Mutations cause disease, but they are the engine of evolution.
DNA works like computer code, albeit written with molecules . The sequence of their nucleotides tells cells exactly what elements to make. But when this code changes, the consequences can be more or less serious, especially if this change goes undetected.
In this article we will talk about mutations, the changes that DNA sequences undergo. We will discuss the types of genetic mutations that may exist, as well as their effects on the body.
What is a genetic mutation?
DNA is a molecular chain made up in part of nucleotides, sugar molecules (pentoses) with a nitrogenous base and a phosphate group. There are two types of nucleotides, purines , such as adenine (A) and guanine (G) , and pyrimidines , such as cytosine (C), thymine (T), and uracil (U), although the latter is not known. commonly found in DNA, if not RNA, substituting for thymine.
Mutations occur when the information contained in the DNA chain is altered , either due to external causes (environmental elements that affect DNA, deteriorating it or preventing its correct reading, such as certain carcinogenic substances) as well as internal causes (errors during DNA duplication, mobile elements such as transposons…).
It is a certainty that mutations will occur . Each living being and cell type has a different mutation rate, depending on its exposure to the environment, duplication speed – tissues that regenerate more will accumulate mutations faster, normally – among other factors. This occurs despite the multiple mechanisms of repair and correction of the genetic material.
- It might interest you: “The 10 most common hereditary diseases (and their characteristics)”
The evolutionary effect of mutation
Without mutations, evolution would not be possible . Without the concept of mutation, life would probably be reduced to the first organism that ever existed, the ancestor of all living things, which would not have changed at all over the ages.
Mutation is a source of disease and disorder, but it is also a necessary factor for living species to evolve . If there are no differences at the genetic level between individuals, theoretically all are equally sensitive or resistant to different environmental factors. A strong environmental change can end up with a population that has little genetic diversity, but a wide variability allows part of the species to adapt more easily.
This adaptive capacity that derives from DNA mutations is a mechanism that living organisms use deliberately in some cases. An example is the “SOS response” of bacteria, whereby they deliberately induce random changes to their DNA in a desperate attempt to adapt to a life-threatening environmental change.
Another curious example of adaptive mutagenesis (mutation production) would be the one observed in tumors . As they grow and develop, some tumors favor a certain chaos and lack of control of the genetic material. This increased mutagenesis is related to increased survival, aggressiveness, and adaptability of the tumor population.
What types of mutations exist?
There are many types of genetic mutations, classifiable according to various parameters , such as the morphology of the mutation, the mechanism through which it occurs, the scale at which it occurs -Genetic, chromosomal, genomic level…-, or also according to the effects that has the mutation, either on the individual or at the population level.
According to its effect on the genetic code
One of the simplest ways to classify mutations is according to the effects they have on the DNA sequence itself and the subsequent interpretation of this sequence. According to this classification, we would find:
1. Missense Mutations
Missense mutations (wrong sense) are those in which a point mutation occurs (that is, only in a nitrogenous base) that ends up changing the “meaning” of the genetic code minimally. DNA is read in small groups of 3 nucleotides at a time and each of these combinations codes for a different amino acid (except for some special combinations). From the reading of these combinations the protein chains are translated.
If the point mutation has changed the meaning of the nucleotide triplet, the missense mutation occurs when this change in meaning also means a change in the amino acid that is integrated into the protein chain. Depending on the role of the amino acid in the protein, this mutation will be more or less serious.
2. Nonsense mutations
Nonsense mutations are those in which the mutation, instead of inducing the change of one protein for another, causes a halt in the construction of the protein . This occurs because there are special nucleotide triplets whose interpretation is to stop the translation process. These triplets are called “STOP codons”.
STOP codons are usually found at the end of genes that make a protein. When a mutation occurs that changes a normal codon to a STOP codon, the protein is truncated. It will be more serious the more portion of the protein is omitted, causing everything from dysfunctions to the complete lack of the encoded protein.
3. Frameshift mutations
Frameshift mutations are those that occur when the “reading frame” of the codons is altered. If the nucleotides are read 3 by 3, deciding which nucleotide is the first to be read is essential for the genetic code to be correctly interpreted.
Frameshift mutations are a type of mutation that usually occurs when an extra nucleotide is mistakenly omitted or added to the genomic sequence. We can easily exemplify: If a DNA sequence is read as CCC-AAA-GGG and an insertion occurs, it could remain as CCT-CAA-AGG-G. The meaning of the codons is altered, and the protein produced loses its structural and functional meaning, if it is produced at all.
4. Repetitive expansions
There are areas of DNA where some small sequences are repeated a certain number of times. Repetitive expansions are mutations in which these repetitive sequences, such as a group of 3 nucleotides that is repeated X times in a row, change their number of repetitions .
A higher number of repeats can cause different structural problems for the final protein . If the repetitions are not 3 nucleotides long, there is also the risk of producing frameshift mutations depending on the number of repetitions and the size of the repetitive unit.
Other types of mutations
Classifying the different types of mutations into a specific number of variants is a complex task, since these, as we have mentioned before, have many classification methods . We show you some of the typical classifications that are made of mutations:
1. Classified according to their effects on the body
This classification divides mutations according to whether they are beneficial to the individual, harm him or are simply neutral changes , which do not affect his life in any way. Whether the effects are positive, negative or neutral depends not only on the effects of the mutation, but also on the environment in which the individual finds himself and the selective pressures that affect him.
For example, let’s say there is a mutation that changes the color of a species of insect. If the insect depends on its color for its way of life (for example, due to its camouflage) it is probably harmed by the mutation. But it is possible that the mutation improves his quality of life (for example, with better camouflage) or it may not affect him at all (such as through minor changes to camouflage, or if he has no need to hide visually)
2. According to its effect on the population
What is good for an individual is not necessarily good at the population level . A mutation that makes an organism larger may be seen as beneficial, but if this organism lives in a resource-poor environment it is probably not a mutation that favors its species.
The analysis of the population effects of mutations is carried out through population genetics, a science that uses statistical modeling and computational techniques to analyze DNA and its fluctuations at the level of groups of organisms or species.
3. According to cell lineage
Mutations that affect somatic cells (the majority that make up our body) may have a greater or lesser impact, but if they have occurred throughout our lives they will not affect our offspring.
On the other hand, mutations that affect the germ cells (those that produce the gametes) will be transmissible to the offspring.
4. According to the genetic material affected
There is not only genomic DNA inside our cells . We also find mitochondrial DNA, isolated from genomic DNA, within the mitochondria, the energy-producing cellular organ. Mutations can occur in any type of DNA and can be differentiated according to the characteristics of the genetic material in which they occur.
Nor does a mutation that affects a gene that is important for life have the same impact as another that affects an area of low importance, either due to low levels of transcription or because it simply does not have functionality. Junk DNA , despite being a controversial idea, represents a portion of our DNA with no apparent function, where mutations tend to be more common (because they don’t have much of an effect, and accumulate between populations).
Bibliographic references
- Genetics Home Reference, NIH. (2019). What kinds of gene mutations are possible?. Genetics Home Reference. Extraído el 13 de septiembre de 2019, de https://ghr.nlm.nih.gov/primer/mutationsanddisorders/possiblemutations
- Michel, B. (2005). After 30 Years of Study, the Bacterial SOS Response Still Surprises Us. Plos Biology, 3(7), e255. doi:10.1371/journal.pbio.0030255.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.