unit of hereditary material, that can alter an inheritable characteristic. Most mutations are not beneficial, since any change in the delicate balance of an organism having a high level of adaptation to its environment tends to be disruptive. As the environment changes, however, mutations can prove advantageous and thus contribute to evolutionary change in the species. In higher animals and many higher plants a mutation may be transmitted to future generations only if it occurs in germ, or sex cell, tissue; somatic, or body cell, mutations cannot be inherited except in plants that propagate asexually (see reproduction). Sometimes the word mutation is used broadly to include variations resulting from aberrations of chromosomes; in chromosomal mutations the number of chromosomes may be altered, or segments of chromosomes may be lost or rearranged. Changes within single genes, called point mutations, are actual chemical changes to the structure of the constituent DNA.

Point Mutations

Each gene is made up of a long sequence of substances called nucleotides; these nucleotides, taken in series of three at a time, specify each amino acid subunit of a protein (see nucleic acid). In a frameshift mutation, a nucleotide is added or deleted to the sequence and the decoding of the entire gene sequence will be radically altered and the amino acid sequence of the protein produced will also be very different. Often the resulting protein is totally ineffective. If one nucleotide substitutes for another in the sequence only one amino acid of the protein will be different, but the effect can be quite dramatic. For example, the inherited sickle cell disease is the result of a mutation that results in the substitution of the amino acid valine for glutamic acid in hemoglobin.

Because proteins called enzymes control most cell activities, a mutation affecting an enzyme can result in alteration of other cell components. A single gene mutation may have many effects if the enzyme it controls is involved in several metabolic processes. Occasionally a mutation can be offset by either another mutation on the same gene or on another gene that suppresses the effect of the first. Certain genes are responsible for producing enzymes that can repair some mutations. While this process is not fully understood, it is believed that if these genes themselves mutate, the result can be a higher mutation rate of all genes in an organism.

Induced Mutations

Mutations may be induced by exposure to ultraviolet rays and alpha, beta, gamma, and X radiation, by extreme changes in temperature, and by certain mutagenic chemicals such as nitrous acid, nitrogen mustard, and chemical substitutes for portions of the nucleotide subunits of genes. H. J. Muller, an American geneticist, pioneered in inducing mutations by X-ray radiation (using the fruit fly, Drosophila) and developed a method of detecting mutations that are lethal.

Mutation and Evolution

In 1901 the observation of mutants, or sports, among evening primrose plants led the Dutch botanist Hugo de Vries to present his theory that new characteristics may appear suddenly and that these characteristics are inheritable; before this time the sources of evolutionary variation were not known and some still believed that evolution resulted from a gradual selection of favorable acquired characteristics. The work of de Vries and of subsequent investigators who demonstrated the distinction between mutation and environmental variations has shown the importance of mutation in the mechanism of evolution.

Bibliography

See W. Gottschalk and G. Wolff, Induced Mutations in Plant Breeding (1983); G. Obe, Mutations in Man (1984).

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Mutagen

 

a physical or chemical substance that causes permanent hereditary change.

Physical mutagens include ultraviolet radiation and all kinds of ionizing radiation, such as gamma rays, X rays, protons, and neutrons. High and low temperatures are much less capable of causing mutations.

As study intensifies, the list of compounds with mutagenic action grows longer. Among the chemical mutagens are many alkylating compounds, for example, mustard gas, dimethyl sulfate, and nitrosomethylurea; analogs of nitrogenous bases of the nucleic acids, for example, 5-bromouracil and 2-aminopurine; acridine dyes; nitrous acid; some alkaloids; formaldehyde; hydrogen peroxide and some organic peroxides; and some bio-polymers, for example, heterologous DNA and, apparently, heterologous RNA.

The most powerful chemical mutagens, which increase the frequency of mutations hundreds of times, are called super-mutagens. Some viruses might also be considered chemical mutagens, since the mutagenic factor in viruses seems to be located in their DNA or RNA.

Mutagens are apparently universal, that is, they can cause mutations in all forms of life—from viruses and bacteria to the higher plants, animals, and man. Various species differ in their mutability, that is, their sensitivity to mutagens. None of the known mutagens appear to have a lower limit of mutagenic action. However, the frequency of induced mutations decreases with the decreasing dose of mutagen to a point that matches the frequency of spontaneous mutations regularly occurring in the absence of any mutagen.

Physical and chemical mutagens are widely used in breeding agriculturally useful plants and useful microorganisms. Once the mutation is induced, the mutant is artificially removed from the population and bred as a separate species. These mutations are used in artificial selection.

REFERENCES

See references under .

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

mutagen

[′myüd·ə·jən]

(genetics)

An agent that raises the frequency of mutation above the spontaneous or background rate.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

mutagen

a substance or agent that can induce genetic mutation

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005