Main characteristics of the gene
It has now been established in molecular biology that genes are
At the same time, each gene is characterized by a number of specific regulatory DNA sequences, such as promoters, which are directly involved in the regulation of gene expression.
Thus, the concept of a gene is not limited only to the coding region of DNA, but is a broader concept that includes regulatory sequences.
Genes can undergo mutations - random ortargeted changes in the nucleotide sequence in the DNA chain. Mutations can lead to a change in the sequence, and therefore a change in the biological characteristics of the protein or RNA, which, in turn, can result in general or local altered or abnormal functioning of the body.
Such mutations in some cases arepathogenic, since they result in disease, or lethal at the embryonic level. However, not all changes in the nucleotide sequence lead to changes in protein structure (due to the effect of degeneracy of the genetic code) or to a significant change in the sequence and are not pathogenic.
How does molecular evolution work?
- Mutation
DNA replication is for the most part extremelyaccurate, but mistakes do happen. The error rate in eukaryotic cells can be as low as 10−8 per nucleotide per replication, whereas for some RNA viruses it can be as high as 10−3. This means that in each generation, each person accumulates 1-2 new mutations in the genome.
Small mutations can be caused by replicationDNA and the consequences of DNA damage and include point mutations, in which a single base is changed, and frameshift mutations, in which a single base is inserted or deleted.
Large mutations can be caused by errors in recombination to cause chromosomal abnormalities, including duplication, rearrangement, or inversion of large sections of a chromosome.
In addition, DNA repair mechanisms canintroduce mutational errors when restoring physical damage to a molecule. Repair, even with a mutation, is more important for survival than replica repair, for example, when repairing double-strand breaks.
- Number of genes
Genome size and number of genes it containscontains, vary significantly among taxonomic groups. The smallest genomes are found in viruses and viroids (which act as a single non-coding RNA gene).
Conversely, plants can have very largegenomes, rice contains over 46,000 genes that code for protein. The total number of protein-coding genes (Earth's proteome), which was estimated in 2007 at 5 million sequences, was reduced to 3.75 million by 2017.
Causes of mutations
There are mutations:
- spontaneous,
- induced.
Spontaneous mutations occur spontaneouslythroughout the life of the organism under normal environmental conditions with a frequency per nucleotide for the cellular generation of the organism from about 10−9 to 10−12
Induced mutations are inheritedchanges in the genome resulting from certain mutagenic influences in artificial (experimental) conditions or under adverse environmental influences.
Mutations appear constantly during processesoccurring in a living cell. The main processes leading to the occurrence of mutations are DNA replication, DNA repair disorders, transcription and genetic recombination.
- Association of mutations with DNA replication
Many spontaneous chemical changesnucleotides lead to mutations that occur during replication. For example, due to deamination of cytosine opposite to guanine, uracil may be included in the DNA chain (a U-G pair is formed instead of the canonical C-G pair).
- Association of mutations with DNA recombination
Of the processes associated with recombination, the mostoften leads to unequal crossing over mutations. It usually occurs when the chromosome contains several duplicated copies of the original gene that retain a similar nucleotide sequence. As a result of unequal crossing over, duplication occurs in one of the recombinant chromosomes, and a deletion occurs in the other.
- Association of mutations with DNA repair
Spontaneous DNA damage is quite common.often, such events take place in every cell. To eliminate the consequences of such damage, there are special repair mechanisms (for example, an erroneous DNA section is cut out and the original one is restored at this place). Mutations arise only when the repair mechanism for some reason does not work or does not cope with the elimination of damage.
What are the mutations
There are several classifications of mutations according tovarious criteria. Möller proposed dividing mutations according to the nature of the change in the functioning of the gene into hypomorphic (altered alleles act in the same direction as wild-type alleles.
Only less protein product is synthesized), amorphous (the mutation looks like a complete loss of gene function, e.g.whitein Drosophila), antimorphic (the mutant trait changes, for example, the color of the corn grain changes from purple to brown) and neomorphic.
Modern educational literature also uses a more formal classification based on the nature of changes in the structure of individual genes, chromosomes and the genome as a whole.
Within the framework of this classification, the following types of mutations are distinguished:
- Genomic— polyploidization and aneuploidy —a change in the number of chromosomes that is not a multiple of the haploid set. Depending on the origin of chromosome sets among polyploids, a distinction is made between allopolyploids, which have chromosome sets obtained by hybridization from different species, and autopolyploids, in which the number of chromosome sets of their own genome increases by a multiple of n.
- Chromosomal. When chromosomal mutations occurmajor rearrangements in the structure of individual chromosomes. In this case, there is a loss (deletion) or doubling of a part (duplication) of the genetic material of one or more chromosomes, a change in the orientation of chromosome segments in individual chromosomes (inversion), as well as a transfer of part of the genetic material from one chromosome to another (translocation).
- Genetic. At the gene level, changes in the primaryDNA structures of genes under the influence of mutations are less significant than with chromosomal mutations, but gene mutations are more common. As a result of gene mutations, substitutions, deletions and insertions of one or more nucleotides, translocations, duplications and inversions of various parts of the gene occur. In the case when only one nucleotide changes under the influence of a mutation, they speak of point mutations.
The consequences of mutations for the cell and the body
Mutations that impair cell activity in a multicellular organism often lead to cell destruction (in particular, programmed cell death - apoptosis).
If intra- and extracellular defense mechanisms are notrecognized a mutation, and the cell underwent division, then the mutant gene will be passed on to all descendants of the cell and, most often, leads to the fact that all these cells begin to function differently.
Mutation in the somatic cell of a complexa multicellular organism can lead to malignant or benign neoplasms, a mutation in the germ cell - to a change in the properties of the entire descendant organism.
In stable (unchanging or slightly changing)conditions of existence, most individuals have a genotype close to the optimal one, and mutations cause disruption of the body’s functions, reduce its fitness and can lead to the death of the individual.
However, in very rare cases, a mutation maylead to the appearance of new useful characteristics in the body, and then the consequences of the mutation are positive; in this case, they are a means of adapting the body to the environment and, accordingly, are calledadaptive.
The role of mutations in evolution
With a significant change in living conditions, those mutations that were previously harmful may turn out to be useful. Thus, mutations are the material for natural selection.
Thus, melanistic mutants (dark-colored individuals) inpopulations of the birch moth in England were first discovered by scientists among typical light-colored individuals in the mid-19th century. Dark coloring occurs as a result of a mutation in one gene. Butterflies spend the day on the trunks and branches of trees, usually covered with lichens, against which the light coloring acts as a camouflage.
As a result of the industrial revolution,accompanied by atmospheric pollution, the lichens died, and the light trunks of birches became covered with soot. As a result, by the middle of the 20th century (over 50-100 generations), in industrial areas the dark morph almost completely replaced the light one.
It was shown that the main reason for the predominant survival of the black form is the predation of birds, which selectively ate light-colored butterflies in contaminated areas.
The problem of randomness of mutations
In the 1940s, it was popular among microbiologiststhe point of view according to which mutations are caused by the influence of an environmental factor (for example, an antibiotic) to which they allow adaptation. To test this hypothesis, a fluctuation test and a replica method were developed.
The Luria-Delbrück fluctuation test consists ofin that small portions of the initial bacterial culture are dispersed into test tubes with a liquid medium, and after several cycles of divisions an antibiotic is added to the test tubes. Then (without subsequent divisions) the surviving antibiotic-resistant bacteria are sown on a Petri dish with solid medium.
The test showed that the number of resistant colonies fromIt is very variable between different tubes - in most cases it is small (or zero), and in some cases it is very high. This means that the mutations that caused antibiotic resistance occurred at random times both before and after exposure.
Thus, by both methods it was proven that“adaptive” mutations arise regardless of the influence of the factor to which they allow adaptation, and in this sense, mutations are random. However, there is no doubt that the possibility of certain mutations depends on the genotype and is channeled by the previous course of evolution.
How are gene mutations detected?
First, biological material is taken from the patient.(blood, urine, muscle biopsy, etc.), DNA is extracted from them using special techniques. Then, using specific methods, the DNA sample obtained by us is prepared for gene sequencing.
Further, it is revealed where exactly in this patient there was a replacement of one or more nucleotides (or any other changes in the deletion, insertion, etc.).
Molecular genetic examination (search for mutations in the gene responsible for the development of the disease) allows you to accurately establish the diagnosis of a hereditary disease.
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