Mutation Definition
At the simplest level, a mutation is a change or transformation. In biology, mutations refer to changes in chromosomes and genes, which typically manifest physically.
The effect of a mutation can depend on the region in which the sequence of genetic material has been changed. The simplest and the most harmless are substitutions of a single base pair with another, with no effect on protein sequence. At the other end are insertion or deletion mutations that lead to non-functional gene products. Mutations can also occur on a large scale, with long stretches of DNA (or RNA when it is the genetic material) being inverted, inserted, duplicated, deleted, transposed or translocated.
The result of a mutation could be harmful, beneficial, neutral or even silent. Mutation can lead to the loss or gain of a specific function, to changes to the expression levels, or in extreme cases, even embryonic lethality.
Types of Mutation
Mutations can be classified in various ways depending on the cause of the mutation, its effect on the function of the gene product or the kind of changes to the structure of the gene itself.
Mutagenic agents such as carcinogens or high-energy radiation lead to changes to the genomic material. Some mutations occur as a natural byproduct of the error rate in DNA or RNA replication mechanisms.
A mutation could be a loss-of-function or gain-of-function mutation, depending on whether the gene product is inactivated or has enhanced activity. In heterozygotes with two copies of every allele, some mutated gene products can suppress the effect of the wild-type allele. These are called dominant negative mutations.
All these effects arise from a change to the structure of a gene or allied chromosomal material. These structural changes can be classified as substitutions, deletions, insertions, amplifications, or translocations.
Substitution Mutations
Substitution mutations are situations where a single nucleotide is changed into another. In organisms having double-stranded DNA or RNA, this usually means that the corresponding base pair is also altered. For example, an A:T base pair could be mutated into a G:C base pair or even a T:A base pair. Depending on the position of this change, it could have a variety of effects.
In highly conserved regions, both in the coding and regulatory stretches of DNA, mutations often lead to deleterious effects. Other, more variable stretches are more accommodating. In the promoter region or in other regulatory parts of the genome, a substitution mutation may change gene expression or the response of the gene to stimulus. Within the coding region, a substitution in the third or wobble position of a codon is called a silent mutation since there is no change to the amino acid sequence. When a substitution mutation results in a new amino acid but with similar properties – it is a neutral or a conserved mutation. For instance, if aspartic acid is substituted with glutamic acid, there is a reasonable chance that there would be very few changes to the biochemistry of the protein.
Lastly, the most drastic substitution mutation is one that results in the premature termination of amino acid elongation because of the sudden appearance of a stop codon in the middle of the coding sequence. For instance, if the UAC codon coding for threonine is mutated into a UAA codon, especially in the 5’ end of the coding sequence, it will likely lead to an extremely short, possibly non-functional protein.
Insertions and Deletions
Insertions and deletions refer to the addition or removal of short stretches of nucleotide sequences. These types of mutations are usually more deleterious than substitutions since they can cause frame shift mutations, altering the entire amino acid sequence downstream of the mutation site. They can lead to a change in polypeptide length, either creating abnormally long proteins that cause aggregates or truncated polypeptides that are non-functional and can clog the translation machinery of the cell.
Insertions and deletions in the regulatory regions of a polypeptide coding sequence or in genes coding for non-coding RNA are less obviously harmful. Here again, the position of the mutation matters – in highly conserved regions, the mutation is more likely to result in negative effects.
Large-scale mutations
Changes to the nucleotide sequence in genetic material can also occur on a large scale, sometimes involving thousands of base pairs and nucleotides. These kinds of mutations include amplifications, where segments of genetic material are present in multiple copies, and deletions, where a large chunk of genetic material is removed. Occasionally, some parts of the genome are translocated to a different chromosome, or reinserted into the same position, but in an inverted orientation. Translocations and deletions can bring together genes that are normally placed far apart from each other, either leading to the formation of mosaic polypeptides, or to the differential regulation of the genes within the segment.
Examples of Mutation
Example #1: Sickle Cell Disease and Malaria
Sickle cell disease (SCD), so-named due to its characteristic sickling effect on red blood cells, usually manifests via blood clots, anemia, and bouts of pain known as “sickle-cell crises.” While many of these symptoms can be treated with medication, they still significantly lower the quality of life of their carriers.
Although considered rare and a mutation, SCD is relatively well-researched. It takes place on the 11th chromosome, and is catalyzed by the inheritance of an abnormal hemoglobin gene from both parents. As far as global prevalence goes, SCD is the most prevalent among West African populations, with an incidence rate of about 4.0%
Research suggests that the prevalence of SCD in West Africa is not a chance phenomenon. Despite its effects on health, SCD has also been shown to reduce risk of contracting malaria from mosquitoes. As West Africa’s climate allows malaria to thrive, SCD serves as a means of protecting the population.
In all, SCD serves as an example of genetic mutation benefiting the populations it effects. This, in part, is why some genetic mutations endure for decades and even centuries.
Example #2: Klinefelter’s Calicos
Klinefelter syndrome, also known as XXY syndrome, is a genetic mutation in which a male subject carries an extra X chromosome, therefore carrying the female genotype XX in addition to the traditional male genotype XY. Likewise, males with Klinefelter syndrome often have feminine features, such as breast tissue, and may not be able to reproduce.
As it lies in the genetic code, which is homologous between most species, Klinefelter syndrome is not exclusive to humans. Therefore, cats, dogs, and even whales can inherit the XXY genotype.
In cats, the X chromosome carries more than sex-related information. Fur color, for instance, is carried on the X chromosome.
Furthermore, fur color is codominant. As male cats typically inherit only one X chromosome and female cats inherit two X chromosomes, female cats are more likely to have multicolored fur patterns than male cats.
This is especially true for the calico, a cat known for its striking orange-and-black fur. The gene for black fur cannot be carried on the same X chromosome as the gene for orange fur, which makes calico cats almost exclusively female.
However, this does not make the existence of a male calico impossible. Male cats with two X chromosomes, or genotype XXY, may very well carry the gene for orange fur on one X chromosome and the gene for black fur, on the other. In this way, they are indeed “Klienfelter’s Calicos.”
Example #3: Lactose Tolerance
We mentioned earlier how SCD, a mutation marked by sometimes life-threatening physical symptoms, also works to prevent malaria in West Africa. Lactose tolerance is another mutation that benefits those who have it.
Human bodies were originally unable to produce lactase, an enzyme that digests the proteins in cow’s milk, after the first months of life. This is because humans often did not consume milk – or other dairy products, for that matter – into adulthood.
The rise of pasteurization, as well as commercial farming, nearly did away with this old habit. As we can see today, humans of all ages eat cheese and drink milk. Of course, this comes after a significant bodily change. A mutation that prolongs lactase production in humans, which is currently more prevalent in Western nations, allows humans to eat dairy products without stomach pain or nausea.
Like SCD, this mutation remains because it helps humans consume vital nutrients, like calcium and potassium, through a more diverse range of sources.
Related Biology Terms
- Chromosome – A part of DNA that carries genetic information.
- Homologous – Having the same function or structure within a body, or between two species.
Quiz
1. Mutations like SCD, which sometimes have deadly side effects, do not become extinct due to natural selection because:
A. The government wants them to stay.
B. Mutations work outside of natural selection. Unlike traits, they cannot be bred out.
C. They provide resistance or immunity to other, more serious illnesses.
D. Mutations are a superbug that drugs cannot combat.
2. Mutations are sometimes carried on the sex chromosomes, X and Y. Why might a male inherit a mutation carried by his mother, even though his mother does not have the mutation, herself?
A. The male inherited the recessive mutation on his X chromosome, while his mother inherited the recessive mutation on one X chromosome and a dominant form of the gene on her other X chromosome.
B. The male inherited the dominant mutation on his X chromosome, because his mother carried the dominant mutation on both of her X chromosomes.
C. The male inherited the dominant mutation on his Y chromosome, because his mother carried the dominant mutation on her Y chromosome.
D. The male inherited the dominant mutation on his Y chromosome, because his mother carried the recessive mutation on her Y chromosome.
3. Male calico cats are rare because:
A. The gene for fur color is carried on the X chromosome, and is inherited exclusively from the mother. The mother would have to carry both the gene for orange fur and the gene for black fur for her male offspring to be a calico.
B. The gene for fur color is carried on the X chromosome, and male cats only have one X chromosome. A male cat would have to have two X chromosomes, or the Klinefelter’s syndrome mutation, to inherit both orange and black fur.
C. The gene for fur color is carried on the X chromosome, and male cats do not always inherit the X chromosome. This is why there are so many albino male cats.
D. The gene for fur color is carried on the Y chromosome, and male cats do not usually inherit two Y chromosomes. A male cat must therefore have an XYY genotype to be a calico.
Mutation
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