What is the genetic code and its properties. Genetic code: properties and functions

The genetic code is a special encoding of hereditary information with the help of molecules. Based on this, genes appropriately control the synthesis of proteins and enzymes in the body, thereby determining metabolism. In turn, the structure of individual proteins and their functions are determined by the location and composition of amino acids - the structural units of the protein molecule.

In the middle of the last century, genes were identified that are separate sections (abbreviated as DNA). The links of nucleotides form a characteristic double chain, assembled in the form of a spiral.

Scientists have found a connection between genes and the chemical structure of individual proteins, the essence of which is that the structural order of amino acids in protein molecules fully corresponds to the order of nucleotides in the gene. Having established this connection, scientists decided to decipher genetic code, i.e. establish the laws of correspondence between the structural orders of nucleotides in DNA and amino acids in proteins.

There are only four types of nucleotides:

1) A - adenyl;

2) G - guanyl;

3) T - thymidyl;

4) C - cytidyl.

Proteins contain twenty types of essential amino acids. Difficulties arose with deciphering the genetic code, since there are much fewer nucleotides than amino acids. In solving this problem, it was suggested that amino acids are encoded various combinations of three nucleotides (the so-called codon or triplet).

In addition, it was necessary to explain exactly how the triplets are located along the gene. Thus, three main groups of theories arose:

1) triplets follow each other continuously, i.e. form a continuous code;

2) triplets are arranged with alternation of "meaningless" sections, i.e. the so-called "commas" and "paragraphs" are formed in the code;

3) triplets can overlap, i.e. the end of the first triplet may form the beginning of the next.

Currently, the theory of code continuity is mainly used.

The genetic code and its properties

1) The code is triplet - it consists of arbitrary combinations of three nucleotides that form codons.

2) The genetic code is redundant - its triplets. One amino acid can be encoded by several codons, since, according to mathematical calculations, there are three times more codons than amino acids. Some codons perform certain termination functions: some may be "stop signals" that program the end of the production of an amino acid chain, while others may indicate the initiation of code reading.

3) The genetic code is unambiguous - only one amino acid can correspond to each of the codons.

4) The genetic code is collinear, i.e. the sequence of nucleotides and the sequence of amino acids clearly correspond to each other.

5) The code is written continuously and compactly, there are no "meaningless" nucleotides in it. It begins with a certain triplet, which is replaced by the next one without a break and ends with a termination codon.

6) The genetic code is universal - the genes of any organism encode information about proteins in exactly the same way. This does not depend on the level of complexity of the organization of the organism or its systemic position.

modern science suggests that the genetic code arises directly from the birth of a new organism from bone matter. Random changes and evolutionary processes make possible any variants of the code, i.e. amino acids can be rearranged in any order. Why did this kind of code survive in the course of evolution, why is the code universal and has a similar structure? The more science learns about the phenomenon of the genetic code, the more new mysteries arise.

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Genetic code is a way of encoding the amino acid sequence of proteins using the sequence of nucleotides in the DNA molecule, characteristic of all living organisms.

The implementation of genetic information in living cells (i.e., the synthesis of a protein encoded in DNA) is carried out using two matrix processes: transcription (i.e., mRNA synthesis on a DNA template) and translation (synthesis of a polypeptide chain on an mRNA template).

DNA uses four nucleotides - adenine (A), guanine (G), cytosine (C), thymine (T). These "letters" make up the alphabet of the genetic code. RNA uses the same nucleotides, except for thymine, which is replaced by uracil (U). In DNA and RNA molecules, nucleotides line up in chains and, thus, sequences of “letters” are obtained.

In the nucleotide sequence of DNA there are code "words" for each amino acid of the future protein molecule - the genetic code. It consists in a certain sequence of nucleotides in the DNA molecule.

Three consecutive nucleotides encode the "name" of one amino acid, that is, each of the 20 amino acids is encoded by a significant code unit - a combination of three nucleotides called a triplet or codon.

At present, the DNA code has been completely deciphered, and we can talk about certain properties that are characteristic of this unique biological system, which provides the translation of information from the "language" of DNA to the "language" of the protein.

The carrier of genetic information is DNA, but since mRNA, a copy of one of the DNA strands, is directly involved in protein synthesis, the genetic code is most often written in the "RNA language".

Properties of the genetic code

Three consecutive nucleotides (nitrogenous bases) encode the "name" of one amino acid, that is, each of the 20 amino acids is encrypted by a significant code unit - a combination of three nucleotides called triplet or codon.

Triplet (codon)- a sequence of three nucleotides (nitrogenous bases) in a DNA or RNA molecule, which determines the inclusion of a certain amino acid in the protein molecule during its synthesis.

• Unambiguity (discreteness)

One triplet cannot encode two different amino acids; it encodes only one amino acid. A certain codon corresponds to only one amino acid.

Each amino acid can be defined by more than one triplet. Exception - methionine and tryptophan. In other words, several codons can correspond to the same amino acid.

• non-overlapping

The same base cannot be present in two adjacent codons at the same time.

Some triplets do not code for amino acids, but are peculiar " road signs”, which determine the beginning and end of individual genes (UAA, UAG, UGA), each of which means the cessation of synthesis and is located at the end of each gene, so we can talk about the polarity of the genetic code.

In animals and plants, in fungi, bacteria and viruses, the same triplet encodes the same type of amino acid, that is, the genetic code is the same for all living beings. In other words, universality is the ability of the genetic code to work in the same way in organisms. different levels complexity from viruses to humans. The universality of the DNA code confirms the unity of the origin of all life on our planet. Genetic engineering methods are based on the use of the universality property of the genetic code.

From the history of the discovery of the genetic code

For the first time the idea of ​​existence genetic code formulated by A. Down and G. Gamow in 1952-1954. Scientists have shown that a nucleotide sequence that uniquely determines the synthesis of a particular amino acid must contain at least three links. Later it was proved that such a sequence consists of three nucleotides, called codon or triplet.

Questions about which nucleotides are responsible for incorporating a particular amino acid into protein molecule and how many nucleotides this inclusion defines remained unresolved until 1961. Theoretical analysis showed that the code cannot consist of one nucleotide, since in this case only 4 amino acids can be encoded. However, the code cannot be doublet, that is, the combination of two nucleotides from the four-letter "alphabet" cannot cover all amino acids, since similar combinations theoretically only 16 are possible (4 2 = 16).

Three consecutive nucleotides are enough to encode 20 amino acids, as well as a “stop” signal, which means the end of the protein sequence, when the number of possible combinations is 64 (4 3 = 64).

Gene classification

1) By the nature of the interaction in the allelic pair:

Dominant (a gene capable of suppressing the manifestation of an allelic recessive gene); - recessive (a gene, the manifestation of which is suppressed by an allelic dominant gene).

2) Functional classification:

2) Genetic code- these are certain combinations of nucleotides and the sequence of their location in the DNA molecule. This is a way of encoding the amino acid sequence of proteins using a sequence of nucleotides, characteristic of all living organisms.

Four nucleotides are used in DNA - adenine (A), guanine (G), cytosine (C), thymine (T), which in Russian-language literature are denoted by the letters A, G, T and C. These letters make up the alphabet of the genetic code. In RNA, the same nucleotides are used, with the exception of thymine, which is replaced by a similar nucleotide - uracil, which is denoted by the letter U (U in Russian-language literature). In DNA and RNA molecules, nucleotides line up in chains and, thus, sequences of genetic letters are obtained.

Genetic code

There are 20 different amino acids used in nature 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 (i.e., the synthesis of a protein encoded by a gene) is carried out using two matrix processes: transcription (i.e., mRNA synthesis on a DNA template) and translation of the genetic code into an amino acid sequence (synthesis of a polypeptide chain on an mRNA template). Three consecutive nucleotides are enough to encode 20 amino acids, as well as the 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. Tripletity- a 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, the information is read continuously.

3. discreteness- the same nucleotide cannot be simultaneously part of 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" - end, that is, RNA polymerase moves along the template DNA chain in the direction 3 "-> 5"

Transcription consists of the stages of initiation, elongation and termination.

Transcription initiation - difficult process, which depends on the DNA sequence near the transcribed sequence (and in eukaryotes also on more distant parts of the genome - enhancers and silencers) and on the presence or absence of various protein factors.

Elongation- Further unwinding of DNA and RNA synthesis along the coding chain continues. it, like DNA synthesis, is carried out in the direction 5-3

Termination- as soon as the polymerase reaches the terminator, it is immediately cleaved from DNA, the local DNA-RNA hybrid is destroyed and the newly synthesized RNA is transported from the nucleus to the cytoplasm, at which transcription is completed.

Processing- a set of reactions leading to the transformation of the primary products of transcription and translation into functioning molecules. Items are subject to functionally inactive precursor molecules decomp. ribonucleic acid (tRNA, rRNA, mRNA) and many others. proteins.

In the process of synthesis of catabolic enzymes (cleaving substrates), prokaryotes undergo induced synthesis of enzymes. This gives the cell the opportunity 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. The enzyme is synthesized only when the cleavage of the corresponding substrate is necessary for the cell.
2. The substrate concentration 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 the repressor protein is located on the operator - transcription of the lac operon is blocked. When the amount of glucose in the environment, 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 RNA polymerase binds; at the same time, excess lactose binds to the repressor protein and releases the operator from it - the path for RNA polymerase is open, transcription of the structural genes of the lac operon begins. Lactose acts as an inductor for the synthesis of those enzymes that break it down.

5) Regulation of gene expression in eukaryotes is much more difficult. different types 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 to cells of this type. The level of production depends on the type of cells, as well as on the stage of development of the organism. Gene expression is regulated at the cell level and at the organism level. The genes of eukaryotic cells are divided into two main types: the first determines the universality of cellular functions, the second determines (determines) specialized cellular functions. Gene Functions first group appear 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 on 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 the DNA molecule. There are informative and non-informative sections, unique and repeatedly repeated informative nucleotide sequences.
4. Eukaryotic genes consist of exons and introns, and mRNA maturation 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 mRNA can long time be stored as infosomes. Transcription and translation occur in different compartments.
7. Some eukaryotic genes have non-permanent localization (labile genes or transposons).
8. Methods of molecular biology revealed the inhibitory effect of histone proteins on the synthesis of mRNA.
9. In the process of 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 should be blocked.

6) reparation genetic material

Genetic repair- the process of eliminating genetic damage and restoring the hereditary apparatus, which occurs 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. Kelner. 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 the 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 pigmentosum) are associated with impaired repair systems.

types of reparations:

Direct repair is the simplest way to eliminate damage in DNA, which usually involves specific enzymes that can 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 ​​residues.

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, representing 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, strange as it may sound, everything is united by one, universal, common genetic code. It was formed over a long time period, and the process ended about 3.5 billion years ago. Therefore, in the structure of the code, traces of its evolution can be traced, from the moment of its inception to the present day.

When talking about the sequence of elements in the genetic code, it means that it is far from being 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 will read on the pages of books or newspapers will turn into ridiculous gibberish.

Basic 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, basic properties the genetic code is:

• triplet;
• degeneracy or redundancy;
• uniqueness;
• continuity;
• the versatility already mentioned above.

Let's take a closer look at each property.

1. Tripletity

This is when three nucleotide compounds 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.

Codons (they are code words!) are distinguished by their connection sequence 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 3 nucleotides each. This is equivalent 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 is observed when several codons are required to encrypt one amino acid, usually within 2-6. And only tryptophan can be encoded with a single triplet.

3. Uniqueness

It is included in the properties of the genetic code as an indicator of healthy gene inheritance. For example, the GAA triplet in sixth place in the chain can tell doctors about a good state of blood, about normal hemoglobin. It is he who carries information about hemoglobin, and it is also encoded by him. And if a person is anemic, one of the nucleotides is replaced by another letter of the code - U, which is a signal of the disease.

4. Continuity

When writing this property of the genetic code, it should be remembered that codons, like chain links, are located not at a distance, but in direct proximity, one after another in the 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 man, in an insect and a bird, a hundred-year-old baobab and a blade of grass barely hatched from the ground, similar amino acids are encoded in identical triplets.

It is in the genes that the basic information about the properties of an organism is stored, a kind of program that the organism inherits from those who lived earlier and which exists as a genetic code.

Ministry of Education and Science Russian Federation Federal Agency for Education

State educational institution higher vocational education"Altai State Technical University named after I.I. Polzunov"

Department of Natural Science and System Analysis

Essay on the topic "Genetic code"

1. The concept of the genetic code

3. Genetic information

Bibliography

1. The concept of the genetic code

The genetic code is a unified system for recording hereditary information in molecules characteristic of living organisms nucleic acids as a sequence of nucleotides. Each nucleotide is denoted by a capital letter, which begins the name of the nitrogenous base that is part of it: - A (A) adenine; - G (G) guanine; - C (C) cytosine; - T (T) thymine (in DNA) or U (U) uracil (in mRNA).

The implementation of the genetic code in the cell occurs in two stages: transcription and translation.

The first of these takes place in the nucleus; it consists in the synthesis of mRNA molecules on the corresponding sections of DNA. In this case, the DNA nucleotide sequence is "rewritten" into the RNA nucleotide sequence. The second stage takes place in the cytoplasm, on ribosomes; in this case, the nucleotide sequence of the i-RNA is translated into the sequence of amino acids in the protein: this stage proceeds with the participation of transfer RNA (t-RNA) and the corresponding enzymes.

2. Properties of the genetic code

1. Tripletity

Each amino acid is encoded by a sequence of 3 nucleotides.

A triplet or codon is a sequence of three nucleotides that codes for one amino acid.

The code cannot be monopleth, since 4 (the number of different nucleotides in DNA) is less than 20. The code cannot be doublet, because 16 (the number of combinations and permutations of 4 nucleotides by 2) is less than 20. The code can be triplet, because 64 (the number of combinations and permutations from 4 to 3) is greater than 20.

2. Degeneracy.

All amino acids except methionine and tryptophan are encoded by more than one triplet: 2 amino acids 1 triplet = 2 9 amino acids 2 triplets each = 18 1 amino acid 3 triplets = 3 5 amino acids 4 triplets each = 20 3 amino acids 6 triplets each = 18 Total 61 triplet codes for 20 amino acids.

3. The presence of intergenic punctuation marks.

A gene is a section of DNA that codes for one polypeptide chain or one molecule of tRNA, rRNA, or sRNA.

The tRNA, rRNA, and sRNA genes do not code for proteins.

At the end of each gene encoding a polypeptide, there is at least one of 3 termination codons, or stop signals: UAA, UAG, UGA. They terminate the broadcast.

Conventionally, the AUG codon also belongs to punctuation marks - the first after the leader sequence. It performs the function of a capital letter. In this position, it codes for formylmethionine (in prokaryotes).

4. Uniqueness.

Each triplet encodes only one amino acid or is a translation terminator.

The exception is the AUG codon. In prokaryotes, in the first position (capital letter) it codes for formylmethionine, and in any other position it codes for methionine.

5. Compactness, or the absence of intragenic punctuation marks.

Within a gene, each nucleotide is part of a significant codon.

In 1961 Seymour Benzer and Francis Crick experimentally proved that the code is triplet and compact.

The essence of the experiment: "+" mutation - the insertion of one nucleotide. "-" mutation - loss of one nucleotide. A single "+" or "-" mutation at the beginning of a gene corrupts the entire gene. A double "+" or "-" mutation also spoils the entire gene. A triple "+" or "-" mutation at the beginning of the gene spoils only part of it. A quadruple "+" or "-" mutation again spoils the entire gene.

The experiment proves that the code is triplet and there are no punctuation marks inside the gene. The experiment was carried out on two adjacent phage genes and showed, in addition, the presence of punctuation marks between the genes.

3. Genetic information

Genetic information is a program of the properties of an organism, received from ancestors and embedded in hereditary structures in the form of a genetic code.

It is assumed that the formation of genetic information proceeded according to the scheme: geochemical processes - mineral formation - evolutionary catalysis (autocatalysis).

It is possible that the first primitive genes were microcrystalline crystals of clay, and each new layer of clay lines up in accordance with the structural features of the previous one, as if receiving information about the structure from it.

Realization of genetic information occurs in the process of synthesis of protein molecules with the help of three RNAs: informational (mRNA), transport (tRNA) and ribosomal (rRNA). The process of information transfer goes: - through the channel of direct communication: DNA - RNA - protein; and - via the feedback channel: environment - protein - DNA.

Living organisms are able to receive, store and transmit information. Moreover, living organisms tend to use the information received about themselves and the world around them as efficiently as possible. Hereditary information embedded in genes and necessary for a living organism for existence, development and reproduction is transmitted from each individual to his descendants. This information determines the direction of development of the organism, and in the process of its interaction with the environment, the reaction to its individual can be distorted, thereby ensuring the evolution of the development of descendants. In the process of evolution of a living organism, new information arises and is remembered, including the value of information for it increases.

During the implementation of hereditary information under certain conditions external environment the phenotype of organisms of a given biological species is formed.

Genetic information determines the morphological structure, growth, development, metabolism, mental warehouse, predisposition to diseases and genetic defects of the body.

Many scientists, rightly emphasizing the role of information in the formation and evolution of living things, noted this circumstance as one of the main criteria of life. So, V.I. Karagodin believes: "The living is such a form of existence of information and the structures encoded by it, which ensures the reproduction of this information in suitable environmental conditions." The connection of information with life is also noted by A.A. Lyapunov: "Life is a highly ordered state of matter that uses information encoded by the states of individual molecules to develop persistent reactions." Our well-known astrophysicist N.S. Kardashev also emphasizes the information component of life: “Life arises due to the possibility of synthesizing a special kind of molecules that are able to remember and use at first the simplest information about environment and their own structure, which they use for self-preservation, for reproduction, and, most importantly for us, for obtaining more more information". Ecologist F. Tipler draws attention to this ability of living organisms to store and transmit information in his book "Physics of Immortality": "I define life as some kind of encoded information that is preserved by natural selection." , then the system life - information is eternal, infinite and immortal.

The discovery of the genetic code and the establishment of the laws of molecular biology showed the need to combine modern genetics and the Darwinian theory of evolution. Thus, a new biological paradigm was born - the synthetic theory of evolution (STE), which can already be considered as non-classical biology.

The main ideas of Darwin's evolution with his triad - heredity, variability, natural selection - in modern view evolution of the living world are complemented by ideas not just natural selection, but such selection, which is determined genetically. The beginning of the development of synthetic or general evolution can be considered the work of S.S. Chetverikov on population genetics, in which it was shown that not individual traits and individuals are subjected to selection, but the genotype of the entire population, but it is carried out through the phenotypic traits of individual individuals. This leads to the spread beneficial changes in the entire population. Thus, the mechanism of evolution is implemented both through random mutations at the genetic level, and through the inheritance of the most valuable traits (the value of information!), which determine the adaptation of mutational traits to the environment, providing the most viable offspring.

Seasonal climate changes, various natural or man-made disasters, on the one hand, lead to a change in the frequency of gene repetition in populations and, as a result, to a decrease in hereditary variability. This process is sometimes called genetic drift. And on the other hand, to changes in the concentration of various mutations and a decrease in the diversity of genotypes contained in the population, which can lead to changes in the direction and intensity of selection.

4. Deciphering the human genetic code

In May 2006, scientists working on sequencing the human genome published a complete genetic map of chromosome 1, which was the last incompletely sequenced human chromosome.

A preliminary human genetic map was published in 2003, marking the formal end of the Human Genome Project. Within its framework, genome fragments containing 99% of human genes were sequenced. The accuracy of gene identification was 99.99%. However, at the end of the project, only four of the 24 chromosomes had been fully sequenced. The fact is that in addition to genes, chromosomes contain fragments that do not encode any traits and are not involved in protein synthesis. The role that these fragments play in the life of the organism is still unknown, but more and more researchers are inclined to believe that their study requires the closest attention.