You probably recognize these costumed comic fans as two of the four Teenage Mutant Ninja Turtles. Can a mutation really turn a reptile into an anthropomorphic superhero? Of course not, but mutations can sometimes result in drastic changes in living things.
What Are Mutations?
Mutations are random changes in the sequence of bases in DNA. The word mutation may make you think of Ninja Turtles, but that's a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens or even hundreds of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.
Causes of Mutations
Is it possible for mutations to occur spontaneously, or does there have to be a cause of the mutation? Well, the answer is that both are possible. Mutagenesis is a process by which the genetic information of an organism is changed in a stable manner, resulting in a mutation. In nature, mutagenesis can lead to changes that are beneficial or harmful or have no effect. Harmful mutations can lead to cancer and various heritable diseases, but beneficial mutations are the driving force of evolution. In 1927, Hermann Muller first demonstrated the effects of mutations with observable changes in chromosomes. He induced mutagenesis by irradiating fruit flies with X-rays,
Mutagenesis may occur spontaneously or be induced. A spontaneous mutation can just happen. These mutations are not caused by an environmental factor but occur during normal cellular processes. A spontaneous mutation may be due to a mistake during DNA replication. Mutations may also occur during mitosis and meiosis. A mutation caused by an environmental factor, or mutagen, is known as an induced mutation. Typical mutagens include chemicals, like those inhaled while smoking, and radiation, such as X-rays, ultraviolet light, and nuclear radiation. Different mutagens have a different mode of damaging DNA. For example, UV radiation may cause bonding between the adjacent nucleotides on one strand of DNA molecule. This prohibits normal bonding between complementary nucleotides of the opposing strand. This causes a bulge in the DNA double helix. If this damage is not repaired, it leads to mutation. Thus, DNA does not replicate, transcribe, and translate properly.
Types of Mutations
There are a variety of types of mutations. Two major categories of mutations are germline mutations and somatic mutations.
- Germline mutations occur in gametes, the sex cells, such as eggs and sperm. These mutations are especially significant because they can be transmitted to offspring and every cell in the offspring will have the mutations.
- Somatic mutations occur in other cells of the body. These mutations may have little effect on the organism because they are confined to just one cell and its daughter cells. Somatic mutations also cannot be passed on to offspring.
Mutations also differ in the way that the genetic material is changed. Mutations may change an entire chromosome or just one or a few nucleotides.
Chromosomal alterations are mutations that change chromosome structure or number. They occur when a section of a chromosome breaks off and rejoins incorrectly or does not rejoin at all. Possible ways these mutations can occur are illustrated in the figure below. Chromosomal alterations are very serious. They often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. An example of a human chromosomal alteration is the mutation that causes Down Syndrome. It is a duplication mutation that leads to developmental delays and other abnormalities. It occurs when the individual inherits an extra copy of chromosome 21. It is also called trisomy ("three-chromosome") 21.
A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as shown in the following table. The effects of point mutations depend on how they change the genetic code.
|Silent||mutated codon codes for the same amino acid||CAA (glutamine) → CAG (glutamine)||none|
|Missense||mutated codon codes for a different amino acid||CAA (glutamine) → CCA (proline)||variable|
|Nonsense||a mutated codon is a premature stop codon||CAA (glutamine) → UAA (stop) usually||serious|
A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:
AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine
Now assume that an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after the start codon AUG. Then the sequence of bases becomes:
AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine
Even though the rest of the sequence is unchanged, this insertion changes the reading frame and thus all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mRNA are read. This can have a drastic effect on the protein product. Another example of the frameshift mutation due to the deletion of a nucleotide is illustrated in the figure below. In this example, a premature stop codon is created by the mutation.
Effects of Mutations
The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations, which are neutral because they do not change the amino acids in the proteins they encode.
Many other DNA damages or errors have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix errors in DNA. One way DNA can be repaired is illustrated in the figure below. If a cell’s DNA is permanently damaged and cannot be repaired, the cell is likely to be prevented from dividing.
Some mutations have a positive effect on the organism in which they occur. They are referred to as beneficial mutations. They generally code for new versions of proteins that help organisms adapt to their environment. If they increase an organism’s chances of surviving or reproducing, the mutations are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are just two:
- Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs. The mutations have led to the evolution of antibiotic-resistant strains of bacteria.
- A unique mutation is found in people in a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels. The individual in which the mutation first appeared has even been identified.
Imagine making a random change in a complicated machine such as a car engine. The chance that the random change would improve the functioning of the car is very small. The change is far more likely to result in a car that does not run well or perhaps does not run at all. By the same token, any random change in a gene's DNA is likely to result in the production of a protein that does not function normally or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.
- A genetic disorder is a disease, syndrome, or other abnormal condition caused by a mutation in one or more genes or by a chromosomal alteration. An example of a genetic disorder is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs.
- Cancer is a disease in which cells grow out of control and form abnormal masses of cells called tumors. It is generally caused by mutations in genes that regulate the cell cycle. Because of the mutations, cells with damaged DNA are allowed to divide without restrictions.
Feature: My Human Body
Inherited mutations are thought to play a role in about 5 to 10 percent of all cancers. Specific mutations that cause many of the known hereditary cancers have been identified. Most of the mutations occur in genes that control the growth of cells or the repair of damaged DNA.
Genetic testing can be done to determine whether individuals have inherited specific cancer-causing mutations. Some of the most common inherited cancers for which genetic testing is available hereditary, breast, and ovarian cancer, caused by mutations in genes named BRCA1 and BRCA2. Besides breast and ovarian cancers, mutations in these genes may also cause pancreatic and prostate cancers. Genetic testing is generally done on a small sample of body fluid or tissue, such as blood, saliva, or skin cells. The sample is analyzed by a lab that specializes in genetic testing, and it usually takes at least a few weeks to get the test results.
Should you get genetic testing to find out whether you have inherited a cancer-causing mutation? Such testing is not done routinely just to screen patients for risk of cancer. Instead, the tests are generally done only when the following three criteria are met:
- The test can determine definitively whether a specific gene is mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
- The test results would be useful to help guide future medical care. For example, if you found out you had a mutation in the BRCA1 or BRCA2 gene, you might get more frequent breast and ovarian cancer screenings than are generally recommended.
- You have a personal or family history that suggests you are at risk of inherited cancer.
Criterion number 3 is based, in turn, on such factors as:
- diagnosis of cancer at an unusually young age.
- several different cancers occurring independently in the same individual.
- several close genetic relatives having the same type of cancer (such as a maternal grandmother, mother, and sister all having breast cancer).
- cancer occurring in both organs in a set of paired organs (such as both kidneys or both breasts).
If you meet the criteria for genetic testing and are advised to undergo it, genetic counseling is highly recommended. A genetic counselor can help you understand what the results mean and how to make use of them to reduce your risk of developing cancer. For example, a positive test result that shows the presence of a mutation may not necessarily mean that you will develop cancer. It may depend on whether the gene is located on an autosome or sex chromosome and whether the mutation is dominant or recessive. Lifestyle factors may also play a role in cancer risk even for hereditary cancers, and early detection can often be life-saving if cancer does develop. Genetic counseling can also help you assess the chances that any children you may have will inherit the mutation.
- Mutations are random changes in the sequence of bases in DNA. Most people have multiple mutations in their DNA without ill effects. Mutations are the ultimate source of all new genetic variation in any species.
- Mutations may happen spontaneously during DNA replication. Other mutations are caused by environmental factors called mutagens. Mutagens include radiation, certain chemicals, and some infectious agents.
- Germline mutations occur in gametes and may be passed on to offspring. Every cell in the offspring will then have the mutation. Somatic mutations occur in other cells than gametes and are confined to just one cell and its daughter cells. These mutations cannot be passed on to offspring.
- Chromosomal alterations are mutations that change chromosome structure or number and usually affect the organism in multiple ways. Down syndrome (trisomy 21) is an example of a chromosomal alteration.
- Point mutations are changes in a single nucleotide. The effects of point mutations depend on how they change the genetic code and may range from no effects to very serious effects.
- Frameshift mutations change the reading frame of the genetic code and are likely to have a drastic effect on the encoded protein.
- Many mutations are neutral and have no effects on the organism in which they occur. Some mutations are beneficial and improve fitness. An example is a mutation that confers antibiotic resistance in bacteria. Other mutations are harmful and decrease fitness. Examples include mutations that cause genetic disorders or cancers.
- Define mutation.
- Identify the causes of mutation.
- Compare and contrast germline and somatic mutations.
- Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation.
- Why are many mutations neutral in their effects?
- Give an example of a beneficial mutation and an example of a harmful mutation.
- Why do you think that exposure to mutagens, such as cigarette smoke, can cause cancer?
- True or False. Mutations are always caused by exposure to toxic substances.
- True or False. Some mutations can make chromosomes longer or shorter.
- Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation.
- Compare and contrast missense and nonsense mutations.
- A mutation that substitutes one nucleotide for another is called a ___________ mutation.
- Which type of mutation is trisomy 21, or Down Syndrome?
- Explain why mutations are important for evolution.
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