48 5.8 Mutations

Created by: CK-12/Adapted by Christine Miller

Figure 5.8.1 Teenage Mutant Ninja Turtles Cosplay: Raphael and Michelangelo.

Mutant Cosplay

You probably recognize these costumed comic fans in Figure 5.8.1 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 often result in other drastic (but more realistic) changes in living things.

What Are Mutations?

 are random changes in the sequence of bases in  or . The word mutation may make you think of the 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 .

Causes of Mutations

Mutations have many possible causes. Some mutations seem to happen spontaneously, without any outside influence. They occur when errors are made during DNA replication or during the transcription phase of protein synthesis. Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is known as a . Examples of mutagens are shown in the figure below.

Examples of Radiation, chemicals and infectious agents: An mage of a sun icon and hand x-ray for UV and x-ray radiation; a picture of hands holding a cigarrette and a vape, 3 smokies on a grill (nitrates/ nitrites and mutagenic BBQ chemicals) and a stylized image of a woman in a green acne face mask with benzoyl peroxide to represent chemicals. To represent infectious agents: an orange spherical virus as human papillomavirus (HPV) and a purple spirilla bacterium with flagella for Helicobacter Pylori - a bacteria spread through contaminated food.
Figure 5.8.2 Examples of Mutagens. Types of mutagens include radiation, chemicals, and infectious agents. Do you know of other examples of each type of mutagen shown here?

 

Types of Mutations

Mutations come in a variety of types. Two major categories of mutations are germline mutations and somatic mutations.

  • occur in gametes (the sex cells), such as eggs and sperm. These mutations are especially significant because they can be transmitted to offspring, causing every cell in the offspring to carry those 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 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 they may alter just one or a few nucleotides.

Chromosomal Alterations

 are mutations that change chromosome structure. They occur when a section of a chromosome breaks off and rejoins incorrectly, or otherwise does not rejoin at all. Possible ways in which 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 disease caused by a chromosomal duplication is Charcot-Marie-Tooth disease type 1 (CMT1). It is characterized by muscle weakness, as well as loss of muscle tissue and sensation. The most common cause of CMT1 is a duplication of part of chromosome 17.

 

Figure 5.8.3 Chromosomal alterations are major changes in the genetic material.

Point Mutations

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 described in Table 5.8.1. The effects of point mutations depend on how they change the genetic code.

Table 5.8.1: The Effects of Point Mutations
Type Description Example Effect
Silent mutated codon codes for the same CAA (glutamine) → CAG (glutamine) none
Missense mutated codon codes for a different amino acid CAA (glutamine) → CCA (proline) variable
Nonsense mutated codon is a premature stop codon CAA (glutamine) → UAA (stop) usually serious

Frameshift Mutations

 is a deletion or insertion of one or more nucleotides, changing the 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. 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, therefore, 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  product.

Effects of Mutations

The majority of  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 mutations have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA.

Beneficial Mutations

Some mutations — known as beneficial mutations — have a positive effect on the organism in which they occur. 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 two such examples:

  1. Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs, leading to the evolution of antibiotic-resistant strains of bacteria.
  2. A unique mutation is found in people in Limone,  a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels despite a high-fat diet. The individual in which this mutation first appeared has even been identified and many of his descendants carry this gene.

Harmful Mutations

Imagine making a random change in a complicated machine, such as a car engine. There is a chance that the random change would result in a car that does not run well — or perhaps does not run at all. By the same token, a random change in a gene’s DNA may 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 .

  • 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.
  • 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 . Because of the mutations, cells with damaged are allowed to divide without restriction.

Feature: My Human Body

Inherited mutations are thought to play a role in roughly five to ten per cent 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 include hereditary breast and ovarian cancer, caused by mutations in genes called 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:

  1. The test can determine definitively whether a specific gene mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
  2. 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.
  3. You have a personal or family history that suggests you are at risk of an 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. 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.

5.8 Summary

  • are random changes in the sequence of bases in or . 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 or . Other mutations are caused by environmental factors called . Mutagens include radiation, certain chemicals, and some infectious agents.
  • occur in gametes and may be passed onto offspring. Every cell in the offspring will then have the mutation. occur in cells other than gametes and are confined to just one cell and its daughter cells. These mutations cannot be passed on to offspring.
  • are mutations that change chromosome structure and usually affect the organism in multiple ways. Charcot-Marie-Tooth disease type 1 is an example of a chromosomal alteration in humans.
  • 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.
  • 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 effect 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, such as the mutations that cause genetic disorders or .

5.8 Review Question

  1. Define mutation.
  2. Identify causes of mutation.
  3. Compare and contrast germline and somatic mutations.
  4. Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation.
  5. Why do many mutations have neutral effects?
  6. Give one example of a beneficial mutation and one example of a harmful mutation.
  7. Why do you think that exposure to mutagens (such as cigarette smoke) can cause cancer?
  8. Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation.
  9. Compare and contrast missense and nonsense mutations.
  10. Explain why mutations are important to evolution.

 

5.8 Explore More

How Radiation Changes Your DNA, Seeker, 2016.

Where do genes come from? – Carl Zimmer, TED-Ed, 2014.

What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin,
TED, 2019.

Attributions

Figure 5.8.1

Ninja Turtles by Pat Loika on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.8.2

Examples of Mutagens by Christine MIller is used under a CC BY SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/) license.
Separate images are all in public domain or CC licensed:

Figure 5.8.3

Scheme of possible chromosome mutations/ Chromosomenmutationen by unknown on Wikimedia Commons is adapted from NIH‘s Talking Glossary of Genetics. [Changes as described by de:user:Dietzel65]. Further use and adapation (text translated to English) by Christine Miller as image is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

 

References

Seeker. (2016, April 23). How radiation changes your DNA. YouTube. https://www.youtube.com/watch?v=PQjL4ZDuq2o&feature=youtu.be

TED. (2019, June 5). What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin. YouTube. https://www.youtube.com/watch?v=a63t8r70QN0&feature=youtu.be

TED-Ed. (2014, September 22). Where do genes come from? – Carl Zimmer. YouTube. https://www.youtube.com/watch?v=z9HIYjRRaDE&feature=youtu.be

Wikipedia contributors. (2020, July 6). Breast cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Breast_cancer&oldid=966366739

Wikipedia contributors. (2020, July 9). Charcot–Marie–Tooth disease. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charcot%E2%80%93Marie%E2%80%93Tooth_disease&oldid=966912915

Wikipedia contributors. (2020, July 7). Cystic fibrosis. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Cystic_fibrosis&oldid=966566921

Wikipedia contributors. (2020, June 4). Limone sul Garda. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Limone_sul_Garda&oldid=960771991

Wikipedia contributors. (2020, June 23). Ovarian cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ovarian_cancer&oldid=964157192

Wikipedia contributors. (2020, May 7). BRCA mutation. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA_mutation&oldid=955463902

Wikipedia contributors. (2020, July 10). Teenage Mutant Ninja Turtles. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Teenage_Mutant_Ninja_Turtles&oldid=967030468

 

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

Share This Book