Mutation is a permanent alteration in the nucleotide sequence of DNA (deoxyribonucleic acid). As a result of mutation, the amino acid sequence of proteins encoded by the stretch of DNA or gene is changed, which in turn, may alter the composition and/or function of body cells and tissues.
Mutation is a major reason for variation in the genetic composition of a population or gene pool. In organisms, mutation can be caused due to cell division (mitosis and meiosis), exposure to mutagens (carcinogens), strong radiations, and viruses.
Mutation in higher organisms is either somatic or germ-line. The former type refers to the mutation in the body cells, which is not usually passed on to the offspring. Germ-line mutation occurs in the germ cells, and is inherited by the offspring via the reproduction cells. Based on the long-term effects of mutation in the particular population, it can be categorized as beneficial (more favorable), deleterious (less favorable), and neutral.
Some Examples of Beneficial Mutation
Beneficial mutation is retained in the population and accumulates in the form of adaptations in the course of evolution, whereas deleterious is not retained and is removed by means of natural selection. Neutral mutation, on the other hand, does not cause significant effects in the population. Generally, neutral mutations are accumulated through genetic drift. The effects of mutation vary depending upon the environment. Let's take a look at some of the examples of favorable mutations that promote the fitness of the organisms.
Nylonase: Nylon BacteriaNylonase is an example of beneficial mutation in bacteria. The nylonase bacteria can eat short molecules of nylon (nylon-6). The mutation in these bacteria involves insertion of a single nucleotide in the genetic material. It is estimated that this frameshift mutation might have occurred in the 1940s when nylon was invented. Nylonase can be used in wastewater treatment plants.
Antibiotic Resistance: Bacteria
Antibiotics are used for the treatment of diseases caused by bacteria. Constant use of antibiotics leads to the development of resistance among the targeted bacteria. Many a time, the antibiotic resistance reduces the fitness of the particular bacterial population, when they are exposed to non-antibiotic environment. These resistant bacteria do not possess the ability to reproduce as fast as those without mutation, thus slowing down the disease progression.
Gene Mutation: Almond Trees
Almond seeds from wild species contain amygdalin, a bitter chemical that converts into cyanide inside the human body. According to researchers, consuming wild almonds is fatal. A single gene mutation in wild almond trees resulted in a variety that no longer synthesizes amygdalin. When humans discovered this non-bitter almond species, they cultivated them, which is continued till today.
Murray Gray: A Breed of Beef Cattle
Murray Gray is a cattle breed, obtained accidentally from a traditional cow species. The calves produced by the specific cow were more productive than those produced by the others. Farmers soon noticed the difference and started breeding from the offspring. This way, the Murray breed with some of the most positive characteristics has become popular all over Australia, which then spread to various other countries.
CCR5-delta32: HIV Immunity in Humans
Cysteine-cysteine chemokine receptor 5 (CCR5) is a receptor molecule, located in the membranes of white blood cells (WBCs) and nerve cells. In a cell, CCR5 permits the entry of chemokines that signals the inflammatory response to any foreign particles. The gene responsible for coding CCR5 is present in the human chromosome 3. A mutation in this gene called CCR5-delta32 (involving deletion of 32 base pairs) affects the normal functioning of the CCR5.
In the initial stages of HIV infection, the virus normally enters through CCR5. However, a mutated CCR5 blocks the entry of HIV. People carrying homozygous mutated CCR5-delta32 are resistant to HIV, while heterozygous ones are beneficial, as they slow down the disease progression. Thus, CCR5-delta32 provides partial or complete immunity to HIV.
Although, the CCR5-delta32 mutation has one drawback. It is strongly associated with a chronic liver disease called primary sclerosing cholangitis (PSC). A long-term progressive liver and gallbladder disorder characterized by inflammation and fibrosis of the intrahepatic and extrahepatic bile ducts causing the bile to drain from the gallbladder.
There is no doubt that some of the most productive plants and animals are evolved as a result of mutation. The effects of mutation are well explained by natural selection in which favorable changes persist in the population, while the harmful alterations are eliminated over a period of time.