Definitions

mutation

mutation

[myoo-tey-shuhn]
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).

Alteration in the genetic material of a cell that is transmitted to the cell's offspring. Mutations may be spontaneous or induced by outside factors (mutagens). They take place in the genes, occurring when one base is substituted for another in the sequence of bases that determines the genetic code, or when one or more bases are inserted or deleted from a gene. Many mutations are harmless, often masked by the presence of a dominant normal gene (see dominance). Some have serious consequences; for example, a particular mutation inherited from both parents results in sickle-cell anemia. Only mutations that occur in the sex cells (eggs or sperm) can be transmitted to the individual's offspring. Alterations caused by these mutations are usually harmful. In the rare instances in which a mutation produces a beneficial change, the percentage of organisms with this gene will tend to increase until the mutated gene becomes the norm in the population. In this way, beneficial mutations serve as the raw material of evolution.

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In biology, mutations are changes to the nucleotide sequence of the genetic material of an organism. Mutations can be caused by copying errors in the genetic material during cell division, by exposure to ultraviolet or ionizing radiation, chemical mutagens, or viruses, or can occur deliberately under cellular control during processes such as hypermutation. In multicellular organisms, mutations can be subdivided into germ line mutations, which can be passed on to descendants, and somatic mutations, which are not transmitted to descendants in animals. Plants sometimes can transmit somatic mutations to their descendants asexually or sexually (in cases where flower buds develop in somatically mutated parts of plants). A new mutation that was not inherited from either parent is called a de novo mutation. The source of the mutation is unrelated to the consequence, although the consequences are related to which cells are affected.

Mutations create variation within the gene pool. Less favorable (or deleterious) mutations can be reduced in frequency in the gene pool by natural selection, while more favorable (beneficial or advantageous) mutations may accumulate and result in adaptive evolutionary changes. For example, a butterfly may produce offspring with new mutations. The majority of these mutations will have no effect; but one might change the color of one of the butterfly's offspring, making it harder (or easier) for predators to see. If this color change is advantageous, the chance of this butterfly surviving and producing its own offspring are a little better, and over time the number of butterflies with this mutation may form a larger percentage of the population.

Neutral mutations are defined as mutations whose effects do not influence the fitness of an individual. These can accumulate over time due to genetic drift. It is believed that the overwhelming majority of mutations have no significant effect on an organism's fitness. Also, DNA repair mechanisms are able to mend most changes before they become permanent mutations, and many organisms have mechanisms for eliminating otherwise permanently mutated somatic cells.

Mutation is generally accepted by the scientific community as the mechanism upon which natural selection acts, providing the advantageous new traits that survive and multiply in offspring or disadvantageous traits that die out with weaker organisms.

Classification

By effect on structure

The sequence of a gene can be altered in a number of ways. Gene mutations have varying effects on health depending on where they occur and whether they alter the function of essential proteins. Structurally, mutations can be classified as:

  • Small-scale mutations, such those as affecting a small gene in one or a few nucleotides, including:
    • Point mutations, often caused by chemicals or malfunction of DNA replication, exchange a single nucleotide for another. Most common is the transition that exchanges a purine for a purine (A ↔ G) or a pyrimidine for a pyrimidine, (C ↔ T). A transition can be caused by nitrous acid, base mis-pairing, or mutagenic base analogs such as 5-bromo-2-deoxyuridine (BrdU). Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/T ↔ A/G). A point mutation can be reversed by another point mutation, in which the nucleotide is changed back to its original state (true reversion) or by second-site reversion (a complementary mutation elsewhere that results in regained gene functionality). These changes are classified as transitions or transversions. An example of a transversion is adenine (A) being converted into a cytosine (C). There are also many other examples that can be found. Point mutations that occur within the protein coding region of a gene may be classified into three kinds, depending upon what the erroneous codon codes for:
    • Insertions add one or more extra nucleotides into the DNA. They are usually caused by transposable elements, or errors during replication of repeating elements (e.g. AT repeats). Insertions in the coding region of a gene may alter splicing of the mRNA (splice site mutation), or cause a shift in the reading frame (frameshift), both of which can significantly alter the gene product. Insertions can be reverted by excision of the transposable element.
    • Deletions remove one or more nucleotides from the DNA. Like insertions, these mutations can alter the reading frame of the gene. They are generally irreversible: though exactly the same sequence might theoretically be restored by an insertion, transposable elements able to revert a very short deletion (say 1–2 bases) in any location are either highly unlikely to exist or do not exist at all. Note that a deletion is not the exact opposite of an insertion: the former is quite random while the latter consists of a specific sequence inserting at locations that are not entirely random or even quite narrowly defined.
  • Large-scale mutations in chromosomal structure, including:
    • Amplifications (or gene duplications) leading to multiple copies of all chromosomal regions, increasing the dosage of the genes located within them.
    • Deletions of large chromosomal regions, leading to loss of the genes within those regions.
    • Mutations whose effect is to juxtapose previously separate pieces of DNA, potentially bringing together separate genes to form functionally distinct fusion genes (e.g. bcr-abl). These include:
      • Chromosomal translocations: interchange of genetic parts from nonhomologous chromosomes.
      • Interstitial deletions: an intra-chromosomal deletion that removes a segment of DNA from a single chromosome, thereby apposing previously distant genes. For example, cells isolated from a human astrocytoma, a type of brain tumor, were found to have a chromosomal deletion removing sequences between the "fused in glioblastoma" (fig) gene and the receptor tyrosine kinase "ros", producing a fusion protein (FIG-ROS). The abnormal FIG-ROS fusion protein has constitutively active kinase activity that causes oncogenic transformation (a transformation from normal cells to cancer cells).
      • Chromosomal inversions: reversing the orientation of a chromosomal segment.
    • Loss of heterozygosity: loss of one allele, either by a deletion or recombination event, in an organism that previously had two different alleles.

By effect on function

  • Loss-of-function mutations are the result of gene product having less or no function. When the allele has a complete loss of function (null allele) it is often called an amorphic mutation. Phenotypes associated with such mutations are most often recessive. Exceptions are when the organism is haploid, or when the reduced dosage of a normal gene product is not enough for a normal phenotype (this is called haploinsufficiency).
  • Gain-of-function mutations change the gene product such that it gains a new and abnormal function. These mutations usually have dominant phenotypes. Often called a neomorphic mutation.
  • Dominant negative mutations (also called antimorphic mutations) have an altered gene product that acts antagonistically to the wild-type allele. These mutations usually result in an altered molecular function (often inactive) and are characterised by a dominant or semi-dominant phenotype. In humans, Marfan syndrome is an example of a dominant negative mutation occurring in an autosomal dominant disease. In this condition, the defective glycoprotein product of the fibrillin gene (FBN1) antagonizes the product of the normal allele.
  • Lethal mutations are mutations that lead the death of the organisms which carry the mutations.

By aspect of phenotype affected

  • Morphological mutations usually affect the outward appearance of an individual. Mutations can change the height of a plant or change it from smooth to rough seeds.
  • Biochemical mutations result in lesions stopping the enzymatic pathway. Often, morphological mutants are the direct result of a mutation due to the enzymatic pathway.

By inheritance

The human genome contains two copies of each gene – a paternal and a maternal allele.

  • A wildtype or homozygous non-mutated organism is one in which neither allele is mutated.
  • A heterozygous mutation is a mutation of only one allele.
  • A homozygous mutation is an identical mutation of both the paternal and maternal alleles.
  • Compound heterozygous mutations or a genetic compound is two different mutations in the paternal and maternal alleles.

Special classes

  • Conditional mutation is a mutation that has wild-type (or less severe) phenotype under certain "permissive" environmental conditions and a mutant phenotype under certain "restrictive" conditions. For example, a temperature-sensitive mutation can cause cell death at high temperature (restrictive condition), but might have no deleterious consequences at a lower temperature (permissive condition).

Causes of mutation

Two classes of mutations are spontaneous mutations (molecular decay) and induced mutations caused by mutagens.

Spontaneous mutations on the molecular level include:

  • Tautomerism – A base is changed by the repositioning of a hydrogen atom.
  • Depurination – Loss of a purine base (A or G).
  • Deamination – Changes a normal base to an atypical base; C → U, (which can be corrected by DNA repair mechanisms), or spontaneous deamination of 5-methycytosine (irreparable), or A → HX (hypoxanthine).
  • Transition – A purine changes to another purine, or a pyrimidine to a pyrimidine.
  • Transversion – A purine becomes a pyrimidine, or vice versa.

Induced mutations on the molecular level can be caused by:

  • Chemicals
  • Radiation
    • Ultraviolet radiation (nonionizing radiation) excites electrons to a higher energy level. DNA molecules are good absorbers of ultraviolet light, especially that with wavelengths in the 260 to 280 nm range. Two nucleotide bases in DNA – cytosine and thymine – are most vulnerable to excitation that can change base-pairing properties. UV light can induce adjacent thymine bases in a DNA strand to pair with each other, as a bulky dimer.
    • Ionizing radiation
  • Viral infections

DNA has so-called hotspots, where mutations occur up to 100 times more frequently than the normal mutation rate. A hotspot can be at an unusual base, e.g., 5-methylcytosine.

Mutation rates also vary across species. Evolutionary biologists have theorized that higher mutation rates are beneficial in some situations, because they allow organisms to evolve and therefore adapt more quickly to their environments. For example, repeated exposure of bacteria to antibiotics, and selection of resistant mutants, can result in the selection of bacteria that have a much higher mutation rate than the original population (mutator strains).

Nomenclature

Nomenclature of mutations specify the type of mutation and base or amino acid changes.

  • Nucleotide Substitution (e.g. 76A>T)- The first number is the position of the nucleotide from 5'end., the letter is the one letter code for Nucleotide Adenine which is the wild type nucleotide and the second letter after '>' denotes the one letter code for the nucleotide which replace the wild type.
  • For differentiating mutations in genomic DNA , mitochondrial DNA , RNA a simple convention is used. For example if the 100th base of a nucleotide sequence mutated from G to C then it will be written as-

g.100G>C if the mutation has occurred in genomic DNA, m.100G>C if the mutation has occurred in mitochondrial DNA, r.100g>c if the mutation has occurred in RNA .(note that for muatations in RNA lower cases of the single letter code is used)

  • Amino acid substitution (e.g. D111E) – The first letter is the one letter code of the wildtype amino acid, the number is the position of the amino acid from the N terminus and the second letter is the one letter code of the amino acid present in the mutation. If the second letter is 'X', any amino acid may replace the wildtype.
  • Amino acid deletion (e.g. ΔF508) – The Greek letter Δ or 'delta' indicates a deletion. The letter refers to the amino acid present in the wildtype and the number is the position from the N terminus of the amino acid were it to be present as in the wildtype.

Types of mutations

Adaptive mutation

In mainstream biological thought it is held that while mutagenesis is non-random in many ways, the utility of a genetic mutation to the organism in which it occurs does not affect the rate at which it occurs. However experimental evidence exists that in some instances the rate of specific mutations arising is greater when they are advantageous to the organism than when they are not.

Back mutation

Back mutation or reversion is a change in a nucleotide pair of a point-mutated DNA sequence that restores the original sequence and hence the original phenotype.

Frameshift mutation

A frameshift mutation is a mutation caused by indels, ie. inserts or deletes a number of nucleotides that is not evenly divisible by three from a DNA sequence. Due to the triplet nature of gene expression by codons, the insertion or deletion can disrupt the reading frame, or the grouping of the codons, resulting in a completely different translation from the original. The earlier in the sequence the deletion or insertion occurs, the more altered the protein produced is.

Missense mutation

Missense mutations or nonsynonymous mutations are types of point mutations where a single nucleotide is changed to cause substitution of a different amino acid. This in turn can render the resulting protein nonfunctional. Such mutations are responsible for diseases such as Epidermolysis bullosa, sickle-cell disease, and SOD1 mediated ALS .

Neutral mutation

A neutral mutation is a mutation that occurs in an amino acid codon (presumably within an mRNA molecule) which results in the use of a different, but chemically similar, amino acid. This is similar to a silent mutation, where a codon mutation may encode the same amino acid (see Wobble Hypothesis); for example, a change from AUU to AUC will still encode leucine, so no discernible change occurs (a silent mutation).

Nonsense mutation

A nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a nonsense codon in the transcribed mRNA, and possibly a truncated, and often nonfunctional protein product.

Point mutation

A point mutation, or substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide. Often the term point mutation also includes insertions or deletions of a single base pair (which have more of an adverse effect on the synthesized protein due to nucleotides still being read in triplets, but in different frames- a mutation called a frameshift mutation).

Silent mutation

Silent mutations are DNA mutations that do not result in a change to the amino acid sequence of a protein. They may occur in a non-coding region (outside of a gene or within an intron), or they may occur within an exon in a manner that does not alter the final amino acid sequence. The phrase silent mutation is often used interchangeably with the phrase synonymous mutation; however, synonymous mutations are a subcategory of the former, occurring only within exons. The name silent could be a misnomer. For example, a silent mutation in the exon/intron border may lead to alternative splicing with creation of an alternative splice site, thereby leading to a changed protein despite the name silent mutation.

Harmful mutations

Changes in DNA caused by mutation can cause errors in protein sequence, creating partially or completely non-functional proteins. To function correctly, each cell depends on thousands of proteins to function in the right places at the right times. When a mutation alters a protein that plays a critical role in the body, a medical condition can result. A condition caused by mutations in one or more genes is called a genetic disorder. Some mutations alter a gene's DNA base sequence but do not change the function of the protein made by the gene. Studies in the fly Drosophila melanogaster suggest that if a mutation does change a protein, this will probably be harmful, with about 70 percent of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.

If a mutation is present in a germ cell, it can give rise to offspring that carries the mutation in all of its cells. This is the case in hereditary diseases. On the other hand, a mutation can occur in a somatic cell of an organism. Such mutations will be present in all descendants of this cell, and certain mutations can cause the cell to become malignant, and thus cause cancer.

Often, gene mutations that could cause a genetic disorder are repaired by the DNA repair system of the cell. Each cell has a number of pathways through which enzymes recognize and repair mistakes in DNA. Because DNA can be damaged or mutated in many ways, the process of DNA repair is an important way in which the body protects itself from disease.

Beneficial mutations

A very small percentage of all mutations actually have a positive effect. These mutations lead to new versions of proteins that help an organism and its future generations better adapt to changes in their environment.

For example, a specific 32 base pair deletion in human CCR5 (CCR5-Δ32) confers HIV resistance to homozygotes and delays AIDS onset in heterozygotes. The CCR5 mutation is more common in those of European descent. One theory for the etiology of the relatively high frequency of CCR5-Δ32 in the European population is that it conferred resistance to the bubonic plague in mid-14th century Europe. People who had this mutation were able to survive infection; thus, its frequency in the population increased. It could also explain why this mutation is not found in Africa where the bubonic plague never reached. Newer theory says the selective pressure on the CCR5 Delta 32 mutation has been caused by smallpox instead of the bubonic plague.

See also

References

  • Leroi A. 2003. Mutants: On the form, varieties & errors of the human body. 1:16-17. Harper Collins 2003
  • Maki H. 2002. Origins of spontaneous mutations: specificity and directionality of base-substitution, frameshift, and sequence-substitution mutageneses. Annual Review of Genetics 36:279-303.
  • Taggart R. Starr C. Biology The Unity and Diversity of Life: Mutated Genes and Their Protein Products. 14.4:227. Thompson Brooks/Cole 2006.

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