Mutations at the gene level are molecular structural changes in DNA that are not visible in the light microscope. These include any transformation of deoxyribonucleic acid, regardless of their effect on viability and localization. Some types of gene mutations have no effect on the function and structure of the corresponding polypeptide (protein). However, most of these transformations provoke the synthesis of a defective compound that has lost its ability to perform its tasks. Next, consider the gene and chromosomal mutations in more detail.
The most common pathologies that provoke human gene mutations are neurofibromatosis, adrenogenital syndrome, cystic fibrosis, phenylketonuria. Hemochromatosis, Duchenne-Becker myopathies and others can also be included in this list. These are not all examples of gene mutations. Their clinical signs are usually metabolic disorders (metabolic process). Gene mutations can consist of:
- Substitution in the base codon. This phenomenon is called missense mutation. In this case, the nucleotide is replaced in the coding part, which, in turn, leads to a change in the amino acid in the protein.
- The codon changes in such a way that the reading of information is suspended. This process is called nonsense. When replacing the nucleotide in this case, the formation of a stop codon and the termination of translation.
- Violation of reading, frame shift. This process is referred to as “frame-shifting”. In the case of molecular DNA change, triplets are transformed during the translation of a polypeptide chain.
In accordance with the type of molecular transformation, the following gene mutations exist:
- Duplication. In this case, there is a repeated duplication or doubling of the DNA fragment from 1 nucleotide to the genes.
- Deletion. In this case, there is a loss of the DNA fragment from the nucleotide to the gene.
- Inversion. In this case, a turn of 180 degrees is noted. DNA segment. Its size can be either two nucleotides or a whole fragment consisting of several genes.
- Insertion. In this case, insertion of DNA from the nucleotide to the gene occurs.
Molecular transformations that capture from 1 to several links are considered as point changes.
Gene mutations have a number of features. First of all, their ability to pass by inheritance should be noted. In addition, mutations can trigger the transformation of genetic information. Some of the changes can be attributed to the so-called neutral. Such gene mutations do not provoke any abnormalities in the phenotype. So, due to the innateness of the code, the same amino acid can be encoded by two triplets, differing only in 1 base. However, a particular gene can mutate (transform) into several different states. It is this kind of changes that provoke most of the hereditary pathologies. If you give examples of gene mutations, you can refer to blood groups. Thus, the element controlling their AB0 system has three alleles: B, A, and 0. Their combination determines blood groups. AB0 is a classic manifestation of the transformation of normal signs in humans.
These transformations have their own classification. Changes in ploidy of structurally unchanged chromosomes and aneuploidy are referred to as genomic mutations. Such transformations are determined by special methods. Aneuploidy is a change (increase - trisomy, decrease - monosomy) of the number of chromosomes in the diploid set, non-multiple haploid. With a multiple increase in the number speak of polyploidy. They and most aneuploidy in humans are considered lethal changes. Among the most common genomic mutations are:
- Monosomy. In this case, only one of the 2 homologous chromosomes is present. Against the background of such a transformation, healthy embryonic development is impossible on any of the autosomes. Monosomy on the X chromosome is the only compatible with life. It provokes Shereshevsky – Turner syndrome.
- Trisomy In this case, three homologous elements are detected in the karyotype. Examples of such gene mutations: Down syndrome, Edwards, Patau.
The reason for which aneuploidy develops is the nondisjunction of chromosomes during cell division against the background of the formation of germ cells or the loss of elements due to anaphase lag, while during movement to the pole the homologous link may lag behind the non-homologous. The term “non-divergence” indicates the absence of separation of chromatids or chromosomes in mitosis or meiosis. This violation can lead to mosaicism. In this case, one cell line will be normal, and the other - monosomal.
Non-occurrence with meiosis
This phenomenon is considered the most frequent. Those chromosomes that should normally share during meiosis remain connected. In anaphase, they move to the same cell pole. As a result, 2 gametes are formed. In one of them there is an additional chromosome, and in the other one there is no element. In the process of fertilization of a normal cell with an extra link, trisomy develops, the gametes with the missing component are monosomy. When a monosomal zygote is formed according to some autosomal element, development stops at the initial stages.
These transformations are structural changes of elements. As a rule, they are visualized in a light microscope. Chromosomal mutations usually involve dozens to hundreds of genes. This provokes changes in the normal diploid set. As a rule, such aberrations do not cause transformation of the sequence into DNA. However, when the number of gene copies changes, a genetic imbalance develops due to a lack or excess of material. There are two major categories of data transformations. In particular, intra- and interchromosomal mutations are isolated.
Humans evolved as groups of isolated populations. They lived long enough in the same environmental conditions. This, in particular, is about the nature of nutrition, climatic and geographical characteristics, cultural traditions, pathogens of pathologies and so on. All this led to the consolidation of specific for each population combinations of alleles, which were the most appropriate for living conditions. However, due to the intensive expansion of the area, migrations, and relocations, situations began to arise when those who were in the same environment had useful combinations of certain genes in another and stopped providing normal functioning of a number of body systems. In this connection, part of the hereditary variability is caused by an unfavorable complex of non-pathological elements. Thus, as the cause of gene mutations in this case are changes in the external environment, living conditions. This, in turn, became the basis for the development of a number of hereditary diseases.
Over time, evolution proceeded in more specific species. It also contributed to the expansion of hereditary diversity. So, those signs remained that could disappear in animals, and vice versa, what was left in animals was swept aside. In the course of natural selection, people also acquired undesirable signs that were directly related to diseases. For example, a person in the process of development appeared genes capable of determining sensitivity to polio or diphtheria toxin. By becoming Homo sapiens, the biological species of people in some way paid for their rationality by accumulation and pathological transformations. This provision is considered the basis of one of the basic concepts of the theory of gene mutations.