Mutations generally fall into two types: point mutations and chromosomal aberrations. In point mutations, one base pair is changed. The human genome, for example, contains over 3. Mistakes, although surprisingly rare, do happen. About one in every 10 10 10,,, base pair is changed.
The most common type of mistake is a point substitution. More uncommon is the failure to copy one of the bases deletion , the making of two copies for a single base point duplication or the addition of a new base or even several bases insertion. Chromosomal aberrations are larger-scale mutations that can occur during meiosis in unequal crossing over events, slippage during DNA recombination or due to the activities of transposable events. Genes and even whole chromosomes can be substituted, duplicated, or deleted due to these errors Figure 1.
Point substitutions are in red, and the yellow box with dashes indicates a deletion of 12 bases. Mutations can have a range of effects. They can often be harmful. Others have little or no detrimental effect. And sometimes, although very rarely, the change in DNA sequence may even turn out to be beneficial to the organism. A mutation that occurs in body cells that are not passed along to subsequent generations is a somatic mutation. A mutation that occurs in a gamete or in a cell that gives rise to gametes are special because they impact the next generation and may not affect the adult at all.
Such changes are called germ-line mutations because they occur in a cell used in reproduction germ cell , giving the change a chance to become more numerous over time. If the mutation has a deleterious affect on the phenotype of the offspring, the mutation is referred to as a genetic disorder.
Alternately, if the mutation has a positive affect on the fitness of the offspring, it is called an adaptation. Thus, all mutations that affect the fitness of future generations are agents of evolution.
Mutations are essential to evolution. Every genetic feature in every organism was, initially, the result of a mutation. The new genetic variant allele spreads via reproduction, and differential reproduction is a defining aspect of evolution. It is easy to understand how a mutation that allows an organism to feed, grow or reproduce more effectively could cause the mutant allele to become more abundant over time. Even deleterious mutations can cause evolutionary change, especially in small populations, by removing individuals that might be carrying adaptive alleles at other genes.
Hyla versicolor , is an example of mutation and its potential effects. When an ancestral Hyla chrysocelis gray treefrog failed to sort its 24 chromosomes during meiosis, the result was H. This treefrog is identical in size, shape and color to H.
All rights reserved. Most mutations occur at single points in a gene, changing perhaps a single protein, and thus could appear unimportant. For instance, genes control the structure and effectiveness of digestive enzymes in your and all other vertebrate salivary glands. At first glance, mutations to salivary enzymes might appear to have little potential for impacting survival.
Yet it is precisely the accumulation of slight mutations to saliva that is responsible for snake venom and therefore much of snake evolution. Natural selection in some ancestral snakes has favored enzymes with increasingly more aggressive properties, but the mutations themselves have been random, creating different venoms in different groups of snakes.
Snake venoms are actually a cocktail of different proteins with different effects, so genetically related species have a different mixture from other venomous snake families.
The ancestors of sea snakes, coral snakes, and cobras family Elapidae evolved venom that attacks the nervous system while the venom of vipers family Viperidae; including rattlesnakes and the bushmaster acts upon the cardiovascular system. Both families have many different species that inherited a slight advantage in venom power from their ancestors, and as mutations accumulate the diversity of venoms and diversity of species increased over time.
Although the history of many species have been affected by the gradual accumulation of tiny point mutations, sometimes evolution works much more quickly. Several types of organisms have an ancestor that failed to undergo meiosis correctly prior to sexual reproduction, resulting in a total duplication of every chromosome pair. Such a process created an "instant speciation" event in the gray treefrog of North America Figure 2. What is the new frequency of A 2 after one generation of mutation?
We find that there is not much change in the frequency of A 2 after one generation of mutation. In general, after t generations, the frequency of the A 1 wild-type allele will be. To calculate the number of generations required to change allele frequencies by a given amount, solve for t, which gives:. We can use this formula to calculate the number of generations needed to change allele frequencies under the assumption that mutation is the only evolutionary force acting on a population.
To move the frequency of A 1 from 1. To move it from 0. In general, as the frequency of the wild-type allele decreases, it takes longer to accomplish the same amount of change. This simple model should convince you that mutation is a very weak force when it comes to changing allele frequencies. But mutation is very important for introducing new alleles new DNA sequences into populations.
The number of alleles in a population will be related to the size of the population. Mutation rates are calculated in units of generations, either per individual, per base pair, or per spore. A mutation rate of 1 x 10 -6 can mean that a mutation for a particular gene will occur once every million cells per generation, or once in every million base pairs of DNA per generation.
The only mutations that are passed to progeny are those that occur in reproductive cells, such as fungal spores or virus particles or sperm or eggs. A mutation rate of 1 x 10 -6 also implies that the mutation occurs at a frequency of one in every million individuals in a population. Mutation rates vary across genes and organisms, but they are usually low and can be considered rare events in most cases Flor , Zimmer , Gassman et al. This means that, on average, in a population of one million individuals spores, bacterial cells, or virus particles , you can expect to find one mutant for any given locus per generation.
In a population of 10 million individuals, you would expect to find 10 mutants for any locus. And in a population of 1 billion individuals, you expect to find mutants for any locus. Consider the barley powdery mildew pathogen Blumeria graminis f.
With a mutation rate of 10 -6 at avirulence loci, there would be approximately 10 7 virulent mutant spores produced in each hectare each day. These virulent mutants can travel out of a field planted to a susceptible barley cultivar and infect a neighboring field planted to a resistant barley cultivar. The virulent mutants that have lost the elicitor encoded by the avirulence allele can infect the resistant cultivar and produce a new generation of virulent progeny.
This process appears to have happened many times with powdery mildew and rust fungi in agricultural ecosystems, leading eventually to boom-and-bust cycles. Thus mutation is the critical first stage in producing the "bust. In general, large populations are expected to have more alleles than small populations because there are more mutants present for selection or genetic drift to operate on.
This is one reason to keep pathogen population sizes as low as possible in agroecosystems. In addition, large populations usually contain more alleles because they experience less genetic drift. Genetic drift leads to a reduction in the number of alleles in a population.
Finally, the diversity of alleles at a locus will be affected by the length of time a population occupies a particular area. Over thousands of generations, many mutations will be introduced into a population and some of these will increase to a detectable frequency as a result of selection or genetic drift.
In cultivated crops like strawberries, farmers may help this process along by selectively growing plants with mutations that make the fruits more resilient against disease, larger, and more flavorful.
A mutation is a change in the structure of a gene , the unit of heredity. Genes are made of deoxyribonucleic acid DNA , a long molecule composed of building blocks called nucleotides. Each nucleotide is built around one of four different subunits called bases.
These bases are known as guanine, cytosine, adenine, and thymine. A gene carries information in the sequence of its nucleotides, just as a sentence carries information in the sequence of its letters. One type of mutation is a change in a base. This is called a point mutation and it is like changing one letter in a word. Most genes carry instructions for making proteins.
When a base is changed in a gene, different results are possible, depending on which base is changed and what it is changed into. The gene may produce an altered protein , it may produce no protein, or it may produce the usual protein.
Most mutations are not harmful, but some can be. A harmful mutation can result in a genetic disorder or even cancer. Another kind of mutation is a chromosomal mutation. Chromosomes, located in the cell nucleus, are tiny threadlike structures that carry genes. A chromosome consists of a molecule of DNA together with proteins. Sometimes, a long segment of DNA is inserted into a chromosome, deleted from a chromosome, flipped around within a chromosome, duplicated, or moved from one chromosome to another.
Such changes are usually very harmful. One example of a chromosomal mutation is a condition called Down syndrome. In each cell, humans normally have forty-six chromosomes, consisting of two copies of the twenty-three kinds of chromosomes.
Down syndrome usually results from the presence of one extra copy of a particular chromosome, or an extra portion of that chromosome. The presence of that extra chromosome leads to problems with certain organs of the body, such as the heart. It can also lead to leukemia—a cancer of the blood-forming cells—and produce mental disabilities. Many people with Down syndrome also have distinct facial features. Mutations can be inherited or acquired during a person's lifetime.
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