It is the basis of the diversity of species and any organism.
They are changes in the genetic sequence and are a significant cause of diversity among organisms. These changes occur at many different levels and can have very other consequences.
In biological systems capable of reproducing, we must first focus on whether they are heritable; specifically, some mutations affect only the individual that carries them, while others affect all the descendants of the carrier organism and other descendants.
For mutations to affect the descendants of an organism, they must:
- Occur in cells that produce the next generation.
- Affect the hereditary material.
Ultimately, the interaction between inherited mutations and environmental pressures generates diversity among species.
Although several molecular changes exist, the word “mutation” typically refers to a change that affects nucleic acids. In cellular organisms, these nucleic acids are the building blocks of DNA, and in viruses, they are the building blocks of DNA or RNA.
One way of thinking about DNA and RNA is that they are substances that carry the long-term memory of the information required for the reproduction of an organism. This article focuses on mutations in DNA, although we must bear in mind that RNA is subject to essentially the same mutation forces.
If the mutations occur in non-germ cells, these changes can be classified as somatic mutations. The word somatic comes from the Greek word soma which means “body,” and somatic mutations only affect the body of the present organism.
From an evolutionary perspective, somatic mutations are not attractive unless they occur systematically and change some fundamental property of an individual, such as the ability to survive. For example, cancer is a full somatic mutation that will affect the survival of a single organism.
As a different approach, the theory of evolution is more interested in DNA changes in the cells that produce the next generation.
Are they random?
The claim that mutations are random is profoundly true and profoundly false at the same time. The essential aspect of this assertion stems from the fact that, to the best of our knowledge, the consequences of a mutation do not influence the likelihood that this mutation will occur or not.
In other words, mutations occur randomly, concerning whether their effects are helpful. Therefore, the beneficial changes in DNA do not happen more often simply because an organism could benefit from them.
Furthermore, even if an organism has acquired a beneficial mutation during its lifetime, the corresponding information will not return to the germline DNA of the organism. This is a fundamental idea that Jean-Baptiste Lamarck was wrong and Charles Darwin was right.
However, the idea that mutations are random can be considered false if one believes that not all types of mutations occur with the same probability. Instead, some occur more frequently than others because low-level biochemical reactions favor them.
These reactions are also the main reason mutations are an unavoidable property of any system capable of reproducing in the real world.
Mutation rates are generally meager, and biological systems go to extraordinary extremes to keep them as low as possible, especially since many mutational effects are harmful.
However, mutation rates never reach zero, despite low-level protection mechanisms, such as DNA repair or correction during DNA replication, and high-level mechanisms, such as the deposition of melanin in cells. Cutaneous to reduce radiation damage.
Beyond a certain point, avoiding the mutation becomes too costly for the cells. Therefore, conversion will always be a powerful force in evolution.
Types of mutations
So, how do mutations happen? The answer to this question is closely related to the molecular details of how DNA and the entire genome are organized. The smallest are point mutations, in which only a single base pair is changed to another pair of bases.
However, another type of mutation is the non-synonymous, in which a sequence of amino acids is modified. These lead to the production of a different protein or the premature termination of a protein.
Unlike non-synonymous mutations, synonymous mutations do not change a sequence of amino acids, although they occur, by definition, only in lines that encode amino acids. There are synonymous mutations because multiple codons encode many amino acids.
Base pairs can also have various regulatory properties found in introns, intergenic regions, or even within the gene coding sequence. For some historical reasons, all these groups are often subsumed with synonymous mutations under the label of “silent.”
Depending on their function, such silent mutations can be from truly silent to extraordinarily important, implying that the purifying selection keeps the working sequences constant.
This is the most likely explanation for the existence of ultra-conserved non-coding elements that have survived for more than 100 million years without substantial changes, as found by comparing the genomes of several vertebrates.
Mutations can also take the form of insertions or deletions, which are known together as indels. Indels can have a variety of lengths.
The indels of one or two base pairs within the coding sequences have the most significant effect at the short end of the spectrum. They will inevitably cause a frame change (only adding one or more codons of three base pairs will maintain a protein approximately intact).
Indels can affect parts of a gene or whole groups of genes at the intermediate level.
At the most significant level, whole chromosomes or even complete copies of the genome may be affected by insertions or deletions. However, such mutations are generally no longer included in the indel tag.
At this high level, it is also possible to invert or translocate entire sections of a chromosome, and the chromosomes can even fuse or break.
If many genes are lost due to one of these processes, the consequences are often very damaging. Of course, different genetic systems react differently to such events.
Finally, other sources of mutations are the many different types of transposable elements, which are small DNA entities that have a mechanism that allows them to move within the genome. Some of these elements are copied and pasted into new locations, while others use a cut and paste method.
Such movements can alter existing genetic functions (by inserting another gene into the medium), activate latent gene functions (by perfect excision of a gene that was disconnected by a previous insertion), or occasionally lead to the production of new genes.
A line chart shows the probability density of the effects. A logarithmic scale of mutational effects is shown on the x-axis, and the probability density is shown on the axis.
The line follows the shape of a bell curve skewed to the right. The probability density increases as the mutational effects increase from 10-10 to 10-4, where the curve reaches its peak.
As mutational effects increase from 10-4 to 1, the probability density decreases. All mutational effects equal to or less than 10-10 are shown as peaks at 10-10 on the x-axis.
This example of a possible distribution of deleterious mutational effects was obtained from DNA sequence polymorphism data of natural populations of two Drosophila species.
The peak at 10-10 includes all the more minor effects, whereas the results are not shown if they induce structural damage equivalent to the selection coefficients that are “super-lethal.”
A single mutation can have a significant effect, but the evolutionary change is based on accumulating many mutations with minor marks in many cases. Depending on context or location, genetic effects can be beneficial, harmful, or neutral. The majority of non-neutral mutations are dangerous.
In general, the more base pairs are affected by a mutation, the greater the effect of the transformation and the greater the likelihood that it will be harmful.
To better understand the impact of mutations, researchers have begun to estimate distributions of mutational effects (DME) that quantify how many mutations occur with what impact a given property of a biological system. In evolutionary studies, the property of interest is aptitude.