DNA functions as a computer code, although written with molecules. The sequence of their nucleotides tells the cells exactly what elements they have to generate. But when this code changes, the consequences can be more or less severe, especially if this change is not detected.
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 organism.
What is a genetic mutation?
DNA is a molecular chain formed in part by nucleotides, sugar molecules (pentoses) with a nitrogen base and a phosphate group. There are two types of nucleotides, the purinessuch as adenine (A) and guanine (G) and the pyrimidinessuch as cytosine (C), thymine (T), and uracil (U), although the latter is not usually found in DNA, if not in RNA, replacing thymine.
Mutations occur when the information contained in the DNA strand is altered.either due to external causes (elements of the environment that affect the DNA, damaging it or preventing its correct reading, such as certain carcinogenic substances) or internal causes (errors during DNA duplication, mobile elements such as transposons, etc.).
It is a certainty that mutations will occur. Each living being and cell type has a different rate of mutation, depending on their exposure to the environment, speed of duplication – the most regenerating tissues will accumulate mutations faster, normally – among other factors. This occurs despite the multiple mechanisms of repair and correction of genetic material.
The evolutionary effect of the 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 beings, which would not have changed at all throughout the ages.
The mutation is a source of diseases and disorders, but also is a necessary factor for living species to evolve. If there are no genetic differences between individuals, theoretically they are all equally sensitive or resistant to different environmental factors. A strong environmental change may wipe out a population with little genetic diversity, but wide variability allows part of the species to adapt more easily.
This adaptive capacity that derives from DNA mutations is a mechanism deliberately used by living organisms in some cases. One 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 (production of mutations) would be the one seen in tumors. As they grow and develop, some tumors favor a certain chaos and uncontrolled genetic material. This increased mutagenesis is related to increased survival, aggressiveness and adaptability of the tumor population.
What types of mutations are there?
There are many types of genetic mutations, classifiable according to various parametersas the morphology of the mutation, the mechanism through which it is produced, the scale at which it is produced -Genetic, chromosomal, genomic level…-, or also according to the effects that the mutation has, 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:
Missense Mutations
Missense mutations (erroneous sense) are those in which occurs a point mutation (i.e. only in a nitrogen base) that ends up changing the “meaning” of the genetic code minimally. DNA is read in small groups of 3 nucleotides each time and each of these combinations codes for a different amino acid (except some special combinations). From the reading of these combinations the protein chains are translated.
If the point mutation has changed the meaning of the triplet of nucleotides, the missense mutation occurs when this change of 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 have greater or lesser severity.
2. Nonsense Mutations
Nonsense mutations (meaningless) are those in which the mutation, rather than inducing the change of one protein for another, causes a halt in the construction of the protein. This happens 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 produce a protein. When a mutation occurs that changes from 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 from dysfunctions to complete lack of coded protein.
3. Frameshift mutations
Frameshift mutations (frame change) are those that occur when the “reading frame” of the codons is altered. If nucleotides are read 3 by 3, deciding which nucleotide is read first is essential for the genetic code to be interpreted correctly.
Frameshift mutations are a type of mutation that usually occurs when a nucleotide is omitted or added in error to the genomic sequence. We can easily exemplify: If a DNA sequence is read as CCC-AAA-GGG and an insertion occurs, it may be left as CCT-CAA-AGG-G. The meaning of the codons is altered, and the protein produced loses its structural and functional sense if it is produced.
4. Repetitive Expansions
There are areas of DNA where 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 greater number of repetitions can cause different structural problems for protein final. If the repeats are not 3 nucleotides, there is also a risk of producing frameshift mutations depending on the number of repeats and size of the repeating unit.
Other types of mutations
Classifying the different types of mutations into a specific number of variants is a complex task, as these, as mentioned above, have many sorting methods. We show you some of the typical classifications that are made of mutations:
Classified according to their effects on the organism
This classification divides the mutations depending on whether they are beneficial to the individual, harmful or simply neutral changesthat don’t affect your life in any way. Whether the effects are positive, negative or neutral does not depend solely 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’s a mutation that changes color to a kind of insect. If the insect depends on its color for its way of life (e.g., because of its camouflage) it is likely to be harmed by the mutation. But the mutation may improve your quality of life (For example, with better camouflage) or that does not affect you at all (such as through minimal changes to the camouflage, or if you do not need to hide visually).
2. According to their 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 favours its species.
Analysis of the population effects of mutations is carried out through population genetics, a science that uses statistical modelling and computational techniques to analyse DNA and its fluctuations at the level of groups of organisms or species.
Depending on the cell lineage
Mutations that affect somatic cells (the majority that form 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, the mutations that affect the germinal cells (those produced by gametes) shall be transmissible to offspring.
4. Depending on the genetic material it affects
There’s not just 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 important to life have the same impact as a mutation that affects a low importance area, either because of low levels of transcription or because it simply has no functionality. Junk DNAAlthough a controversial idea, it represents a portion of our DNA with no apparent function, where mutations tend to be more common (because they don’t have much effect, and accumulate among populations).
Bibliographic references
- Genetics Home Reference, NIH. (2019). What kinds of gene mutations are possible? Genetics Home Reference. Retrieved September 13, 2019, from 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.