In any cell and organism, all features of the anatomical, morphological and functional nature are determined by the structure of the proteins that are included in them. The hereditary property of the body is the ability to synthesize certain proteins. The amino acids are located in the polypeptide chain, on which biological traits depend.
Each cell has its own sequence of nucleotides in the polynucleotide DNA chain. That's what it is genetic code DNA. Through it, information about the synthesis of certain proteins is recorded. This article describes what the genetic code is, its properties and genetic information.
A bit of history
The idea that the genetic code may exist was formulated by J. Gamow and A. Down in the middle of the twentieth century. They described that the sequence of nucleotides responsible for the synthesis of a particular amino acid contains at least three units. Later, they proved the exact number of three nucleotides (this is a unit of the genetic code), which was called a triplet or codon. There are sixty-four nucleotides in total, because the acid molecule where RNA occurs is composed of residues of four different nucleotides.
What is the genetic code
The way of coding the protein sequence of amino acids due to the nucleotide sequence is characteristic of all living cells and organisms. That's what the genetic code is.
There are four nucleotides in DNA:
- adenine - A;
- guanine - G;
- cytosine - C;
- thymine - T.
They are designated by capital letters in Latin or (in Russian-language literature) Russian.
There are also four nucleotides in RNA, but one of them differs from DNA:
- adenine - A;
- guanine - G;
- cytosine - C;
- uracil - U.
All nucleotides are lined up in chains, and a double helix is \u200b\u200bobtained in DNA, and a single helix in RNA.
Proteins are built on where they are located in a certain sequence, determine its biological properties.
Properties of the genetic code
Tripletness. The unit of the genetic code consists of three letters, it is triplet. This means that the twenty existing amino acids are encoded with three specific nucleotides called codons or trilpets. There are sixty-four combinations that can be made from four nucleotides. This amount is more than enough to encode twenty amino acids.
Degeneracy. Each amino acid corresponds to more than one codon, with the exception of methionine and tryptophan.
Unambiguity. One codon encrypts one amino acid. For example, in the gene of a healthy person with information about the beta target of hemoglobin, the triplet GAG and GAA encodes A in everyone who is sick with sickle cell anemia, one nucleotide is replaced.
Collinearity. The amino acid sequence always matches the nucleotide sequence that the gene contains.
The genetic code is continuous and compact, which means that it has no "punctuation marks". That is, starting at a certain codon, there is a continuous reading. For example, AUGGUGTSUUAAUGUG will be read as: AUG, GUG, TSUU, AAU, GUG. But not AUG, UGG and so on or in any other way.
Versatility. It is the same for absolutely all terrestrial organisms, from humans to fish, fungi and bacteria.
Table
Not all available amino acids are present in the table shown. Hydroxyproline, hydroxylysine, phosphoserine, iodine derivatives of tyrosine, cystine and some others are absent, since they are derivatives of other amino acids encoded by mRNA and formed after protein modification as a result of translation.
It is known from the properties of the genetic code that one codon is capable of encoding one amino acid. The exception is the genetic code, which performs additional functions and encodes valine and methionine. IRNA, being at the beginning with a codon, attaches t-RNA, which carries formylmethion. Upon completion of the synthesis, it cleaves off by itself and captures the formyl residue, being converted into the methionine residue. Thus, the aforementioned codons are the initiators of the synthesis of the polypeptide chain. If they are not at the beginning, then they are no different from others.
Genetic information
This concept refers to a property program that is passed down from ancestors. It is embedded in heredity as a genetic code.
The genetic code is implemented during protein synthesis:
- informational i-RNA;
- ribosomal r-RNA.
Information is transmitted by direct communication (DNA-RNA-protein) and reverse (environment-protein-DNA).
Organisms can receive, store, transfer it and use it most efficiently.
Being inherited, information determines the development of an organism. But due to interaction with the environment, the reaction of the latter is distorted, due to which evolution and development occurs. Thus, new information is put into the body.
The calculation of the laws of molecular biology and the discovery of the genetic code illustrated that it is necessary to combine genetics with Darwin's theory, on the basis of which a synthetic theory of evolution - non-classical biology - appeared.
Darwin's heredity, variability, and natural selection are complemented by genetically determined selection. Evolution is implemented on genetic level by random mutations and inheritance of the most valuable traits that are most adapted to environment.
Decoding the code in a person
In the nineties, the Human Genome project was launched, as a result of which in the two thousandths were discovered fragments of the genome containing 99.99% of human genes. Fragments that are not involved in protein synthesis and are not encoded remain unknown. Their role is still unknown.
The last chromosome 1 discovered in 2006 is the longest in the genome. More than three hundred and fifty diseases, including cancer, appear as a result of disorders and mutations in it.
The role of such studies can hardly be overestimated. When they discovered what the genetic code is, it became known by what patterns the development takes place, how the morphological structure, psyche, predisposition to certain diseases, metabolism and vices of individuals are formed.
Under the genetic code, it is customary to understand such a system of signs denoting the sequential arrangement of nucleotide compounds in DNA and RNA, which corresponds to another sign system that displays the sequence of amino acid compounds in a protein molecule.
It is important!
When scientists managed to study the properties of the genetic code, universality was recognized as one of the main ones. Yes, as strange as it sounds, everything is united by one, universal, common genetic code. It was formed over a long time interval, and the process ended about 3.5 billion years ago. Consequently, in the structure of the code, traces of its evolution can be traced, from the moment of its inception to the present day.
When we talk about the sequence of the arrangement of elements in the genetic code, we mean that it is far from chaotic, but has a strictly defined order. And this also largely determines the properties of the genetic code. This is equivalent to the arrangement of letters and syllables in words. It is worth breaking the usual order, and most of what we read on the pages of books or newspapers will turn into ridiculous gibberish.
The main properties of the genetic code
Usually the code carries some information encrypted in a special way. In order to decipher the code, you need to know distinctive features.
So, the main properties of the genetic code are:
- tripletness;
- degeneracy or redundancy;
- unambiguity;
- continuity;
- already mentioned above universality.
Let's dwell on each property in more detail.
1. Triplet
This is when three compounds of nucleotides form a sequential chain within a molecule (i.e. DNA or RNA). As a result, a triplet compound is created or encodes one of the amino acids, its location in the peptide chain.
Distinguish codons (they are also code words!) By their sequence of connection and by the type of those nitrogenous compounds (nucleotides) that are part of them.
In genetics, it is customary to distinguish 64 codon types. They can form combinations of four types of nucleotides, 3 in each. This is tantamount to raising the number 4 to the third power. Thus, the formation of 64 nucleotide combinations is possible.
2. Redundancy of the genetic code
This property can be traced when several codons are required to encrypt one amino acid, usually in the range of 2-6. And only tryptophan can be encoded with a single triplet.
3. Unambiguity
It is included in the properties of the genetic code as an indicator of healthy genetic inheritance. For example, the GAA triplet, which is in sixth place in the chain, can tell doctors about the good condition of the blood, about normal hemoglobin. It is he who carries information about hemoglobin, and it is also encoded by it. And if a person is sick with anemia, one of the nucleotides is replaced by another letter of the code - Y, which is a signal of the disease.
4. Continuity
When recording this property of the genetic code, it should be remembered that codons, as chain links, are not located at a distance, but in direct proximity, one after another in a nucleic acid chain, and this chain is not interrupted - it has no beginning or end.
5. Versatility
It should never be forgotten that everything on Earth is united by a common genetic code. And therefore, in a primate and a person, in an insect and a bird, a hundred-year-old baobab and a blade of grass barely hatching from the ground, similar amino acids are encoded by the same triplets.
It is in the genes that the basic information about the properties of an organism is laid, a kind of program that the organism inherits from those who lived earlier and which exists as a genetic code.
Gene classification
1) By the nature of the interaction in the allelic pair:
Dominant (a gene capable of suppressing the manifestation of a recessive gene allelic to it); - recessive (gene, the expression of which is suppressed by the dominant gene allelic to it).
2) Functional classification:
2) Genetic code are certain combinations of nucleotides and the sequence of their location in the DNA molecule. This is a way inherent in all living organisms to encode the amino acid sequence of proteins using a sequence of nucleotides.
In DNA, four nucleotides are used - adenine (A), guanine (G), cytosine (C), thymine (T), which in Russian literature are designated by the letters A, G, T and C. These letters make up the alphabet of the genetic code. The RNA uses the same nucleotides, with the exception of thymine, which is replaced by a similar nucleotide - uracil, which is denoted by the letter U (Y in Russian literature). In DNA and RNA molecules, nucleotides are arranged in chains and, thus, sequences of genetic letters are obtained.
Genetic code
In nature, 20 different amino acids are used to build proteins. Each protein is a chain or several chains of amino acids in a strictly defined sequence. This sequence determines the structure of the protein, and therefore all of its biological properties. The set of amino acids is also universal for almost all living organisms.
The implementation of genetic information in living cells (that is, the synthesis of the protein encoded by the gene) is carried out using two matrix processes: transcription (that is, the synthesis of mRNA on the DNA matrix) and translation of the genetic code into the amino acid sequence (synthesis of the polypeptide chain on the mRNA matrix). Three consecutive nucleotides are enough to encode 20 amino acids, as well as a stop signal, which means the end of the protein sequence. A set of three nucleotides is called a triplet. Accepted abbreviations corresponding to amino acids and codons are shown in the figure.
Properties of the genetic code
1. Tripletness - the significant unit of the code is a combination of three nucleotides (triplet, or codon).
2. Continuity - there are no punctuation marks between the triplets, that is, information is read continuously.
3. Discreteness - the same nucleotide cannot be simultaneously included in two or more triplets.
4. Specificity - a certain codon corresponds to only one amino acid.
5. Degeneracy (redundancy) - several codons can correspond to the same amino acid.
6. Versatility - genetic code works the same way in organisms of different levels of complexity - from viruses to humans. (genetic engineering methods are based on this)
3) transcription - the process of RNA synthesis using DNA as a template that occurs in all living cells. In other words, it is the transfer of genetic information from DNA to RNA.
Transcription is catalyzed by the enzyme DNA-dependent RNA polymerase. The process of RNA synthesis proceeds in the direction from 5 "- to 3" - the end, that is, along the template DNA chain, RNA polymerase moves in the direction 3 "-\u003e 5"
Transcription consists of the stages of initiation, elongation and termination.
Transcription initiation - a complex process that depends on the DNA sequence near the transcribed sequence (and in eukaryotes also from more distant regions of the genome - enhancers and silencers) and on the presence or absence of various protein factors.
Elongation - further unweaving of DNA and RNA synthesis along the coding strand continues. it, like DNA synthesis, is carried out in the direction of 5-3
Termination- as soon as the polymerase reaches the terminator, it is immediately cleaved from the DNA, the local DNA-RNA hybrid is destroyed and the newly synthesized RNA is transported from the nucleus to the cytoplasm, and transcription is completed.
Processing - a set of reactions leading to the conversion of the primary products of transcription and translation into functioning molecules. P. are exposed to functionally inactive precursor molecules decomp. ribonucleic to-t (tRNA, rRNA, mRNA) and many others. proteins.
In the process of synthesis of catabolic enzymes (cleaving substrates) in prokaryotes, inducible enzyme synthesis occurs. This enables the cell to adapt to environmental conditions and save energy by stopping the synthesis of the corresponding enzyme if the need for it disappears.
To induce the synthesis of catabolic enzymes, the following conditions are required:
1. An enzyme is synthesized only when the cleavage of the appropriate substrate is necessary for the cell.
2. The concentration of the substrate in the medium must exceed a certain level before the corresponding enzyme can be formed.
The mechanism of regulation of gene expression in Escherichia coli is best studied using the example of the lac operon, which controls the synthesis of three catabolic enzymes that break down lactose. If there is a lot of glucose and little lactose in the cell, the promoter remains inactive, and a repressor protein is located on the operator - the transcription of the lac operon is blocked. When the amount of glucose in the medium, and therefore in the cell, decreases, and lactose increases, the following events occur: the amount of cyclic adenosine monophosphate increases, it binds to the CAP protein - this complex activates the promoter to which the RNA polymerase binds; at the same time, the excess of lactose combines with the repressor protein and releases the operator from it - the pathway for RNA polymerase is open, transcription of the structural genes of the lac-operone begins. Lactose acts as an inducer of the synthesis of those enzymes that break down it.
5) Regulation of gene expression in eukaryotes is much more complicated. Different types of cells of a multicellular eukaryotic organism synthesize a number of identical proteins and at the same time they differ from each other in a set of proteins specific for cells of this type. The level of production depends on the type of cells as well as the stage of development of the organism. The regulation of gene expression is carried out at the level of the cell and at the level of the organism. The genes of eukaryotic cells are divided into two main types: the first determines the universality of cellular functions, the second determines (defines) specialized cellular functions. Functions of genes first group manifest in all cells... To carry out differentiated functions, specialized cells must express a certain set of genes.
Chromosomes, genes and operons of eukaryotic cells have a number of structural and functional features, which explains the complexity of gene expression.
1. Operons of eukaryotic cells have several genes - regulators, which can be located in different chromosomes.
2. Structural genes that control the synthesis of enzymes of one biochemical process can be concentrated in several operons located not only in one DNA molecule, but also in several.
3. Complex sequence of a DNA molecule. There are informative and non-informative sections, unique and repetitive informative nucleotide sequences.
4. Eukaryotic genes consist of exons and introns, and the maturation of i-RNA is accompanied by excision of introns from the corresponding primary RNA transcripts (pro-i-RNA), i.e. splicing.
5. The process of gene transcription depends on the state of chromatin. Local compaction of DNA completely blocks RNA synthesis.
6. Transcription in eukaryotic cells is not always associated with translation. The synthesized m-RNA can be stored for a long time in the form of informosomes. Transcription and translation take place in different compartments.
7. Some genes of eukaryotes have inconsistent localization (labile genes or transposons).
8. Methods of molecular biology have revealed the inhibitory effect of histone proteins on the synthesis of i-RNA.
9. During the development and differentiation of organs, the activity of genes depends on hormones circulating in the body and causing specific reactions in certain cells. In mammals, the action of sex hormones is important.
10. In eukaryotes, 5-10% of genes are expressed at each stage of ontogenesis, the rest must be blocked.
6) repair of genetic material
Genetic repair - the process of eliminating genetic damage and restoring the hereditary apparatus, which takes place in the cells of living organisms under the action of special enzymes. The ability of cells to repair genetic damage was first discovered in 1949 by the American geneticist A. Kellner. Repair - a special function of cells, which consists in the ability to correct chemical damage and breaks in DNA molecules damaged during normal DNA biosynthesis in a cell or as a result of exposure to physical or chemical agents. It is carried out by special enzyme systems of the cell. A number of hereditary diseases (eg, xeroderma pigmentosa) are associated with disorders of the repair systems.
types of reparations:
Direct repair is the simplest way of repairing damage in DNA, which usually involves specific enzymes that are able to quickly (usually in one stage) repair the corresponding damage, restoring the original structure of nucleotides. This is how, for example, O6-methylguanine-DNA methyltransferase acts, which removes the methyl group from the nitrogenous base to one of its own cysteine \u200b\u200bresidues.
Genetic code - a unified system for recording hereditary information in molecules nucleic acids as a sequence of nucleotides. The genetic code is based on the use of an alphabet consisting of only four letters A, T, C, G, corresponding to DNA nucleotides. There are 20 types of amino acids in total. Of the 64 codons, three - UAA, UAG, UGA - do not encode amino acids, they were called nonsense codons, and they function as punctuation marks. Codon (encoding a trinucleotide) is a unit of the genetic code, a triplet of nucleotide residues (triplet) in DNA or RNA, encoding the inclusion of one amino acid. Genes themselves are not involved in protein synthesis. The mediator between gene and protein is mRNA. The structure of the genetic code is characterized by the fact that it is triplet, that is, it consists of triplets (triplets) of nitrogenous bases of DNA, called codons. Of 64
Gene properties. code
1) Triplet: one amino acid is encoded by three nucleotides. These 3 nucleotides in DNA
called a triplet, in mRNA - a codon, in tRNA - an anticodon.
2) Redundancy (degeneracy): there are only 20 amino acids, and the triplets encoding amino acids 61, therefore each amino acid is encoded by several triplets.
3) Unambiguity: each triplet (codon) encodes only one amino acid.
4) Versatility: the genetic code is the same for all living organisms on Earth.
5.) continuity and consistency of codons during reading. This means that the sequence of nucleotides is read triplet by triplet without gaps, while adjacent triplets do not overlap.
88. Heredity and variability are fundamental properties of living things. Darwin's understanding of the phenomena of heredity and variability.
Heredity they call the common property of all organisms to preserve and transmit traits from the parent to the offspring. Heredity - this is the property of organisms to reproduce in generations a similar type of metabolism that has developed in the process of the historical development of a species and manifests itself under certain environmental conditions.
Variability there is a process of the emergence of qualitative differences between individuals of the same species, which is expressed either in a change under the influence of the external environment of only one phenotype, or in genetically determined hereditary variations arising from combinations, recombinations and mutations that occur in a number of successive generations and populations.
Darwin's understanding of heredity and variability.
Under heredity Darwin understood the ability of organisms to preserve their species, varietal and individual characteristics in their offspring. This feature was well known and represented hereditary variation. Darwin analyzed in detail the importance of heredity in the evolutionary process. He drew attention to cases of uniformity of hybrids of the first generation and splitting of traits in the second generation, he was aware of heredity associated with sex, hybrid atavisms and a number of other phenomena of heredity.
Variability. Comparing many breeds of animals and varieties of plants, Darwin noticed that within any species of animals and plants, and in culture within any variety and breed, there are no identical individuals. Darwin concluded that variability is inherent in all animals and plants.
Analyzing the material on the variability of animals, the scientist noticed that any change in the conditions of housing is enough to cause variability. Thus, Darwin understood variability as the ability of organisms to acquire new characters under the influence of environmental conditions. He distinguished the following forms of variability:
Specific (group) variability (now called modification) - a similar change in all individuals of the offspring in one direction due to the influence of certain conditions. Certain changes are usually non-hereditary.
Uncertain individual variability (now called genotypic) - the appearance of various insignificant differences in individuals of the same species, variety, breed, by which, existing in similar conditions, one individual differs from others. Such multidirectional variability is a consequence of the indefinite influence of the conditions of existence on each individual individual.
Correlative (or relative) variability. Darwin understood the organism as an integral system, the individual parts of which are closely interconnected. Therefore, a change in the structure or function of one part often leads to a change in another or others. An example of such variability is the relationship between the development of a functioning muscle and the formation of a crest on the bone to which it attaches. In many wading birds, there is a correlation between neck length and limb length: birds with a long neck also have long limbs.
Compensatory variability consists in the fact that the development of some organs or functions is often the cause of the oppression of others, that is, there is an inverse correlation, for example, between milk yield and meatiness of cattle.
89. Modification variability. The rate of reaction of genetically determined traits. Phenocopies.
Phenotypic variability encompasses changes in the state of directly signs that occur under the influence of developmental conditions or environmental factors. The range of modification variability is limited by the normal response. The resulting specific modification change in the trait is not inherited, but the range of modification variability is determined by heredity, while the hereditary material is not involved in the change.
Reaction rate is the limit of the modification variability of the trait. The reaction norm is inherited, but not the modifications themselves, i.e. the ability to develop a trait, and the form of its manifestation depends on environmental conditions. The reaction rate is a specific quantitative and qualitative characteristic of the genotype. There are signs with a wide reaction rate, a narrow () and an unambiguous rate. Reaction rate has limits or boundaries for each species (lower and upper) - for example, increased feeding will lead to an increase in the weight of the animal, but it will be within the range of the reaction rate typical for this species or breed. The rate of reaction is genetically determined and inherited. For different signs, the limits of the reaction norm are very different. For example, the milk yield, productivity of cereals and many other quantitative traits have wide limits for the reaction rate, narrow limits are the color intensity of most animals and many other qualitative traits. Under the influence of some harmful factors that a person does not encounter in the process of evolution, the possibility of modification variability that determines the reaction rate is excluded.
Phenocopies - changes in the phenotype under the influence of unfavorable environmental factors, in manifestation similar to mutations. The resulting phenotypic modifications are not inherited. It was found that the appearance of phenocopy is associated with the influence of external conditions on a certain limited stage of development. Moreover, the same agent, depending on which phase it acts on, can copy different mutations, or one stage reacts to one agent, the other to another. Different agents can be used to induce the same phenocopy, which indicates that there is no connection between the result of the change and the influencing factor. The most complex genetic developmental disorders are relatively easy to reproduce, while traits are much more difficult to copy.
90. The adaptive nature of the modification. The role of heredity and environment in the development, education and upbringing of a person.
Modification variability corresponds to habitat conditions and is adaptive in nature. Such characteristics as the growth of plants and animals, their mass, color, etc. are subject to modification variability. The emergence of modification changes is associated with the fact that environmental conditions affect the enzymatic reactions occurring in the developing organism, and to a certain extent change its course.
Since the phenotypic manifestation of hereditary information can be modified by environmental conditions, only the possibility of their formation is programmed in the genotype of the organism within certain limits, called the reaction norm. The reaction rate represents the limits of the modification variability of the trait allowed for a given genotype.
The degree of expression of a trait during the realization of a genotype under various conditions is called expressivity. It is associated with the variability of the trait within the normal reaction range.
The same trait may appear in some organisms and absent in others with the same gene. The quantitative indicator of the phenotypic manifestation of a gene is called penetrance.
Expressiveness and penetrance are supported by natural selection. Both patterns must be borne in mind when studying heredity in humans. By changing environmental conditions, it is possible to influence penetrance and expressivity. The fact that one and the same genotype can be the source of the development of different phenotypes is essential for medicine. This means that the burdened one does not have to manifest itself. Much depends on the conditions in which the person is. In some cases, the disease as a phenotypic manifestation of hereditary information can be prevented by adherence to diet or taking medications. The implementation of hereditary information depends on the environment. Formed on the basis of a historically formed genotype, modifications are usually of an adaptive nature, since they are always the result of responses of the developing organism to environmental factors affecting it. The nature of mutational changes is different: they are the result of changes in the structure of the DNA molecule, which causes a disruption in the previously established process of protein synthesis. when mice are kept in conditions of elevated temperatures, they give birth to offspring with elongated tails and enlarged ears. This modification is adaptive in nature, since the protruding parts (tail and ears) play a thermoregulatory role in the body: an increase in their surface makes it possible to increase heat transfer.
Human genetic potential is limited in time, and rather harshly. If you miss the term of early socialization, it will fade away, not having time to be realized. A prime example of this statement are numerous cases when babies by force of circumstances fell into the jungle and spent several years among the animals. After their return to the human community, they could no longer fully make up for lost time: master speech, acquire sufficiently complex skills of human activity, their mental functions of a person were poorly developed. This is evidence that the characteristic features of human behavior and activity are acquired only through social inheritance, only through the transfer of a social program in the process of education and training.
Identical genotypes (in identical twins), being in different environments, can give different phenotypes. Taking into account all the factors of influence, the human phenotype can be represented as consisting of several elements.
These include:biological inclinations encoded in genes; environment (social and natural); the activity of the individual; mind (consciousness, thinking).
The interaction of heredity and environment in human development plays an important role throughout his life. But it acquires particular importance during the periods of the formation of the organism: embryonic, breast, child, adolescent and youth. It was at this time that an intensive process of the development of the organism and the formation of the personality was observed.
Heredity determines what an organism can become, but a person develops under the simultaneous influence of both factors - heredity and the environment. Today it is becoming generally accepted that human adaptation is carried out under the influence of two programs of heredity: biological and social. All signs and properties of any individual are the result of the interaction of his genotype and environment. Therefore, each person is both a part of nature and a product of social development.
91. Combinative variability. The value of combinative variability in ensuring the genotypic diversity of people: Systems of marriage. Medical and genetic aspects of the family.
Combinative variability associated with obtaining new combinations of genes in the genotype. This is achieved as a result of three processes: a) independent divergence of chromosomes during meiosis; b) their accidental combination during fertilization; c) gene recombination thanks to Crossover. The hereditary factors (genes) themselves do not change, but new combinations of them appear, which leads to the emergence of organisms with other genotypic and phenotypic properties. Due to combinative variability a variety of genotypes is created in the offspring, which is of great importance for the evolutionary process due to the fact that: 1)
the variety of material for the evolutionary process increases without reducing the viability of individuals; 2)
the possibilities of adaptation of organisms to changing environmental conditions expand and thereby ensure the survival of a group of organisms (population, species) as a whole
The composition and frequency of alleles in humans, in populations largely depend on the types of marriages. In this regard, the study of types of marriages and their medico-genetic consequences is of great importance.
Marriages can be: electoral, indiscriminate.
To the indiscriminate include panmix marriages. Panmixia (Greek nixis - mixture) - intermarriages between people with different genotypes.
Electoral marriages: 1. Outbreeding - marriages between people who do not have family ties according to a predetermined genotype, 2.Inbreeding - marriages between relatives, 3.Positively assortative - marriages between individuals with similar phenotypes between (deaf and dumb, undersized with undersized, tall with tall, feeble-minded with feeble-minded, etc.). 4.Negative-assortative-marriages between people with dissimilar phenotypes (deaf-mute-normal; short-tall; normal - with freckles, etc.). 4 incest - marriages between close relatives (between brother and sister).
Inbred and incest marriage is illegal in many countries. Unfortunately, there are regions with a high frequency of inbred marriages. Until recently, the rate of inbred marriages in some regions of Central Asia reached 13-15%.
Medical and genetic significance inbred marriages are very negative. With such marriages, homozygotization is observed, the frequency of autosomal recessive diseases increases by 1.5-2 times. Inbred populations are characterized by inbred depression, i.e. the frequency increases sharply, the frequency of unwanted recessive alleles increases, and infant mortality increases. Positive-assortative marriages also lead to similar phenomena. Outbreeding is genetically positive. With such marriages, heterozygotization is observed.
92. Mutational variability, classification of mutations by the level of changes in the lesion of hereditary material. Mutations in germ and somatic cells.
Mutation is called a change due to the reorganization of the reproducing structures, a change in its genetic apparatus. Mutations occur spasmodically and are inherited. Depending on the level of change in the hereditary material, all mutations are divided into gene, chromosomal and genomic.
Gene mutations, or transgenations, affect the structure of the gene itself. Mutations can change parts of the DNA molecule of different lengths. The smallest site, a change in which leads to the appearance of a mutation, is called a muton. It can only be a couple of nucleotides. A change in the sequence of nucleotides in DNA determines a change in the sequence of triplets and, ultimately, a protein synthesis program. It should be remembered that violations in the structure of DNA lead to mutations only when there is no repair.
Chromosomal mutations, chromosomal rearrangements or aberrations consist in a change in the number or redistribution of the hereditary material of chromosomes.
Restructures are subdivided into nutrichromosomal and interchromosomal... Intrachromosomal rearrangements consist in the loss of a part of the chromosome (deletion), doubling or multiplication of some of its sections (duplication), rotation of the chromosome fragment by 180 ° with a change in the sequence of the gene arrangement (inversion).
Genomic mutations associated with a change in the number of chromosomes. Genomic mutations include aneuploidy, haploidy, and polyploidy.
Aneuploidy the change in the number of individual chromosomes is called - the absence (monosomy) or the presence of additional (trisomy, tetrasomy, in the general case polysomy) chromosomes, that is, an unbalanced chromosome set. Cells with an altered number of chromosomes appear as a result of disturbances in the process of mitosis or meiosis, in connection with which mitotic and meiotic aneuplody are distinguished. A multiple decrease in the number of chromosome sets of somatic cells in comparison with diploid is called haploidy... The multiple increase in the number of chromosome sets of somatic cells in comparison with the diploid one is called polyploidy.
The listed types of mutations are found both in germ cells and in somatic ones. Mutations that occur in the germ cells are called generative... They are passed on to subsequent generations.
Mutations that occur in body cells at one stage or another individual development organism are called somatic... Such mutations are inherited by the descendants of only the cell in which it occurred.
93. Gene mutations, molecular mechanisms of occurrence, frequency of mutations in nature. Biological anti-mutation mechanisms.
Modern genetics emphasizes that gene mutations consist in changing the chemical structure of genes. Specifically, gene mutations are substitutions, insertions, drops and losses of base pairs. The smallest part of a DNA molecule, a change in which leads to a mutation, is called a muton. It is equal to one pair of nucleotides.
There are several classifications of gene mutations ... Spontaneous (spontaneous) is a mutation that occurs outside of direct connection with any physical or chemical factor in the environment.
If mutations are caused deliberately by exposure to factors of a known nature, they are called induced... The mutation-inducing agent is called mutagen.
The nature of mutagens is diverse are physical factors, chemical compounds... The mutagenic effect of some biological objects - viruses, protozoa, helminths - has been established when they enter the human body.
As a result of dominant and recessive mutations in the phenotype, dominant and recessive altered traits appear. Dominantmutations appear in the phenotype already in the first generation. Recessive mutations are hidden in heterozygotes from action natural selection, therefore, they accumulate in the gene pools of species in large numbers.
An indicator of the intensity of the mutational process is the mutation frequency, which is calculated on average per genome or separately for specific loci. The average mutation frequency is comparable in a wide range of living things (from bacteria to humans) and does not depend on the level and type of morphophysiological organization. It is equal to 10 -4 - 10 -6 mutations per 1 locus per generation.
Anti-mutation mechanisms.
The chromosome pairing in the diploid karyotype of eukaryotic somatic cells serves as a defense factor against the adverse effects of gene mutations. The paired allele genes prevent the phenotypic manifestation of mutations if they are recessive.
The phenomenon of extracopying of genes encoding vital macromolecules contributes to the reduction of the harmful effects of gene mutations. For example, the genes of rRNA, tRNA, histone proteins, without which the vital activity of any cell is impossible.
These mechanisms contribute to the preservation of genes selected during evolution and, at the same time, the accumulation of alleles in the gene pool of the population, forming a reserve of hereditary variability.
94. Genomic mutations: polyploidy, haploidy, heteroploidy. Mechanisms of their occurrence.
Genomic mutations are associated with changes in the number of chromosomes. Genomic mutations include heteroploidy, haploidyand polyploidy.
Polyploidy - an increase in the diploid number of chromosomes by adding whole chromosome sets as a result of a violation of meiosis.
In polyploid forms, there is an increase in the number of chromosomes, a multiple of the haploid set: 3n - triploid; 4n - tetraploid, 5n - pentaploid, etc.
Polyploid forms phenotypically differ from diploid ones: along with a change in the number of chromosomes, hereditary properties also change. In polyploids, cells are usually large; sometimes the plants are gigantic.
Forms resulting from the multiplication of chromosomes in one genome are called autoploid. However, another form of polyploidy is also known - alloploidy, in which the number of chromosomes of two different genomes is multiplied.
A multiple decrease in the number of chromosomal sets of somatic cells in comparison with diploid is called haploidy... Haploid organisms in natural habitat are found mainly among plants, including higher ones (dope, wheat, corn). The cells of such organisms have one chromosome of each homologous pair, so all recessive alleles appear in the phenotype. This explains the reduced viability of haploids.
Heteroploidy... As a result of a violation of mitosis and meiosis, the number of chromosomes may change and not become a multiple of the haploid set. The phenomenon when any of the chromosomes, instead of being paired, turns out to be in a triple number, has received the name trisomies... If trisomy is observed on one chromosome, then such an organism is called a trisomic and its chromosomal set is 2n + 1. Trisomy can be on any of the chromosomes, and even on several. With Double trisomy, it has a set of chromosomes 2n + 2, triple - 2n + 3, etc.
The opposite phenomenon trisomies, i.e. the loss of one of the chromosomes from a pair in a diploid set is called monosomy, the organism is a monosomic; its genotypic formula is 2n-1. In the absence of two different chromosomes, the organism is a double monosome with the genotypic formula 2n-2, etc.
From what has been said, it is clear that aneuploidy, i.e. violation of the normal number of chromosomes, leads to changes in the structure and to a decrease in the viability of the organism. The larger the violation, the lower the viability. In humans, a violation of the balanced set of chromosomes entails painful conditions known collectively as chromosomal diseases.
Mechanism of occurrence genomic mutations are associated with the pathology of a violation of the normal separation of chromosomes in meiosis, as a result of which abnormal gametes are formed, which leads to mutation. Changes in the body are associated with the presence of genetically dissimilar cells.
95. Methods for studying human heredity. Genealogical and twin methods, their importance for medicine.
The main methods for studying human heredity are genealogical, twin, population-statistical, dermatoglyphics method, cytogenetic, biochemical, somatic cell genetics method, modeling method
Genealogical method. This method is based on the compilation and analysis of pedigrees. A pedigree is a diagram that reflects the bonds between family members. Analyzing pedigrees, they study any normal or (more often) pathological sign in generations of people in family ties.
Genealogical methods are used to determine the hereditary or non-hereditary nature of a trait, dominance or recessiveness, chromosome mapping, sex linkage, to study the mutational process. As a rule, the genealogical method forms the basis for conclusions in medical genetic counseling.
When compiling pedigrees, standard designations are used. The person who starts the research is a proband. A descendant of a married couple is called a sibling, siblings are called siblings, cousins \u200b\u200bare called cousins \u200b\u200bsiblings, etc. Descendants who have a common mother (but different fathers) are called consanguineous, and descendants who have a common father (but different mothers) are called consanguineous; if the family has children from different marriages, moreover, they do not have common ancestors (for example, a child from a mother's first marriage and a child from a father's first marriage), then they are called half-hearted.
With the help of the genealogical method, the hereditary conditionality of the trait under study, as well as the type of its inheritance, can be established. When analyzing pedigrees for several signs, the linked nature of their inheritance can be revealed, which is used when compiling chromosome maps. This method allows one to study the intensity of the mutation process, to assess the expressivity and penetrance of the allele.
Twin method... It consists in studying the patterns of inheritance of traits in pairs of single and double twins. Twins are two or more children conceived and born by the same mother almost simultaneously. Distinguish between identical and fraternal twins.
Identical (monozygous, identical) twins appear at the earliest stages of zygote cleavage, when two or four blastomeres retain the ability to develop into a full-fledged organism during separation. Since the zygote divides by mitosis, the genotypes of identical twins are, at least initially, completely identical. Identical twins are always of the same sex, during intrauterine development they have one placenta.
Different eggs (dizygotic, non-identical) occur when two or more simultaneously matured eggs are fertilized. Thus, they share about 50% of the genes in common. In other words, they are similar to regular brothers and sisters in their genetic constitution and can be either same-sex or heterosexual.
When comparing identical and fraternal twins raised in the same environment, one can conclude about the role of genes in the development of traits.
The twin method allows you to make reasonable conclusions about the heritability of traits: the role of heredity, environment and random factors in determining certain traits of a person
Prevention and diagnosis of hereditary pathology
Currently, the prevention of hereditary pathology is carried out at four levels: 1) pregametic; 2) prezygotic; 3) prenatal; 4) neonatal.
1.) Pregametic level
Carried out:
1. Sanitary control of production - exclusion of the effect of mutagens on the body.
2. Exemption of women of childbearing age from work in hazardous work.
3. Creation of lists of hereditary diseases that are common in a particular
territory with def. frequent.
2.Presygotic level
The most important element of this level of prevention is medical genetic counseling (MGC) of the population, which informs the family about the degree of possible risk of having a child with an investigational pathology and help in making the right decision about childbirth ..
Prenatal level
It consists in conducting prenatal (antenatal) diagnostics.
Prenatal diagnosis Is a set of measures that is carried out in order to determine the hereditary pathology in the fetus and terminate this pregnancy. The methods of prenatal diagnosis include:
1. Ultrasonic scanning (USS).
2. Fetoscopy - a method of visual observation of the fetus in the uterine cavity through an elastic probe equipped with an optical system.
3... Chorionic biopsy... The method is based on the collection of chorionic villi, cell cultivation and their study using cytogenetic, biochemical and molecular genetic methods.
4. Amniocentesis- puncture of the amniotic fluid through the abdominal wall and taking
amniotic fluid. It contains fetal cells that can be examined
cytogenetically or biochemically, depending on the alleged pathology of the fetus.
5. Cordocentesis- puncture of the vessels of the umbilical cord and taking fetal blood. Fetal lymphocytes
cultivated and tested.
4.Neonatal level
At the fourth level, newborns are screened for the detection of autosomal recessive metabolic diseases in the preclinical stage, when timely started treatment makes it possible to ensure the normal mental and physical development of children.
Principles of treating hereditary diseases
There are the following types of treatment.
1. Symptomatic (impact on disease symptoms).
2. Pathogenetic (impact on the mechanisms of development of the disease).
Symptomatic and pathogenetic treatment does not eliminate the causes of the disease, because does not eliminate
genetic defect.
In symptomatic and pathogenetic treatment, the following techniques can be used.
· Correction malformations by surgical methods (syndactyly, polydactyly,
non-closure of the upper lip ...
Substitution therapy, the meaning of which is to introduce into the body
missing or insufficient biochemical substrates.
· Metabolism induction - the introduction into the body of substances that enhance synthesis
some enzymes and, therefore, speed up the processes.
· Inhibition of metabolism - the introduction into the body of drugs that bind and remove
abnormal metabolic products.
· Diet therapy (medical nutrition) - the elimination of substances from the diet that
cannot be absorbed by the body.
Perspectives: In the near future, genetics will develop rapidly, although it is still in our days.
very widespread in agricultural crops (selection, cloning),
medicine (medical genetics, genetics of microorganisms). In the future, scientists hope
use genetics to eliminate defective genes and eradicate diseases transmitted by
by inheritance, be able to treat such serious diseases as cancer, viral
infections.
With all the shortcomings of the modern assessment of the radiogenetic effect, there is no doubt about the seriousness of the genetic consequences that await humanity in the event of an uncontrolled increase in the radioactive background in the environment. The danger of further testing of atomic and hydrogen weapons is obvious.
At the same time, the use of atomic energy in genetics and breeding makes it possible to create new methods of controlling the heredity of plants, animals and microorganisms, to better understand the processes of genetic adaptation of organisms. In connection with manned flights into outer space, it becomes necessary to study the influence of the cosmic reaction on living organisms.
98. Cytogenetic method for the diagnosis of human chromosomal disorders. Amniocentesis. Karyotype and idiogram of human chromosomes. Biochemical method.
The cytogenetic method consists in studying chromosomes using a microscope. More often, the object of study is mitotic (metaphase), less often meiotic (prophase and metaphase) chromosomes. Cytogenetic methods are used when studying the karyotypes of individual individuals
The material of the developing intrauterine organism is obtained in different ways. One of them is amniocentesis, with the help of which amniotic fluid is obtained at 15-16 weeks of pregnancy, containing waste products of the fetus and cells of its skin and mucous membranes
The material taken during amniocentesis is used for biochemical, cytogenetic and molecular chemical studies. Cytogenetic methods determine the sex of the fetus and identify chromosomal and genomic mutations. The study of amniotic fluid and fetal cells using biochemical methods makes it possible to detect a defect in protein gene products, but it does not make it possible to determine the localization of mutations in the structural or regulatory part of the genome. The use of DNA probes plays an important role in the detection of hereditary diseases and the exact localization of damage to the hereditary material of the fetus.
Currently, with the help of amniocentesis, all chromosomal abnormalities, over 60 hereditary metabolic diseases, incompatibility of the mother and the fetus for erythrocyte antigens are diagnosed.
The diploid set of chromosomes of a cell, characterized by their number, size and shape, is called karyotype... The normal human karyotype includes 46 chromosomes, or 23 pairs: of which 22 are autosomes and one pair is sex chromosomes
In order to make it easier to understand the complex complex of chromosomes that make up the karyotype, they are arranged in the form idiograms... IN idiogramchromosomes are arranged in pairs in decreasing order of magnitude, with the exception of sex chromosomes. The largest pair was assigned No. 1, the smallest - No. 22. The identification of chromosomes only by size encounters great difficulties: a number of chromosomes have similar sizes. However, in recent times through the use of various kinds of dyes, a clear differentiation of human chromosomes by their length into dyeing special methods and non-staining streaks. The ability to accurately differentiate chromosomes is of great importance for medical genetics, as it allows you to accurately establish the nature of violations in a person's karyotype.
Biochemical method
99. Human karyotype and idiogram. Characteristics of the human karyotype is normal
and pathology.
Karyotyp - a set of signs (number, size, shape, etc.) of a complete set of chromosomes,
inherent in the cells of a given biological species (species karyotype), a given organism
(individual karyotype) or line (clone) of cells.
To determine the karyotype, a micrograph or a sketch of chromosomes with microscopy of dividing cells is used.
Each person has 46 chromosomes, two of which are sex. A woman has two X chromosomes
(karyotype: 46, XX), and males have one X chromosome and the other Y (karyotype: 46, XY). Study
a karyotype is performed using a technique called cytogenetics.
Idiogram - a schematic representation of the haploid set of chromosomes of an organism, which
arranged in a row according to their size, in pairs in descending order of their size. An exception is made for sex chromosomes, which stand out especially.
Examples of the most common chromosomal abnormalities.
Down syndrome is a trisomy on the 21st pair of chromosomes.
Edwards syndrome is trisomy on the 18th pair of chromosomes.
Patau syndrome is a trisomy on the 13th pair of chromosomes.
Klinefelter's syndrome is an X chromosome polysomy in boys.
100. The importance of genetics for medicine. Cytogenetic, biochemical, population-statistical methods for studying human heredity.
The role of genetics in human life is very important. It is implemented with the help of medical genetic counseling. Medical genetic counseling is designed to save humanity from the suffering associated with hereditary (genetic) diseases. The main goals of medical genetic counseling are to establish the role of the genotype in the development of this disease and to predict the risk of having sick offspring. Recommendations given in medico-genetic consultations regarding marriage or prognosis of the genetic usefulness of offspring are aimed at ensuring that they are taken into account by the consulted persons who voluntarily make the appropriate decision.
Cytogenetic (karyotypic) method. The cytogenetic method consists of studying chromosomes using a microscope. More often, the object of study is mitotic (metaphase), less often meiotic (prophase and metaphase) chromosomes. This method is also used to study sex chromatin ( calf barra) Cytogenetic methods are used when studying the karyotypes of individual individuals
The use of the cytogenetic method allows not only to study the normal morphology of chromosomes and the karyotype in general, to determine the genetic sex of the organism, but, most importantly, to diagnose various chromosomal diseases associated with a change in the number of chromosomes or a violation of their structure. In addition, this method allows you to study the processes of mutagenesis at the level of chromosomes and karyotype. Its use in medical and genetic counseling for the purposes of prenatal diagnosis of chromosomal diseases makes it possible by timely termination of pregnancy to prevent the appearance of offspring with gross developmental disorders.
Biochemical method consists in determining in the blood or urine the activity of enzymes or the content of certain metabolic products. With the help of this method, metabolic disorders are detected and caused by the presence of an unfavorable combination of allelic genes in the genotype, more often recessive alleles in a homozygous state. With the timely diagnosis of such hereditary diseases, preventive measures allow avoiding serious developmental disorders.
Population-statistical method. This method allows you to assess the probability of birth of persons with a certain phenotype in a given population group or in closely related marriages; calculate the frequency of carriage in a heterozygous state of recessive alleles. The method is based on the Hardy - Weinberg law. Hardy-Weinberg law Is the law of population genetics. The law says: "In an ideal population, the frequencies of genes and genotypes remain constant from generation to generation."
The main features of human populations are: common territory and the possibility of free marriage. Factors of isolation, that is, limiting the freedom of choice of spouses, a person may have not only geographical, but also religious and social barriers.
In addition, this method allows one to study the mutational process, the role of heredity and the environment in the formation of phenotypic polymorphism in humans according to normal characteristics, as well as in the occurrence of diseases, especially with a hereditary predisposition. The population-statistical method is used to determine the significance of genetic factors in anthropogenesis, in particular in race formation.
101. Structural aberrations (aberrations) of chromosomes. Classification based on changes in genetic material. Significance for biology and medicine.
Chromosomal aberrations result from the rearrangement of chromosomes. They are a consequence of the rupture of the chromosome, leading to the formation of fragments, which are subsequently reunited, but the normal structure of the chromosome is not restored. There are 4 main types of chromosomal aberrations: shortages, doubling, inversion, translocations, deletion - the loss of a certain area by the chromosome, which is then usually destroyed
Shortages arise due to the loss of a chromosome of a particular site. The deficiencies in the middle part of the chromosome are called deletions. The loss of a significant part of the chromosome leads the body to death, the loss of insignificant areas causes a change in hereditary properties. So. When one of the chromosomes in corn is lacking, its seedlings are devoid of chlorophyll.
Doubling associated with the inclusion of an extra, duplicate portion of the chromosome. This also leads to the emergence of new signs. So, in Drosophila, the stripe-eye gene is due to the duplication of a section of one of the chromosomes.
Inversions are observed when the chromosome is broken and the detached area is turned over by 180 degrees. If the rupture occurs in one place, the detached fragment is attached to the chromosome with the opposite end, if in two places, then the middle fragment, turning over, is attached to the places of rupture, but with different ends. According to Darwin, inversions play an important role in the evolution of species.
Translocations arise in cases when a chromosome section from one pair is attached to a non-homologous chromosome, i.e. chromosome from another pair. Translocationsections of one of the chromosomes are known in humans; it can be the cause of Down's disease. Most translocations involving large sections of chromosomes render the organism unviable.
Chromosomal mutations change the dose of some genes, cause a redistribution of genes between linkage groups, change their localization in the linkage group. By doing this, they disrupt the gene balance of the cells of the body, resulting in deviations in the somatic development of the individual. Typically, changes affect multiple organ systems.
Chromosomal aberrations are of great importance in medicine. When chromosomal aberrations, there is a delay in the overall physical and mental development... Chromosomal diseases are characterized by a combination of many congenital defects. Such a defect is the manifestation of Down syndrome, which is observed in the case of trisomy in a small segment of the long arm of chromosome 21. The picture of the crying syndrome develops with the loss of a section of the short arm of chromosome 5. In humans, malformations of the brain, musculoskeletal, cardiovascular, and genitourinary systems are most often observed.
102. The concept of a species, modern views on speciation. View criteria.
ViewIs a collection of individuals that are similar in terms of the criteria of the species to such an extent that they can
naturally interbreed and produce fertile offspring.
Fertile offspring - that which itself can reproduce. An example of infertile offspring is a mule (a hybrid of a donkey and a horse), it is sterile.
View criteria - these are signs by which two organisms are compared to determine whether they belong to the same species or to different ones.
· Morphological - internal and external structure.
· Physiological and biochemical - how organs and cells work.
· Behavioral - behavior, especially at the time of reproduction.
Environmental - a set of environmental factors necessary for life
species (temperature, humidity, food, competitors, etc.)
· Geographic - area (area of \u200b\u200bdistribution), i.e. the territory in which this species lives.
· Genetic-reproductive - the same number and structure of chromosomes, which allows organisms to produce fertile offspring.
View criteria are relative, i.e. one criterion cannot be used to judge the species. For example, there are sibling species (in the malaria mosquito, in rats, etc.). They do not differ morphologically from each other, but they have a different number of chromosomes and therefore do not give offspring.
103. Population. Its ecological and genetic characteristics and role in speciation.
Population - a minimal self-reproducing grouping of individuals of the same species, more or less isolated from other similar groups, inhabiting a certain area for a long series of generations, forming its own genetic system and forming its own ecological niche.
Environmental indicators of the population.
Number - the total number of individuals in the population. This value is characterized by a wide range of variability, but it cannot be below some limits.
Density - the number of individuals per unit area or volume. As the number increases, the population density tends to increase
Spatial structure The population is characterized by the peculiarities of the distribution of individuals in the occupied territory. It is determined by the properties of the habitat and the biological characteristics of the species.
Gender structure reflects a certain ratio of males and females in the population.
Age structure reflects the ratio of different age groups in populations, depending on life expectancy, the time of puberty, the number of offspring.
Genetic indicators of the population... Genetically, a population is characterized by its gene pool. It is represented by a set of alleles that form the genotypes of organisms in a given population.
When describing populations or comparing them with each other, a number of genetic characteristics are used. Polymorphism... A population is called polymorphic at a given locus if two or more alleles are found in it. If a locus is represented by a single allele, one speaks of monomorphism. Examining many loci, one can determine the proportion of polymorphic ones among them, i.e. assess the degree of polymorphism, which is an indicator of the genetic diversity of a population.
Heterozygosity... An important genetic characteristic of a population is heterozygosity - the frequency of heterozygous individuals in a population. It also reflects genetic diversity.
Inbreeding coefficient... This coefficient is used to estimate the prevalence of closely related crossings in the population.
Gene association... The allele frequencies of different genes can depend on each other, which is characterized by the association coefficients.
Genetic distances. Different populations differ from each other in allele frequencies. To quantify these differences, indicators called genetic distances have been proposed.
Population- elementary evolutionary structure. In the range of any species, individuals are distributed unevenly. Areas of dense concentration of individuals are interspersed with spaces where there are not many of them or are absent. The result is more or less isolated populations in which random free crossing (panmixia) occurs systematically. Crossbreeding with other populations is very rare and irregular. Thanks to panmixia, each population creates a characteristic gene pool that is different from other populations. It is the population that should be recognized as an elementary unit of the evolutionary process
The role of populations is great, since almost all mutations occur within it. These mutations are primarily associated with the isolation of populations and the gene pool, which differs due to their isolation from each other. The material for evolution is mutational variability, which begins in the population and ends with the formation of a species.
GENETIC CODE, a method of recording hereditary information in nucleic acid molecules in the form of a sequence of nucleotides forming these acids. A specific sequence of nucleotides in DNA and RNA corresponds to a specific sequence of amino acids in the polypeptide chains of proteins. It is customary to write the code using capital letters of the Russian or Latin alphabet. Each nucleotide is designated by a letter with which the name of the nitrogenous base included in its molecule begins: A (A) - adenine, G (G) - guanine, C (C) - cytosine, T (T) - thymine; in RNA instead of thymine, uracil is U (U). Each is encoded by a combination of three nucleotides - a triplet, or codon. Briefly, the path of transfer of genetic information is summarized in the so-called. central dogma of molecular biology: DNA `RNA f protein.
In special cases, information can be transferred from RNA to DNA, but never from protein to genes.
The implementation of genetic information is carried out in two stages. In the cell nucleus, informational, or matrix, RNA (transcription) is synthesized on DNA. In this case, the DNA nucleotide sequence is "rewritten" (recoded) into the mRNA nucleotide sequence. Then mRNA passes into the cytoplasm, attaches to the ribosome, and on it, as on a template, a protein polypeptide chain is synthesized (translation). Amino acids with the help of transport RNA are attached to the building chain in a sequence determined by the order of nucleotides in mRNA.
Four “letters” can be used to make 64 different three-letter “words” (codons). Of the 64 codons, 61 code for certain amino acids, and three are responsible for the end of the synthesis of the polypeptide chain. Since there are 61 codons per 20 amino acids in proteins, some amino acids are encoded by more than one codon (the so-called degeneracy of the code). This redundancy increases the reliability of the code and the entire mechanism of protein biosynthesis. Another property of the code is its specificity (unambiguity): one codon encodes only one amino acid.
In addition, the code does not overlap - information is read in one direction sequentially, triplet by triplet. The most surprising property of the code is its universality: it is the same in all living things - from bacteria to humans (with the exception of the genetic code of mitochondria). Scientists see this as a confirmation of the concept of the origin of all organisms from one common ancestor.
The decoding of the genetic code, that is, the definition of the "meaning" of each codon and the rules by which information is read, was carried out in 1961-1965. and is considered one of the most striking achievements of molecular biology.