The genetic code is. The concept of a gene, genetic code. DNA code system

Genetic functions of DNA are that it provides storage, transmission and implementation hereditary information, which represents information about the primary structure of proteins (i.e. their amino acid composition). The connection between DNA and protein synthesis was predicted by biochemists J. Beadle and E. Tatum back in 1944 when studying the mechanism of mutations in the mold Neurospora. Information is recorded as a specific sequence of nitrogenous bases in a DNA molecule using a genetic code. Deciphering the genetic code is considered one of the great discoveries of natural science of the twentieth century. and in significance is equated to the discovery nuclear energy in physics. Success in this area is associated with the name of the American scientist M. Nirenberg, in whose laboratory the first codon, YYY, was deciphered. However, the entire decryption process took more than 10 years, many famous scientists from different countries, and not only biologists, but also physicists, mathematicians, and cybernetics. A decisive contribution to the development of the mechanism for recording genetic information was made by G. Gamow, who was the first to suggest that a codon consists of three nucleotides. Through the joint efforts of scientists, a complete description of the genetic code was given.

Letters in the inner circle are bases in the 1st position in the codon, letters in the second circle are
the bases are in the 2nd position and the letters outside the second circle are the bases in the 3rd position.
In the last circle are the abbreviated names of amino acids. NP - non-polar,
P - polar amino acid residues.

The main properties of the genetic code are: triplicity, degeneracy And non-overlapping. Triplety means that a sequence of three bases determines the inclusion of a specific amino acid in a protein molecule (for example, AUG - methionine). The degeneracy of the code is that the same amino acid can be encoded by two or more codons. Non-overlap means that the same base cannot appear in two adjacent codons.

It has been determined that the code is universal, i.e. The principle of recording genetic information is the same in all organisms.

Triplets encoding the same amino acid are called synonymous codons. Usually they have identical grounds in the 1st and 2nd positions and differ only in the third base. For example, the inclusion of the amino acid alanine in a protein molecule is encoded by synonymous codons in the RNA molecule - GCA, GCC, GCG, GCY. The genetic code contains three non-coding triplets (nonsense codons - UAG, UGA, UAA), which play the role of stop signals in the process of reading information.

It has been established that the universality of the genetic code is not absolute. While maintaining the principle of coding common to all organisms and the features of the code, in a number of cases a change in the semantic load of individual code words is observed. This phenomenon was called the ambiguity of the genetic code, and the code itself was called quasi-universal.

Read also other articles Topic 6 "Molecular basis of heredity":

Continue reading other topics in the book "Genetics and selection. Theory. Assignments. Answers".

The genetic code is a system for recording hereditary information in nucleic acid molecules, based on a certain alternation of nucleotide sequences in DNA or RNA, forming codons corresponding to amino acids in a protein.

Properties of the genetic code.

The genetic code has several properties.

Tripletity.

Degeneracy or redundancy.

Unambiguity.

Polarity.

Non-overlapping.

Compactness.

Versatility.

It should be noted that some authors also propose other properties of the code related to the chemical characteristics of the nucleotides included in the code or the frequency of occurrence of individual amino acids in the body’s proteins, etc. However, these properties follow from those listed above, so we will consider them there.

A. Tripletity. The genetic code, like many complexly organized systems, has the smallest structural and smallest functional unit. A triplet is the smallest structural unit of the genetic code. It consists of three nucleotides. A codon is the smallest functional unit of the genetic code. Typically, triplets of mRNA are called codons. In the genetic code, a codon performs several functions. Firstly, its main function is that it encodes a single amino acid. Secondly, the codon may not code for an amino acid, but, in this case, it performs another function (see below). As can be seen from the definition, a triplet is a concept that characterizes elementary structural unit genetic code (three nucleotides). Codon – characterizes elementary semantic unit genome - three nucleotides determine the attachment of one amino acid to the polypeptide chain.

The elementary structural unit was first deciphered theoretically, and then its existence was confirmed experimentally. Indeed, 20 amino acids cannot be encoded with one or two nucleotides because there are only 4 of the latter. Three out of four nucleotides give 4 3 = 64 variants, which more than covers the number of amino acids available in living organisms (see Table 1).

The 64 nucleotide combinations presented in table have two features. Firstly, of the 64 triplet variants, only 61 are codons and encode any amino acid; they are called sense codons. Three triplets do not encode

amino acids a are stop signals indicating the end of translation. There are three such triplets - UAA, UAG, UGA, they are also called “meaningless” (nonsense codons). As a result of a mutation, which is associated with the replacement of one nucleotide in a triplet with another, a nonsense codon can arise from a sense codon. This type of mutation is called nonsense mutation. If such a stop signal is formed inside the gene (in its information part), then during protein synthesis in this place the process will be constantly interrupted - only the first (before the stop signal) part of the protein will be synthesized. A person with this pathology will experience a lack of protein and will experience symptoms associated with this deficiency. For example, this kind of mutation was identified in the gene encoding the hemoglobin beta chain. A shortened inactive hemoglobin chain is synthesized, which is quickly destroyed. As a result, a hemoglobin molecule devoid of a beta chain is formed. It is clear that such a molecule is unlikely to fully fulfill its duties. A serious disease occurs that develops as hemolytic anemia (beta-zero thalassemia, from the Greek word “Thalas” - Mediterranean Sea, where this disease was first discovered).

The mechanism of action of stop codons differs from the mechanism of action of sense codons. This follows from the fact that for all codons encoding amino acids, corresponding tRNAs have been found. No tRNAs were found for nonsense codons. Consequently, tRNA does not take part in the process of stopping protein synthesis.

CodonAUG (in bacteria sometimes GUG) not only encode the amino acids methionine and valine, but are alsobroadcast initiator .

b. Degeneracy or redundancy.

61 of the 64 triplets encode 20 amino acids. This three-fold excess of the number of triplets over the number of amino acids suggests that two coding options can be used in the transfer of information. Firstly, not all 64 codons can be involved in encoding 20 amino acids, but only 20 and, secondly, amino acids can be encoded by several codons. Research has shown that nature used the latter option.

His preference is obvious. If out of 64 variant triplets only 20 were involved in encoding amino acids, then 44 triplets (out of 64) would remain non-coding, i.e. meaningless (nonsense codons). Previously, we pointed out how dangerous it is for the life of a cell to transform a coding triplet as a result of mutation into a nonsense codon - this significantly disrupts the normal functioning of RNA polymerase, ultimately leading to the development of diseases. Currently, three codons in our genome are nonsense, but now imagine what would happen if the number of nonsense codons increased by about 15 times. It is clear that in such a situation the transition of normal codons to nonsense codons will be immeasurably higher.

A code in which one amino acid is encoded by several triplets is called degenerate or redundant. Almost every amino acid has several codons. Thus, the amino acid leucine can be encoded by six triplets - UUA, UUG, TSUU, TsUC, TsUA, TsUG. Valine is encoded by four triplets, phenylalanine by two and only tryptophan and methionine encoded by one codon. The property that is associated with recording the same information with different symbols is called degeneracy.

The number of codons designated for one amino acid correlates well with the frequency of occurrence of the amino acid in proteins.

And this is most likely not accidental. The higher the frequency of occurrence of an amino acid in a protein, the more often the codon of this amino acid is represented in the genome, the higher the likelihood of its damage by mutagenic factors. Therefore, it is clear that a mutated codon has a greater chance of encoding the same amino acid if it is highly degenerate. From this perspective, the degeneracy of the genetic code is a mechanism that protects the human genome from damage.

It should be noted that the term degeneracy is used in molecular genetics in another sense. Thus, the bulk of the information in a codon is contained in the first two nucleotides; the base in the third position of the codon turns out to be of little importance. This phenomenon is called “degeneracy of the third base.” The latter feature minimizes the effect of mutations. For example, it is known that the main function of red blood cells is to carry oxygen from the lungs to the tissues and carbon dioxide from tissues to lungs. This function is performed by the respiratory pigment - hemoglobin, which fills the entire cytoplasm of the erythrocyte. It consists of a protein part - globin, which is encoded by the corresponding gene. In addition to protein, the hemoglobin molecule contains heme, which contains iron. Mutations in globin genes lead to the appearance of different variants of hemoglobins. Most often, mutations are associated with replacing one nucleotide with another and the appearance of a new codon in the gene, which may encode a new amino acid in the hemoglobin polypeptide chain. In a triplet, as a result of mutation, any nucleotide can be replaced - the first, second or third. Several hundred mutations are known that affect the integrity of the globin genes. Near 400 of which are associated with the replacement of single nucleotides in a gene and the corresponding amino acid replacement in a polypeptide. Of these only 100 replacements lead to instability of hemoglobin and various kinds of diseases from mild to very severe. 300 (approximately 64%) substitution mutations do not affect hemoglobin function and do not lead to pathology. One of the reasons for this is the above-mentioned “degeneracy of the third base,” when a replacement of the third nucleotide in a triplet encoding serine, leucine, proline, arginine and some other amino acids leads to the appearance of a synonymous codon encoding the same amino acid. Such a mutation will not manifest itself phenotypically. In contrast, any replacement of the first or second nucleotide in a triplet in 100% of cases leads to the appearance of a new hemoglobin variant. But even in this case, there may not be severe phenotypic disorders. The reason for this is the replacement of an amino acid in hemoglobin with another one similar to the first one. physical and chemical properties. For example, if an amino acid with hydrophilic properties is replaced by another amino acid, but with the same properties.

Hemoglobin consists of the iron porphyrin group of heme (oxygen and carbon dioxide molecules are attached to it) and protein - globin. Adult hemoglobin (HbA) contains two identical -chains and two -chains. Molecule -chain contains 141 amino acid residues, -chain - 146, - And -chains differ in many amino acid residues. The amino acid sequence of each globin chain is encoded by its own gene. Gene encoding -the chain is located in the short arm of chromosome 16, -gene - in the short arm of chromosome 11. Substitution in the gene encoding -the hemoglobin chain of the first or second nucleotide almost always leads to the appearance of new amino acids in the protein, disruption of hemoglobin functions and serious consequences for the patient. For example, replacing “C” in one of the triplets CAU (histidine) with “Y” will lead to the appearance of a new triplet UAU, encoding another amino acid - tyrosine. Phenotypically this will manifest itself in a severe disease.. A similar substitution in position 63 -chain of histidine polypeptide to tyrosine will lead to destabilization of hemoglobin. The disease methemoglobinemia develops. Replacement, as a result of mutation, of glutamic acid with valine in the 6th position -chain is the cause of the most severe disease - sickle cell anemia. Let's not continue the sad list. Let us only note that when replacing the first two nucleotides, an amino acid with physicochemical properties similar to the previous one may appear. Thus, replacement of the 2nd nucleotide in one of the triplets encoding glutamic acid (GAA) in -chain with “U” leads to the appearance of a new triplet (GUA), encoding valine, and replacing the first nucleotide with “A” forms the triplet AAA, encoding the amino acid lysine. Glutamic acid and lysine are similar in physicochemical properties - they are both hydrophilic. Valine is a hydrophobic amino acid. Therefore, replacing hydrophilic glutamic acid with hydrophobic valine significantly changes the properties of hemoglobin, which ultimately leads to the development of sickle cell anemia, while replacing hydrophilic glutamic acid with hydrophilic lysine changes the function of hemoglobin to a lesser extent - patients develop a mild form of anemia. As a result of the replacement of the third base, the new triplet can encode the same amino acids as the previous one. For example, if in a CAC triplet uracil was replaced by cytosine and a CAC triplet appeared, then virtually no phenotypic changes would be detected in humans. This is understandable, because both triplets code for the same amino acid – histidine.

In conclusion, it is appropriate to emphasize that the degeneracy of the genetic code and the degeneracy of the third base from a general biological point of view are protective mechanisms that are inherent in evolution in the unique structure of DNA and RNA.

V. Unambiguity.

Each triplet (except nonsense) encodes only one amino acid. Thus, in the direction codon - amino acid the genetic code is unambiguous, in the direction amino acid - codon it is ambiguous (degenerate).

Unambiguous

Amino acid codon

Degenerate

And in this case, the need for unambiguity in the genetic code is obvious. In another option, when translating the same codon, different amino acids would be inserted into the protein chain and, as a result, proteins with different primary structures and different functions would be formed. Cell metabolism would switch to the “one gene – several polypeptides” mode of operation. It is clear that in such a situation the regulatory function of genes would be completely lost.

g. Polarity

Reading information from DNA and mRNA occurs only in one direction. Polarity has important to determine higher order structures (secondary, tertiary, etc.). Earlier we talked about how lower-order structures determine higher-order structures. Tertiary structure and structures more high order in proteins, they are formed immediately as soon as the synthesized RNA chain leaves the DNA molecule or the polypeptide chain leaves the ribosome. While the free end of an RNA or polypeptide acquires a tertiary structure, the other end of the chain continues to be synthesized on DNA (if RNA is transcribed) or a ribosome (if a polypeptide is transcribed).

Therefore, the unidirectional process of reading information (during the synthesis of RNA and protein) is essential not only for determining the sequence of nucleotides or amino acids in the synthesized substance, but for the strict determination of secondary, tertiary, etc. structures.

d. Non-overlapping.

The code may be overlapping or non-overlapping. Most organisms have a non-overlapping code. Overlapping code is found in some phages.

The essence of a non-overlapping code is that a nucleotide of one codon cannot simultaneously be a nucleotide of another codon. If the code were overlapping, then the sequence of seven nucleotides (GCUGCUG) could encode not two amino acids (alanine-alanine) (Fig. 33, A) as in the case of a non-overlapping code, but three (if there is one nucleotide in common) (Fig. . 33, B) or five (if two nucleotides are common) (see Fig. 33, C). In the last two cases, a mutation of any nucleotide would lead to a violation in the sequence of two, three, etc. amino acids.

However, it has been established that a mutation of one nucleotide always disrupts the inclusion of one amino acid in a polypeptide. This is a significant argument that the code is non-overlapping.

Let us explain this in Figure 34. Bold lines show triplets encoding amino acids in the case of non-overlapping and overlapping code. Experiments have clearly shown that the genetic code is non-overlapping. Without going into details of the experiment, we note that if you replace the third nucleotide in the sequence of nucleotides (see Fig. 34)U (marked with an asterisk) to some other thing:

1. With a non-overlapping code, the protein controlled by this sequence would have a substitution of one (first) amino acid (marked with asterisks).

2. With an overlapping code in option A, a substitution would occur in two (first and second) amino acids (marked with asterisks). Under option B, the replacement would affect three amino acids (marked with asterisks).

However, numerous experiments have shown that when one nucleotide in DNA is disrupted, the disruption in the protein always affects only one amino acid, which is typical for a non-overlapping code.

GZUGZUG GZUGZUG GZUGZUG

GCU GCU GCU UGC GCU GCU GCU UGC GCU GCU GCU

*** *** *** *** *** ***

Alanin - Alanin Ala - Cis - Ley Ala - Ley - Ley - Ala - Ley

A B C

Non-overlapping code Overlapping code

Rice. 34. A diagram explaining the presence of a non-overlapping code in the genome (explanation in the text).

The non-overlap of the genetic code is associated with another property - the reading of information begins from a certain point - the initiation signal. Such an initiation signal in mRNA is the codon encoding methionine AUG.

It should be noted that humans still have a small number of genes that deviate from general rule and overlap.

e. Compactness.

There is no punctuation between codons. In other words, triplets are not separated from each other, for example, by one meaningless nucleotide. The absence of “punctuation marks” in the genetic code has been proven in experiments.

and. Versatility.

The code is the same for all organisms living on Earth. Direct evidence of the universality of the genetic code was obtained by comparing DNA sequences with corresponding protein sequences. It turned out that all bacterial and eukaryotic genomes use the same sets of code values. There are exceptions, but not many.

The first exceptions to the universality of the genetic code were found in the mitochondria of some animal species. This concerned the terminator codon UGA, which reads the same as the codon UGG, encoding the amino acid tryptophan. Other rarer deviations from universality were also found.

DNA code system.

The genetic code of DNA consists of 64 triplets of nucleotides. These triplets are called codons. Each codon codes for one of the 20 amino acids used in protein synthesis. This gives some redundancy in the code: most amino acids are coded for by more than one codon.
One codon performs two interrelated functions: it signals the beginning of translation and encodes the inclusion of the amino acid methionine (Met) in the growing polypeptide chain. The DNA coding system is designed so that the genetic code can be expressed either as RNA codons or DNA codons. RNA codons are found in RNA (mRNA) and these codons are able to read information during the synthesis of polypeptides (a process called translation). But each mRNA molecule acquires a nucleotide sequence in transcription from the corresponding gene.

All but two amino acids (Met and Trp) can be encoded by 2 to 6 different codons. However, the genome of most organisms shows that certain codons are favored over others. In humans, for example, alanine is encoded by GCC four times more often than by GCG. This probably indicates greater translation efficiency of the translation apparatus (for example, the ribosome) for some codons.

The genetic code is almost universal. The same codons are assigned to the same section of amino acids and the same start and stop signals are overwhelmingly the same in animals, plants and microorganisms. However, some exceptions have been found. Most involve assigning one or two of the three stop codons to an amino acid.

The genetic code is a way of encoding the sequence of amino acids in a protein molecule using the sequence of nucleotides in a nucleic acid molecule. The properties of the genetic code arise from the characteristics of this coding.

Each protein amino acid is matched to three consecutive nucleic acid nucleotides - triplet, or codon. Each nucleotide can contain one of four nitrogenous bases. In RNA it is adenine(A), uracil(U), guanine(G), cytosine(C). By combining nitrogenous bases (in this case, nucleotides containing them) in different ways, you can get many different triplets: AAA, GAU, UCC, GCA, AUC, etc. The total number of possible combinations is 64, i.e. 4 3 .

The proteins of living organisms contain about 20 amino acids. If nature “planned” to encode each amino acid not with three, but with two nucleotides, then the variety of such pairs would not be enough, since there would be only 16 of them, i.e. 4 2 .

Thus, the main property of the genetic code is its triplicity. Each amino acid is encoded by a triplet of nucleotides.

Since there are significantly more possible different triplets than the amino acids used in biological molecules, the following property has been realized in living nature: redundancy genetic code. Many amino acids began to be encoded not by one codon, but by several. For example, the amino acid glycine is encoded by four different codons: GGU, GGC, GGA, GGG. Redundancy is also called degeneracy.

The correspondence between amino acids and codons is shown in tables. For example, these:

In relation to nucleotides, the genetic code has the following property: unambiguity(or specificity): each codon corresponds to only one amino acid. For example, the GGU codon can only code for glycine and no other amino acid.

Again. Redundancy means that several triplets can code for the same amino acid. Specificity - each specific codon can code for only one amino acid.

There are no special punctuation marks in the genetic code (except for stop codons, which indicate the end of polypeptide synthesis). The function of punctuation marks is performed by the triplets themselves - the end of one means that another will begin next. This implies the following two properties of the genetic code: continuity And non-overlapping. Continuity refers to the reading of triplets immediately after each other. Non-overlapping means that each nucleotide can be part of only one triplet. So the first nucleotide of the next triplet always comes after the third nucleotide of the previous triplet. A codon cannot begin with the second or third nucleotide of the preceding codon. In other words, the code does not overlap.

The genetic code has the property versatility. It is the same for all organisms on Earth, which indicates the unity of the origin of life. There are very rare exceptions to this. For example, some triplets in mitochondria and chloroplasts encode amino acids other than their usual ones. This may suggest that at the dawn of life there were slightly different variations of the genetic code.

Finally, the genetic code has noise immunity, which is a consequence of its property as redundancy. Point mutations, which sometimes occur in DNA, usually result in the replacement of one nitrogenous base with another. This changes the triplet. For example, it was AAA, but after the mutation it became AAG. However, such changes do not always lead to a change in the amino acid in the synthesized polypeptide, since both triplets, due to the redundancy property of the genetic code, can correspond to one amino acid. Considering that mutations are often harmful, the property of noise immunity is useful.

Gene classification

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

Dominant (a gene capable of suppressing the manifestation of a recessive gene allelic to it); - recessive (a gene whose expression is suppressed by its 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 method characteristic of all living organisms of encoding the amino acid sequence of proteins using a sequence of nucleotides.

DNA uses four nucleotides - 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. RNA uses the same nucleotides, with the exception of thymine, which is replaced by a similar nucleotide - uracil, which is designated by the letter U (U in Russian literature). In DNA and RNA molecules, nucleotides are arranged in chains and, thus, sequences of genetic letters are obtained.

Genetic code

To build proteins in nature, 20 different amino acids are used. 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 a protein encoded by a gene) is carried out using two matrix processes: transcription (that is, the synthesis of mRNA on a DNA matrix) and translation of the genetic code into an amino acid sequence (synthesis of a polypeptide chain on an mRNA matrix). Three consecutive nucleotides are sufficient to encode 20 amino acids, as well as the stop signal indicating 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. Triplety- a meaningful unit of code is a combination of three nucleotides (a triplet, or codon).

2. Continuity- there are no punctuation marks between triplets, that is, the information is read continuously.

3. Discreteness- the same nucleotide cannot simultaneously be part of two or more triplets.

4. Specificity- a specific 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 in organisms of different levels of complexity - from viruses to humans. (the methods are based on this genetic engineering)

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 the 5" to the 3" end, that is, along the DNA template strand, RNA polymerase moves in the direction 3"->5"

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

Initiation of transcription- a complex process that 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 synthesis of RNA along the coding chain continues. it, like DNA synthesis, occurs in the 5-3 direction

Termination- as soon as the polymerase reaches the terminator, it immediately splits off 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 primary products of transcription and translation into functioning molecules. Functionally inactive precursor molecules are exposed to P. ribonucleic acids (tRNA, rRNA, mRNA) and many others. proteins.

In the process of synthesis of catabolic enzymes (breaking down substrates), inducible synthesis of enzymes occurs in prokaryotes. This gives the cell the ability to adapt to conditions environment 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 breakdown of the corresponding 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 coli 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 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 RNA polymerase binds; at the same time, excess lactose binds to the repressor protein and releases the operator from it - the path is open for RNA polymerase, transcription of the structural genes of the lac operon begins. Lactose acts as an inducer of the synthesis of those enzymes that break it down.

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 to cells of a given type. The level of production depends on the cell type, as well as the stage of development of the organism. Regulation of gene expression occurs at the cellular and organism levels. 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 implement differentiated functions specialized cells must express a specific 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 a 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 the maturation of mRNA is accompanied by excision of introns from the corresponding primary RNA transcripts (pro-RNA), i.e. splicing.
5. The process of gene transcription depends on the state of chromatin. Local DNA compaction completely blocks RNA synthesis.
6. Transcription in eukaryotic cells is not always associated with translation. The synthesized mRNA can long time stored in the form of informationosomes. Transcription and translation occur in different compartments.
7. Some eukaryotic genes have inconsistent localization (labile genes or transposons).
8. Methods molecular biology revealed the inhibitory effect of histone proteins on mRNA synthesis.
9. During the development and differentiation of organs, gene activity 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, at each stage of ontogenesis, 5-10% of genes are expressed, the rest must be blocked.

6) repair of genetic material

Genetic reparation- the process of eliminating genetic damage and restoring the hereditary apparatus, occurring in the cells of living organisms under the influence 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 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 disorders of 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) eliminate the corresponding damage, restoring the original structure of nucleotides. This is the case, for example, with O6-methylguanine DNA methyltransferase, which removes a methyl group from a nitrogenous base onto one of its own cysteine ​​residues.

Substances responsible for storing and transmitting genetic information are nucleic acids(DNA and RNA).

All functions of cells and the body as a whole are determined a set of proteins providing

• formation of cellular structures,
• synthesis of all other substances (carbohydrates, fats, nucleic acids),
• the course of life processes.

The genome contains information about the amino acid sequence of all proteins in the body. This information is called genetic information .

Due to gene regulation, the time of protein synthesis, their quantity, and location in the cell or in the body as a whole are regulated. Regulatory sections of DNA are largely responsible for this, enhancing and weakening gene expression in response to certain signals.

Information about a protein can be recorded in nucleic acid in only one way - in the form of a sequence of nucleotides. DNA is built from 4 types of nucleotides (A, T, G, C), and proteins are made from 20 types of amino acids. Thus, the problem arises of translating the four-letter record of information in DNA into the twenty-letter record of proteins. The relations on the basis of which such a translation is carried out are called genetic code.

The outstanding physicist was the first to theoretically consider the problem of the genetic code Georgy Gamov. The genetic code has a certain set of properties, which will be discussed below.

Why is a genetic code necessary?

Previously, we said that all reactions in living organisms are carried out under the action of enzymes, and it is the ability of enzymes to couple reactions that allows cells to synthesize biopolymers using the energy of ATP hydrolysis. In the case of simple linear homopolymers, that is, polymers consisting of identical units, one enzyme is sufficient for such synthesis. To synthesize a polymer consisting of two alternating monomers, two enzymes are needed, three - three, etc. If the polymer is branched, additional enzymes are needed to form bonds at the branching points. Thus, in the synthesis of some complex polymers, more than ten enzymes are involved, each of which is responsible for the addition of a specific monomer in a specific place and with a specific bond.

However, when synthesizing irregular heteropolymers (that is, polymers without repeating regions) with a unique structure, such as proteins and nucleic acids, such an approach is in principle impossible. The enzyme can attach a specific amino acid, but cannot determine where in the polypeptide chain it should be placed. This is the main problem of protein biosynthesis, the solution of which is impossible using conventional enzymatic apparatus. An additional mechanism is needed that uses some source of information about the order of amino acids in the chain.

To solve this problem Koltsov suggested matrix mechanism of protein synthesis. He believed that a protein molecule is the basis, a matrix for the synthesis of the same molecules, i.e., opposite each amino acid residue in the polypeptide chain the same amino acid is placed in the new molecule being synthesized. This hypothesis reflected the level of knowledge of that era, when all functions of living things were associated with certain proteins.

However, it later turned out that the substance storing genetic information, are nucleic acids.

PROPERTIES OF THE GENETIC CODE

COLINEARITY (linearity)

First, we'll look at how the nucleotide sequence records the sequence of amino acids in proteins. It is logical to assume that since the sequences of nucleotides and amino acids are linear, there is a linear correspondence between them, i.e., adjacent nucleotides in DNA correspond to adjacent amino acids in the polypeptide. This is also indicated by the linear nature of genetic maps. Proof of such a linear correspondence, or collinearity, is the coincidence of the linear arrangement of mutations on genetic map and amino acid substitutions in proteins of mutant organisms.

triplicity

When considering the properties of a code, the question that comes up least often is the code number. It is necessary to encode 20 amino acids with four nucleotides. Obviously, 1 nucleotide cannot encode 1 amino acid, since then it would be possible to encode only 4 amino acids. To encode 20 amino acids, combinations of several nucleotides are needed. If we take combinations of two nucleotides, we get 16 different combinations ($4^2$ = 16). This is not enough. There will already be 64 combinations of three nucleotides ($4 ^3 $ = 64), i.e. even more than needed. It is clear that combinations of more nucleotides could also be used, but for reasons of simplicity and economy they are unlikely, i.e. the code is triplet.

degeneracy and uniqueness

In the case of 64 combinations, the question arises whether all combinations encode amino acids or whether each amino acid corresponds to only one triplet of nucleotides. In the second case, most of the triplets would be meaningless, and nucleotide substitutions as a result of mutations would lead to protein loss in two thirds of cases. This is not consistent with the observed rates of protein loss by mutation, indicating the use of all or almost all triplets. Later it was found that there are three triplets, not coding for amino acids. They serve to mark the end of a polypeptide chain. They are called stop codons. 61 triplets encode different amino acids, i.e. one amino acid can be encoded by several triplets. This property of the genetic code is called degeneracy. Degeneracy occurs only in the direction from amino acids to nucleotides, in the opposite direction the code is unambiguous, i.e. Each triplet codes for one specific amino acid.

punctuation marks

An important question, which theoretically turned out to be impossible to solve, is how triplets encoding neighboring amino acids are separated from each other, i.e., whether there are punctuation marks in the genetic text.

Missing commas - experiments

Ingenious experiments by Crick and Brenner made it possible to find out whether there are “commas” in genetic texts. During these experiments, scientists used mutagenic substances (acridine dyes) to cause the occurrence of a certain type of mutation - the loss or insertion of 1 nucleotide. It turned out that the loss or insertion of 1 or 2 nucleotides always causes a breakdown of the encoded protein, but the loss or insertion of 3 nucleotides (or a multiple of 3) has virtually no effect on the function of the encoded protein.

Let's imagine that we have a genetic text built from a repeating triplet of ABC nucleotides (Fig. 1, a). If there are no punctuation marks, inserting one additional nucleotide will lead to complete distortion of the text (Fig. 1, a). Bacteriophage mutations were obtained that were located close to each other on the genetic map. When crossing two phages carrying such mutations, a hybrid arose that carried two single-letter inserts (Fig. 1, b). It is clear that the meaning of the text was lost in this case as well. If you introduce another one-letter insert, then after a short incorrect section the meaning will be restored and there is a chance to obtain a functioning protein (Fig. 1, c). This is true for triplet code in the absence of punctuation. If the code number is different, then the number of insertions necessary to restore the meaning will be different. If there are punctuation marks in the code, then the insertion will disrupt the reading of only one triplet, and the rest of the protein will be synthesized correctly and will remain active. Experiments have shown that single-letter insertions always lead to the disappearance of the protein, and restoration of function occurs when the number of insertions is a multiple of 3. Thus, the triplet nature of the genetic code and the absence of internal punctuation marks were proven.

non-overlapping

Gamow assumed that the code was overlapping, i.e. the first, second and third nucleotides coded for the first amino acid, the second, third and fourth - for the second amino acid, the third, fourth and fifth - for the third, etc. This hypothesis created the appearance of solving spatial difficulties, but it created another problem. With this coding, a given amino acid could not be followed by any other, since in the triplet encoding it, the first two nucleotides had already been determined, and the number of possible triplets was reduced to four. Analysis of amino acid sequences in proteins showed that all possible pairs of neighboring amino acids occur, i.e. the code should be non-overlapping.

versatility

decoding the code

When the basic properties of the genetic code were studied, work began on deciphering it and the meanings of all triplets were determined (see figure). The triplet encoding a specific amino acid is called codon. As a rule, codons are indicated in mRNA, sometimes in the sense strand of DNA (the same codons, but with Y replaced by T). For some amino acids, such as methionine, there is only one codon. Others have two codons (phenylalanine, tyrosine). There are amino acids that are encoded by three, four and even six codons. Codons of one amino acid are similar to each other and, as a rule, differ in one last nucleotide. This makes the genetic code more stable, since replacing the last nucleotide in a codon during mutations does not lead to a replacement of the amino acid in the protein. Knowledge of the genetic code allows us, knowing the sequence of nucleotides in a gene, to deduce the sequence of amino acids in a protein, which is widely used in modern research.