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What are harmful mutations?
Harmful mutations are variations in an organism's genetic code that cause harmful changes in gene expression. Harmful mutations arise from exposure to harmful chemicals, viruses, traumatic injury, radiation, UV light, or hereditarily. Such mutations can be caused in two ways: induced or spontaneous.1
Induced mutations are caused by exposure to harmful things in the environment, such as chemicals, UV light, and radiation, while spontaneous mutations occur randomly within the body due to natural reactions that take place within the body. The majority of spontaneous mutations are harmless, though a minority can be quite harmful to the organism. Mutations can be point mutations, frameshift mutations, substitution mutations, nonsense mutations, missense mutations, addition mutations, or subtraction mutations. See the point mutations article for more discussion on these types of mutations.
Harmless mutations are usually not expressed, meaning they do not alter the organism's gene expression. These types of mutations are known as silent mutations. A silent mutation is a type of substitution or point mutation where the organism's gene expression is not affected. Typically silent mutations arise when a base pair is changed, but the new codon still codes for the same amino acid as the original codon.
An example of a silent mutation is changing the original codon AAA to AAG by changing the last base within the codon. This mutation has no effect on the organism because AAA and AAG both code for the amino acid lysine.1 This change will not affect the organism because the amino acid lysine will still be produced in its original place in the nucleotide sequence.
Examples of harmful mutations
Point mutations are usually harmless if they are silent mutations. Missense and nonsense point mutations, however, can cause serious disease because these mutations can change the codon completely, where the new codon codes for an entirely different amino acid.2 This phenomenon is seen in the case of sickle cell anaemia which is a debilitating disease characterized by poor blood flow to bodily organs and chronic pain.5
Sickle cell anaemia is caused by a missense point mutation in the haemoglobin gene. Within the normal haemoglobin gene, the codon GAA codes for glutamic acid leading to a healthy round haemoglobin A molecule. However, when the sickle cell point mutation is present, GAA is converted to GUA.5 GUA codes for the amino acid valine, which produces haemoglobin S a sticky sickle-shaped haemoglobin molecule that causes the organism's red blood cells to stick together, drastically reducing blood flow to areas of the body. 5
People with sickle cell disease inherit this point mutation from family members as the mutation is carried on the DNA level. The mutated gene is a recessive gene which means that the offspring must have both mutated genes to have the full sickle cell anaemia disorder. Offspring with only one mutated gene still have sickle cells in their system however, only a portion of their cells are misshapen, while their reminding cells are fully healthy.5
The most common harmful mutations are substitution point mutations, as seen in sickle cell anaemia. These mutations result when one base is replaced by another base. There are two types of substitution mutations: transitions and transversions.1 Transition substitutions occur when a purine or pyrimidine is replaced by a base of the same kind.1 For example, a purine such as adenine may be replaced by another purine such as guanine. A transversion substitution on the other hand occurs when a purine is replaced by a pyrimidine.1 For example, the purine adenine may be replaced by the pyrimidine cytosine.
It may be a while since you reviewed the purines and pyrimidines, so here is a quick refresher. Purines and pyrimidines are nitrogenous bases that form two kinds of nucleotide bases in DNA. Purines have two carbon-nitrogen rings, while pyrimidines only have one carbon-nitrogen ring. Adenine and guanine are purines, while thymine and cytosine are pyrimidines. See Figure 1 for a visual illustration.
Other types of harmful mutations include nonsense and frameshift mutations. Nonsense mutations result when an amino acid coding codon is substituted for a stop codon.2 Nonsense mutations can be very harmful to organisms because it prevents the entire gene from being transcribed due to a premature stop codon being inserted into the gene. Nonsense mutations can cause rare genetic diseases such as Duchenne muscular dystrophy, cystic fibrosis, and various cancers and neurological disorders.2 We will discuss some of these specific diseases in a later section.
Frameshift mutations are arguably the most harmful type of mutation because they result in a shift in the gene reading frame.1 Frameshift mutations are caused by random insertions or deletions of bases in the DNA. These mutations have the potential to change every codon in a genetic sequence or create a premature stop codon. Let's take a look at an example.
Frameshift mutations can change the entire reading frame of a gene. For example, a normal gene may be AGG-TAC-CCT-TAC a random insertion of another A at the beginning of the gene will cause each base to shift one space resulting in AAG-GTA-CCC-TTA-C. Notice how the insertion of only one base changes the entire gene.
See Figure 2 for an illustration of the various types of point mutations.
Harmful effects of genetic mutations
As previously mentioned, genetic mutations have many harmful effects on organisms. Genetic mutations can cause various rare diseases such as muscular dystrophy, Huntington's disease, cancer, and much more.2 Also mutations that occur during fetal development can cause physical disabilities such as microcephaly, cleft lips, spina bifida, and other congenital disorders. It is arguable that mutations in the fetal brain cause disorders such as Autism, ADHD, and other mental disorders, though there are no conclusive research findings.
Spina bifida: A rare neurological condition characterized by irregular development of the central nervous system. People with spina bifida have spinal cords that are not protected by the bones in their spine since their spinal cord developed outside of their spine.
Negative effects of mutation
As previously mentioned, mutations can have negative effects on the affected organism, such as disease, deformity, and even death. Mutations can also affect a person in ways that are not obvious. For example, pathogens such as viruses infect host organisms and attempt to take over the host's cells. Once your body is infected with a virus, your immune system works hard to kill the virus to prevent it from infecting any more of your healthy cells. Once the virus is dead, your body deploys antibodies specific to antigens on the virus to prevent it from entering your cell if you were to get infected again. The reason why we are always getting sick from the same virus is that viruses have the ability to evolve.
Antibodies: A protein produced by specialized immune cells called B cells. These proteins are produced in response to a specific microbe and help the body prevent subsequent infections from the same microbe.
Viruses are just strings of genetic code that hijack your cells in order to reproduce. Just like your cells need to transcribe DNA in your to reproduce, viruses too require transcriptional machinery to do so. However, viruses do not have their own translational machinery, which is why they target host cells. Since the virus is made up of genetic material, it has the ability to mutate and develop changes in its protein structure and function. these changes allow viruses to evade antibodies and mechanisms placed by your immune system, which is why we continue to get sick from the same virus and why we need to get vaccinated every year. Sometimes we need to get vaccinated more than once a year in the case of highly mutagenic viruses like COVID-19.
List of harmful mutations
We have already discussed how harmful mutations can be to an organism. Let's move on to discussing specific examples of harmful mutations. In a previous section, we discussed that mutations can cause many diseases such as cancer and cystic fibrosis. These diseases can be very debilitating to the affected organism, as their quality of life and life expectancy will suffer.
The human genome contains certain genes called proto-oncogenes.3 These are genes that are capable of turning into oncogenes upon certain mutations. Oncogenes are genes that cause a cell to become cancerous and divide uncontrollably. In the case of chronic myelogenous leukaemia, chromosomes actually exchange parts, with results in the merging of two separate genes into one gene. Chromosome 9 carries the ABL1 gene while the end of chromosome 22 carries the BCR gene. Chromosomes 9 and 22 exchange parts to create the Philadelphia chromosome containing the newly fused BCR-ABL gene. This gene is a potent stimulator of cell division and leads to various forms of leukaemia in the affected organism.
In the case of cystic fibrosis, it is caused by mutations in the gene that produces the cystic fibrosis transmembrane conductance regulator protein (CFTR).4 The most common mutation associated with CF is the deletion of a phenylalanine at position 508. This mutation leads to lung disease due to the build-up of mucus.
Harmful Mutations - Key takeaways
- Harmful mutations arise from exposure to harmful chemicals, viruses, traumatic injury, radiation, UV light, or hereditarily.
- Induced mutations are caused by exposure to harmful things in the environment such as chemicals, UV light, and radiation, while spontaneous mutations occur randomly within the body due to natural reactions that take place within the body.
- Harmless mutations are usually not expressed, meaning that they do not alter the organism's gene expression. These types of mutations are known as silent mutations.
- When the sickle cell point mutation is present, GAA is converted to GUA. GUA codes for the amino acid valine which produces haemoglobin S a sticky sickle-shaped haemoglobin molecule that causes the organism's red blood cells to stick together.
References
- Eggebrecht, J (2018) Biology for AP Courses. Rice University.
- Benhabiles H et.al. (2017)Optimized approach for the identification of highly efficient correctors of nonsense mutations in human diseases. Plos One
- Chial, H. (2008) Proto-oncogenes to oncogenes to cancer. Nature Education 1(1):33
- Ostedgaard, L. S., Meyerholz, D. K., Chen, J. H., Pezzulo, A. A., Karp, P. H., Rokhlina, T., Ernst, S. E., Hanfland, R. A., Reznikov, L. R., Ludwig, P. S., Rogan, M. P., Davis, G. J., Dohrn, C. L., Wohlford-Lenane, C., Taft, P. J., Rector, M. V., Hornick, E., Nassar, B. S., Samuel, M., Zhang, Y., … Stoltz, D. A. (2011). The ΔF508 mutation causes CFTR misprocessing and cystic fibrosis-like disease in pigs. Science translational medicine, 3(74), 74ra24. https://doi.org/10.1126/scitranslmed.3001868
- National Human Genome Research Institute (2020)About Sickle Cell Disease
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Frequently Asked Questions about Harmful Mutations
Are all mutations harmful?
No. Only mutations that change the amino acid being coded or mutations in oncogenes are considered harmful.
Which mutation is harmful to organisms?
Missense mutations, nonsense mutations, frameshift mutations, deletion and addition mutations. Mutations in oncogenes are also harmful to an organism. Also, mutations in viruses can lead to death or serious illness in infected organisms.
How can mutations be harmful?
A mutation can be harmful if it results in serious illness or deformity in the affected organism.
What are examples of harmless mutations?
Silent mutations are harmless mutations because they do not change the amino acid being coded.
What type of DNA mutation is most harmful?
Missense, nonsense, frameshift, and oncogenetic mutations are considered the most harmful mutations as they can cause serious diseases in the affected organism.
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