It is the basis of the diversity of species and any type of organism.
They are changes in the genetic sequence and are a major cause of diversity among organisms. These changes occur at many different levels, and can have very different consequences.
In biological systems that are 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 there are several types of molecular changes, 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 interesting, unless they occur systematically and change some fundamental property of an individual, such as the ability to survive. For example, cancer is a powerful 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 deeply true and profoundly false at the same time. The true aspect of this assertion stems from the fact that, to the best of our knowledge, the consequences of a mutation have no influence on the likelihood that this mutation will occur or not.
In other words, mutations occur at random with respect to whether their effects are useful. Therefore, the beneficial changes of DNA do not occur 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 takes into account the fact that not all types of mutations occur with the same probability. Rather, some occur more frequently than others because they are favored by low-level biochemical reactions.
These reactions are also the main reason why mutations are an unavoidable property of any system that is capable of reproducing in the real world.
Mutation rates are generally very low, 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, even in spite of 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 simply becomes too costly for the cells. Therefore, the mutation will always be present as 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 sequences that encode amino acids. There are synonymous mutations because many amino acids are encoded by multiple codons.
Base pairs can also have various regulatory properties if they are 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, which implies that the working sequences are kept constant by the purifying selection.
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.
At the short end of the spectrum, the indels of one or two base pairs within the coding sequences have the greatest effect, because they will inevitably cause a frame change (only the addition of one or more codons of three base pairs will maintain a protein approximately intact).
At the intermediate level, indels can affect parts of a gene or whole groups of genes.
At the largest level, whole chromosomes or even whole copies of the genome may be affected by insertions or deletions, although 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 a large number of genes are lost as a result of 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 a peak 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 smaller effects, whereas the effects are not shown if they induce structural damage that is equivalent to the selection coefficients that are “super-lethal”.
A single mutation can have a great effect, but in many cases, the evolutionary change is based on the accumulation of many mutations with small effects. Mutational effects can be beneficial, harmful or neutral, depending on their context or location. The majority of non-neutral mutations are harmful.
In general, the more base pairs are affected by a mutation, the greater the effect of the mutation 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 effect on a given property of a biological system. In evolutionary studies, the property of interest is aptitude.