In biology, mutation is a sudden change in the base pair sequence of the genetic material of a living organism, whether the genetic material be deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). In multicellular organisms that reproduce sexually, mutations can be subdivided into germ line mutations, which can be passed on to descendants, and somatic mutations, which cannot be transmitted to descendants in animals. Plants sometimes can transmit somatic mutations to their descendants asexually or sexually (in case when flower buds develop in somatically mutated part of plant). A new mutation that was not inherited from either parent is called a de novo mutation.
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 evolutionary theory, specifically the theory of evolution by natural selection, mutation is considered the main source of new variation in a population. Although most phenotypic variation is a product of genetic recombination, all new genes are considered to be produced by mutations. For example, a mutation can be postulated for the development of blue eyes in humans. In the two step evolutionary process, production of genetic variation is the chance element, whereas natural selection is not a chance event, but an antichance event. Natural selection is proposed to increase beneficial mutations and select against and eliminate deleterious mutations. However, most mutations actually are harmful, such as seen in genetic diseases, leading to speculation that the role of mutations may be overstated by evolutionary biologists and that other factors, perhaps less random, may be of greater importance in the origin of new designs and macroevolutionary changes.
Mutations involve a change in the base pair of an organism's genetic material. In most organisms, this means the mutation impacts the base pairs of deoxyribonucleic acid (DNA). In some cases, it may mean a change in the base pairs of ribonucleic acid (RNA).
DNA and RNA are nucleic acids. Nucleic acids are complex, high-molecular-weight macromolecule composed of polymers of repeating units (called monomers). Specifically, they consist of long chains of nucleotide monomers connected by covalent chemical bonds. A nucleotide is a chemical compound with three components: a nitrogen-containing base, a pentose (five-carbon) sugar, and one or more phosphate groups. The nitrogen-containing base of a nucleotide (also called the nucleobase) is typically a derivative of either purine or pyrimidine. The most common nucleotide bases are the purines adenine and guanine and the pyrimidines cytosine and thymine (or uracil in RNA). RNA molecules may contain as few as 75 nucleotides or more than 5,000 nucleotides, while a DNA molecule may be composed of more than 1,000,000 nucleotide units. The sugar component is either deoxyribose or ribose, giving the name of DNA and RNA. (“Deoxy” simply indicates that the sugar lacks an oxygen atom present in ribose, the parent compound.)
The main role of DNA is the long-term storage of genetic information. DNA is often compared to a blueprint, since it contains instructions for constructing other components of the cell, such as proteins and RNA molecules. The DNA segments that carry genetic information are called genes, but other DNA sequences have structural purposes or are involved in regulating the expression of genetic information.
RNA serves as a genetic blueprint for certain viruses. However, it plays a diversity of roles in other organisms. RNA may be thought of as the intermediate between the DNA blueprint and the actual workings of the cell, serving as the template for the synthesis of proteins from the genetic information stored in DNA. Some RNA molecules (called ribozymes) are also involved in the catalysis of biochemical reactions.
In other words, chemically, DNA is a long polymer of simple units called nucleotides, with a backbone made of sugars (deoxyribose) and phosphate atoms joined by ester bonds. Attached to each sugar is one of four types of molecules called bases: adenine (A), guanine (G), cytosine (C), or thymine (T). Likewise, RNA is a nucleic acid consisting of chains of nucleotides also forming a polymer, with each nucleotide consisting of a nitrogenous base (adenine, cytosine, guanine, or uracil, U), ribose as the sugar component, and a phosphate.
It is the sequence of these four bases along the backbone of DNA that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. It reads it in a triplet of bases, with one triplet translating into a particular amino acid. That is, the codon adenine-guanine-cytosine may translate to one particular amino acid, while the codon adenine-guanine-adenine may translate to another amino acid. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription. Most of these RNA molecules are used to synthesize proteins.
A mutation is a change in the sequence of the four bases along the backbone of DNA (or RNA). As a result, the sequence of amino acids may be changed, which would affect the structure of the protein that is encoded. However, not all changes in the bases (such as a mutation involving a change from adenine to guanine) necessarily results in the replacement with another amino acid, since there is code redundancy, with some different sequences of bases translating into the same amino acid.
The functionality of a protein is highly dependent on its three-dimensional structure (how it "folds") and this is highly dependent on the order of the amino acids that make it up. A change in a single amino acid could make the protein non-functional.
Within cells, DNA is organized into structures called chromosomes and the set of chromosomes within a cell make up a genome. These chromosomes are duplicated before cells divide, in a process called DNA replication. Eukaryotic organisms such as animals, plants, and fungi store their DNA inside the cell nucleus, while in prokaryotes such as bacteria it is found in the cell's cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA, which helps control its interactions with other proteins and thereby control which genes are transcribed.
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:
The human genome contains two copies of each gene—a paternal and a maternal allele.
Two classes of mutations are spontaneous mutations (molecular decay) and induced mutations caused by mutagens.
Spontaneous mutations. Spontaneous mutations on the molecular level include:
Induced mutations. Induced mutations on the molecular level can be caused by:
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 of mutations specify the type of mutation and base or amino acid changes.
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.
A frameshift mutation is a mutation caused by indels, i.e.. inserts or deletes in 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 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.
A neutral mutation is a mutation that occurs in an amino acid codon (presumably within an mRNA molecule) that results in the substitution 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 discernable change occurs (a silent 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.
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 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.
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. However, only a small percentage of mutations cause genetic disorders; most have no impact on health. For example, some mutations alter a gene's DNA base sequence but do not change the function of the protein made by the gene.
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.
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-fourteenth 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. A more recent theory says the selective pressure on the CCR5 Delta 32 mutation has been caused by smallpox instead of bubonic plague.
|Basic topics in evolutionary biology||(edit)|
|Processes of evolution: evidence - macroevolution - microevolution - speciation|
|Mechanisms: natural selection - genetic drift - gene flow - mutation - phenotypic plasticity|
|Modes: anagenesis - catagenesis - cladogenesis|
|History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis|
|Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics|
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