One of the most important properties of living things is variability. It is variability that allows species to acquire new properties that can potentially help them form a new species. Mutations that occur in the genome provide this variability. Mutations can be global, and affect the entire chromosome, or local, up to the replacement of a single nucleotide. But the consequences can be significant for both. In this topic, we will discuss what the consequences of mutations are, how they can be classified, and whether a mutation is the only mechanism that facilitates provide evolution.
Classification of consequenses
There are several ways to classify the effects of mutations. Firstly, this is a classification according to the effect on the final protein product, that is, as an effect at the micro level. Second, mutations can manifest themselves at the cellular level and finally, mutations can affect the macro level. It must be noted that all types of mutations can have the same consequences. Even "large" mutations, such as those affecting an entire chromosome, may have little or even no consequences (as in the case of X-trisomy in women – most often they phenotypically look like ordinary women and remain fertile).
Mutation on the micro level
Let's imagine that a mutation has occurred in one single place in the nucleotide sequence. It's just "one letter", but what would be the consequences of changing this "letter"?
1) A mutation may not affect the protein in any way – for example, if there was a single nucleotide change from the triplet UUU to UUC – this will in any case lead to the inclusion of the amino acid phenylalanine, so the final protein will not change at all. Such mutations are called synonymous mutations; 2) A mutation can "break" an individual amino acid – this may not affect the final protein at all, or it may lead to a violation of its folding. Imagine that glycine has been replaced by alanine. Both of the molecules are small, so they may not affect how the protein folds into the final product at all. Such mutations are called missense mutations. Amino acid substitution, however, does not completely break down the protein; 3) A mutation can completely "break" a protein, due to which it will not be able to perform its functions. In this case, a new nucleotide turns into a stop codon and abruptly interrupts protein synthesis (this is called a nonsense mutation).
Mutation cellular level
So we "got" the wrong protein. How can it affect metabolism at the level of cells and organs?
1) The wrong protein does not affect the metabolism of cells and tissues. This happens if the absence or irregularity of the protein is successfully compensated for by the fact that it is correctly synthesized by other cells or other proteins; 2) The wrong protein significantly affects the metabolism of cells and tissues. This happens if the wrong protein performed an important function – for example, it was a signal protein that acts as a messenger between different signal cascades.
Mutation on the macro level
Finally, mutations can also affect the macro level or the phenotype level.
1) A mutation may not lead to any changes in the body of the organism. An example of such a mutation would be a Brittain toe, an enlarged second toe. In people with this mutation, the first metatarsal bone of the foot is shortened compared to the second. Such a mutation occurs in almost 10% of the population, but does not lead to any consequences; 2) A mutation may manifest itself, but weakly or compensated. In some cases, the work of the "wrong" protein may be noticeable, but not affect the quality of human life. An example is the development of cancer. There are oncogenes, which, however, are not activated in everyone. The fact is that such oncogenes are compensated by the work of oncosuppressors – genes whose products prevent the development of cancer. Such genes regulate excessive division, prevent the cell from going into apoptosis (programmed cell death), and so on; 3) A mutation significantly affects the life of the organism. There are many such mutations, and all of them lead to the emergence of severe incurable diseases – for example, Down's syndrome, cystic fibrosis, and Huntington's disease.
The evolutionary significance of mutations
It is necessary to understand exactly how mutations affect the evolutionary process. Mutations are often divided conditionally into beneficial, neutral, and harmful. Conditionally beneficial ones help an individual to reproduce better and, accordingly, spread in the population, which can ultimately lead to the isolation of a certain group from the general species and the formation of a new species. The fact is that the occurrence of a mutation in the genome of an individual is not enough to start the process of "separation" of species from species. This mutation should also provide its owner with some kind of advantage – this is how it can leave more offspring, and therefore pass it on to more individuals.
So, it is important how exactly the mutation affects the life of the individual. In addition, the conditions in which this individual lives are also important. Imagine that a mutation has occurred in the gene encoding actin, one of the muscle proteins, that allows muscles to contract faster, stronger, and more efficiently. If the carrier of this mutation, for example, is a bird, then its life will definitely become better – it will be able to fly farther and find more food, it will be able to fly away from predators faster, and so on. And if the carrier, for example, is a sloth who spends most of its life eating leaves on one tree, then this mutation is absolutely useless for him. That is, mutations cannot be divided into strictly positive and strictly negative ones – their action strongly depends on the ecological niche, the way food is obtained, the terrain, and so on.
Also, there are cases when even those mutations are often fixed and remain, which, although they appear, do not affect the life of the individual in any way. An example of such a case is albinism – when the synthesis of the protein responsible for the synthesis of the melanin pigment is disrupted. People with albinism can live quite happily, although h the influence of the "wrong" protein is noticeable. There are also cases when the work of the "wrong" protein is compensated by the shock work of other proteins, or this work does not affect the body at all.
Conclusion
So, all the consequences of mutations can be classified at three levels of the organization – the micro level, the level of cells and organs, and the macro level. In addition, mutations can have no effect, have little effect, be compensated, or have a strong effect on the entire organism or individual cells/organs.