Mutations at the gene level are molecular, not visible in the light microscope structural changes in DNA. These include any transformations of deoxyribonucleic acid, regardless of their effect on viability and localization. Some species of gene mutations have no effect on the function and structure of the corresponding polypeptide (protein). However, most of such transformations provoke the synthesis of a defective connection that has lost the ability to perform its tasks. Next, we'll look at gene and chromosome mutations in more detail.
Characteristics of transformations
The most common pathologies that are provoked by human gene mutations are neurofibromatosis, adreno-genital syndrome, cystic fibrosis, phenylketonuria. This list can also include hemochromatosis, Duchenne-Becker myopathies and others. This is not all examples of gene mutations. Their clinical signs are usually disorders of metabolism (metabolic process). Gene mutations can consist of:
- Substitution in the base codon. This phenomenon is called a 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.
- Changing the codon in such a way that the information is paused. This process is called nonsense. When the nucleotide is replaced in this case, the stop codon is formed and the translation stops.
- Violation of reading, frame shift. This process is called "frame-shift". When the DNA changes molecularly, triplets are transformed during the translation of the polypeptide chain.
In accordance with the type of molecular transformation, the following gene mutations exist:
- Duplication. In this case, duplication or doubling of the DNA fragment from 1 nucleotide to genes occurs.
- Deletion. In this case, there is a loss of the DNA fragment from the nucleotide to the gene.
- Inversion. In this case, there is a turn of 180 degrees. section of DNA. Its size can be either two nucleotides or a whole fragment consisting of several genes.
- Insertion. In this case, DNA segments are inserted from the nucleotide to the gene.
Molecular transformations that capture from 1 to several links are considered as point changes.
Gene mutations have a number of characteristics. First of all, it should be noted their ability to inherit. 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 disturbances in the phenotype. Thus, due to the innate nature of the code, the same amino acid can be encoded by two triplets, which differ only in 1 base. At the same time, a certain gene can mutate (transform) into several different states. It is this kind of change that provokes most of the hereditary pathologies. If you give examples of gene mutations, you can refer to blood groups. Thus, in the element that controls their systems AB0, there are three alleles: B, A and 0. Their combination determines the blood groups. Relating to the system AB0 is considered a classic manifestation of the transformation of normal traits in humans.
These transformations have their own classification. The category of genomic mutations include changes in ploidy of unchanged structural chromosomes and aneuploidy. Such transformations are determined by special methods. Aneuploidy is a change (an increase in trisomy, a decrease in monosomy) of the number of chromosomes of the diploid set, not a single haploid set. With a multiple increase in numbers, they speak of polyploidy. They and most of the 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 for any of the autosomes. As the only compatible with life is monosomy on the chromosome X. It provokes the Shereshevsky-Turner syndrome.
- Trisomy. In this case, three homologous elements are revealed in the karyotype. Examples of such gene mutations: Down syndrome, Edwards syndrome, Patau.
The provoking factor
The reason for the development of aneuploidy is the non-dissociation of chromosomes in the process of cell division against the background of the formation of sex cells or the loss of elements due to anaphase lag, while during the movement to the pole the homologous link may lag behind the non-homologous one. The concept of "nondisjunction" 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.
Nondisjunction for meiosis
This phenomenon is considered the most frequent. Those chromosomes, which should normally be divided during meiosis, remain connected. In anaphase, they go to a single cellular pole. As a result, two gametes are formed. In one of them there is an additional chromosome, and in the other one it lacks an element. In the process of fertilization of a normal cell with an extra link, trisomy develops, gametes with the missing component - monosomy. In the formation of a monosomal zygote for some autosomal element, development stops at the initial stages.
These transformations are structural changes in the elements. As a rule, they are visualized in a light microscope. The chromosomal mutation is usually involved from tens to hundreds of genes. This provokes changes in the normal diploid set. As a rule, such aberrations do not cause sequence transformation in DNA. However, when the number of gene copies changes, a genetic imbalance develops due to a lack or overabundance of the material. There are two large categories of these transformations. In particular, intra- and interchromosomal mutations are isolated.
The influence of environment
People evolved as groups of isolated populations. They lived long enough in the same environmental conditions. Speech, in particular, is about the nature of nutrition, climatogeographic characteristics, cultural traditions, causative agents of pathologies and so on. All this led to the fixation of specific for each population combinations of alleles, which were most appropriate for living conditions. However, due to the intensive expansion of the range, migrations, relocation, situations began to arise when the beneficial combinations of certain genes in one environment failed to ensure the normal functioning of a number of body systems. In this regard, part of the hereditary variability is due to an unfavorable complex of non-pathological elements. Thus, as a cause of gene mutations in this case 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 took place in more specific forms. This also contributed to the expansion of hereditary diversity. So, those signs which could disappear at animals were kept, and on the contrary, that was left at animals. In the course of natural selection, people also acquired undesirable symptoms that were directly related to illness. For example, in a person in the development of genes appeared, capable of determining sensitivity to poliomyelitis or diphtheria toxin. Having become Homo sapiens, the biological species of people in some way "paid for their wisdom" by accumulation and pathological transformations. This provision is considered the basis of one of the basic concepts of the theory of gene mutations.