Mutation
A mutation is a sudden change in DNA (deoxyribonucleic acid), the genetic material of life. Mutation is a major evolutionary force that results from wide range of factors and that carries a wide range of results. Mutations may carry no discernable effects or act to significantly increase or decrease genetic variation within a population.
Although some mutations have no visible effects, there are mutations that can cause dramatic changes in an organism's appearance, behavior, or health. Organisms born with mutations can look very different from their parents. Albinism, for example, results from mutations that eliminate skin pigment. Dwarfism can be the result of mutations that affect growth hormones. Mutations are usually harmful, but some may help an organism survive, by proving to be beneficial to the species. Regardless, mutations are a major driving force behind biological change (evolution).
Every cell contains DNA on threadlike structures called chromosomes. Stretches of DNA that code for specific proteins are known as genes. If the DNA of a particular gene is altered, that gene is considered a mutant gene. Such mutations may have little or no impact on protein synthesis. If, however, the mutation adversely impacts protein synthesis (i.e., if the mutation causes the protein for which it codes to be missing or defective) it is possible that the loss or alteration of the protein will result in either the death of the organism (usually early in embryonic development) or a significant phenotypic change (a visible difference in the organism). Albinism, for instance, is the result of one missing protein. This understanding of genetic inheritance is based upon experiments conducted by Thomas H. Morgan in 1910 with fruit fly mutations, and experiments conducted by George W. Beadle and Edward L. Tatum in the 1940s on bread mold mutations.
Errors in DNA take several forms. DNA itself is made up of subunits known as nucleotide bases. There are four kinds of bases: adenine, cytosine, guanine, and thymine. They are referred to by their initials: A, C, G, and T. DNA can be thought of as a code (the genetic code) written with these four letters. Mutations, in the strictest sense, are changes in the genetic sequence or code.
The term mutation is also more broadly used in genetics. For example, errors in all or part of a chromosome are another form of mutation termed chromosomal abnormalities. Humans normally have 23 pairs of chromosomes. (Each pair of chromosomes is distinct under the microscope and scientists have numbered them for ease of identification.) An extra chromosome can have an enormous effect. Three copies of chromosome 21, for instance, results in Down syndrome. People with Down syndrome have a unique physical appearance and are developmentally disabled. If parts of non-homologous chromosomes swap pieces, the result is a translocation. Such translocations can also carry a significant impact. A translocation between chromosome 9 and chromosome 22, for example, is associated with a certain type of leukemia.
Mutations that occur in an organism's egg or sperm cells are known as germinal mutations. Germinal mutations can be passed on to the organism's offspring. Mutations that occur in cells other than the sex cells are known as somatic mutations and can not be passed on. Accordingly, some causes of mutation will affect only the somatic cells of the organism exposed. Other causes will affect the germ cells and may be passed on to many succeeding generations. In this manner, a mutation can become common in certain populations.
Mutations are a normal occurrence. Mutation rates vary depending on the gene in question. The opportunity for mutation exists every time a cell replicates. Almost always, DNA reproduces itself correctly. Yet if the genetic code is somehow altered--if part of it is deleted, duplicated, or switched --the result is a mutation. Generally, cells that divide many, many times in a lifetime are more at risk for errors than cells that divide less frequently.
Uncontrolled cell growth, known as cancer, is also a kind of mutation. Some types of cancers are associated with environmental factors such as smoking. Many researchers assert that repeated exposure to cigarette smoke may cause a somatic mutation in the lung cells that leads to lung cancer. Other environmental factors that are known to cause mutations include exposure to radiation, pesticides, asbestos, and some (now banned) food additives. Factors that cause mutations are termed mutagens. Those that cause cancer are known as carcinogens.
Scientists have found that carcinogens work by engineering mutations in important genes. A gene known as p53, for instance, helps prevent the growth of tumors. Yet, exposure to ultraviolet light and other environmental stimuli may cause that gene to mutate. In its mutated form, the gene no longer prevents tumors. People with two copies of the mutated gene are at greater risk develop some forms of cancer. If mutated genes responsible for cancer are present in the egg or sperm cells, then a susceptibility to cancer may be passed on to future generations. This mechanism may account for cancers that have a hereditary association in families, and points to the existence of a germinal mutation.
Developing embryos and fetuses are especially at risk for mutation. Their cells divide very rapidly and become increasingly specialized for specific tasks. Accordingly, physicians often warn pregnant women to avoid excessive x rays and some medications in order to protect developing fetal cells.
For every human trait, there was once a person or small group in which the genetic mutation first appeared. Certain mutations occurred long ago and are now part of what is considered the normal genetic constitution shared by the majority of humans. Other traits occur only in certain populations of people. Cystic fibrosis, for instance, is most common in people of northern European descent. Sickle cell anemia, a serious blood disease, occurs frequently in people of African and Mediterranean ancestry. Tay-Sachs disease, a fatal disorder, is found primarily in ethnic Jews with eastern European ancestors.
Whether a mutation is detrimental (deleterious) to useful to a population may depend on external or environmental circumstances. For example, while two copies of the mutant sickle cell anemia gene may result in serious illness, one copy confers a resistance to malaria. Such selective advantage proves useful to people living in the tropics where malaria is common and, as a group, outweighs the deleterious effects of sickle cell. For these reasons, sickle cell genes are preserved in some populations. Scientists have hypothesized that some advantage must be conferred upon people with single copies of the cystic fibrosis gene or the Tay-Sachs disease gene.
Over millions of years, advantageous mutations have allowed life to develop and diversify from primitive cells into the multitude of species on Earth. Evolution is led by mutation. Mutations that allow an organism to survive and reproduce better than other members of its species are valuable. Mutations become especially important when an organism's environment is changing.
Animal and plant breeders use mutations to produce new or improved species of crops and livestock. Manipulation of mutations can result in crops that are resistant to drought or insects and that have a high yield per acre.
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