Genetic code: description, characteristics, research history. The unambiguity of the genetic code is manifested in the fact that

In the body's metabolism leading role belongs to proteins and nucleic acids.
Protein substances form the basis of all vital cell structures, have an unusually high reactivity, and are endowed with catalytic functions.
Nucleic acids are part of the most important organ of the cell - the nucleus, as well as the cytoplasm, ribosomes, mitochondria, etc. Nucleic acids play an important, primary role in heredity, body variability, and protein synthesis.

Plan synthesis protein is stored in the cell nucleus, and direct synthesis occurs outside the nucleus, so it is necessary delivery service encoded plan from the nucleus to the site of synthesis. This delivery service is performed by RNA molecules.

The process starts at core cells: part of the DNA "ladder" unwinds and opens. Due to this, the RNA letters form bonds with the open DNA letters of one of the DNA strands. The enzyme transfers the letters of the RNA to connect them into a thread. So the letters of DNA are "rewritten" into the letters of RNA. The newly formed RNA chain is separated, and the DNA "ladder" twists again. The process of reading information from DNA and synthesizing its RNA template is called transcription , and the synthesized RNA is called informational or i-RNA .

After further modifications, this kind of encoded mRNA is ready. i-RNA exits the nucleus and goes to the site of protein synthesis, where the letters i-RNA are deciphered. Each set of three letters of i-RNA forms a "letter" that stands for one particular amino acid.

Another type of RNA looks for this amino acid, captures it with the help of an enzyme, and delivers it to the site of protein synthesis. This RNA is called transfer RNA, or tRNA. As the mRNA message is read and translated, the chain of amino acids grows. This chain twists and folds into a unique shape, creating one kind of protein. Even the process of protein folding is remarkable: to use a computer to calculate all options it would take 1027 (!) years to fold a medium-sized protein consisting of 100 amino acids. And for the formation of a chain of 20 amino acids in the body, it takes no more than one second, and this process occurs continuously in all cells of the body.

Genes, genetic code and its properties.

About 7 billion people live on Earth. Except for 25-30 million pairs of identical twins, then genetically all people are different : each is unique, has unique hereditary characteristics, character traits, abilities, temperament.

Such differences are explained differences in genotypes-sets of genes of an organism; each one is unique. The genetic traits of a particular organism are embodied in proteins - consequently, the structure of the protein of one person differs, although quite a bit, from the protein of another person.

It does not mean that humans do not have exactly the same proteins. Proteins that perform the same functions may be the same or very slightly differ by one or two amino acids from each other. But does not exist on the Earth of people (with the exception of identical twins), in which all proteins would be are the same .

Information about the primary structure of a protein encoded as a sequence of nucleotides in a section of a DNA molecule, gene - a unit of hereditary information of an organism. Each DNA molecule contains many genes. The totality of all the genes of an organism makes up its genotype . In this way,

A gene is a unit of hereditary information of an organism, which corresponds to a separate section of DNA

Hereditary information is encoded using genetic code , which is universal for all organisms and differs only in the alternation of nucleotides that form genes and code for proteins of specific organisms.

Genetic code consists of triplets (triplets) of DNA nucleotides that combine in different sequences (AAT, HCA, ACG, THC, etc.), each of which encodes a specific amino acid (which will be built into the polypeptide chain).

Actually code counts sequence of nucleotides in an i-RNA molecule , because it removes information from DNA (the process transcriptions ) and translates it into a sequence of amino acids in the molecules of synthesized proteins (process broadcasts ).
The composition of mRNA includes nucleotides A-C-G-U, the triplets of which are called codons : the CHT DNA triplet on mRNA will become the HCA triplet, and the AAG DNA triplet will become the UUC triplet. Exactly i-RNA codons reflects the genetic code in the record.

In this way, genetic code - a unified system for recording hereditary information in nucleic acid molecules in the form of a sequence of nucleotides . The genetic code is based on the use of an alphabet consisting of only four nucleotide letters that differ in nitrogenous bases: A, T, G, C.

The main properties of the genetic code:

1. Genetic code triplet. A triplet (codon) is a sequence of three nucleotides that codes for one amino acid. Since proteins contain 20 amino acids, it is obvious that each of them cannot be encoded by one nucleotide ( since there are only four types of nucleotides in DNA, in this case 16 amino acids remain uncoded). Two nucleotides for coding amino acids are also not enough, since in this case only 16 amino acids can be encoded. This means that the smallest number of nucleotides encoding one amino acid must be at least three. In this case, the number of possible nucleotide triplets is 43 = 64.

2. Redundancy (degeneracy) The code is a consequence of its triplet nature and means that one amino acid can be encoded by several triplets (since there are 20 amino acids, and there are 64 triplets), with the exception of methionine and tryptophan, which are encoded by only one triplet. In addition, some triplets perform specific functions: in the mRNA molecule, the triplets UAA, UAG, UGA are terminating codons, i.e. stop-signals that stop the synthesis of the polypeptide chain. The triplet corresponding to methionine (AUG), standing at the beginning of the DNA chain, does not encode an amino acid, but performs the function of initiating (exciting) reading.

3. Unambiguity code - along with redundancy, the code has the property uniqueness : each codon matches only one specific amino acid.

4. Collinearity code, i.e. sequence of nucleotides in a gene exactly corresponds to the sequence of amino acids in the protein.

5. Genetic code non-overlapping and compact , i.e. does not contain "punctuation marks". This means that the reading process does not allow for the possibility of overlapping columns (triples), and, starting at a certain codon, the reading goes continuously triple by triple until stop-signals ( termination codons).

6. Genetic code universal , i.e., the nuclear genes of all organisms encode information about proteins in the same way, regardless of the level of organization and the systematic position of these organisms.

Exist genetic code tables for decryption codons i-RNA and building chains of protein molecules.

Matrix synthesis reactions.

In living systems, there are reactions unknown in inanimate nature - matrix synthesis reactions.

The term "matrix" in technology they denote the form used for casting coins, medals, typographic type: the hardened metal exactly reproduces all the details of the form used for casting. Matrix synthesis resembles a casting on a matrix: new molecules are synthesized in strict accordance with the plan laid down in the structure of already existing molecules.

The matrix principle lies at the core the most important synthetic reactions of the cell, such as the synthesis of nucleic acids and proteins. In these reactions, an exact, strictly specific sequence of monomeric units in the synthesized polymers is provided.

This is where directional pulling monomers to a specific location cells - into molecules that serve as a matrix where the reaction takes place. If such reactions occurred as a result of a random collision of molecules, they would proceed infinitely slowly. The synthesis of complex molecules based on the matrix principle is carried out quickly and accurately. The role of the matrix macromolecules of nucleic acids play in matrix reactions DNA or RNA .

monomeric molecules, from which the polymer is synthesized - nucleotides or amino acids - in accordance with the principle of complementarity are arranged and fixed on the matrix in a strictly defined, predetermined order.

Then comes "crosslinking" of monomer units into a polymer chain, and the finished polymer is dropped from the matrix.

Thereafter matrix ready to the assembly of a new polymer molecule. It is clear that just as only one coin, one letter can be cast on a given mold, so on a given matrix molecule only one polymer can be "assembled".

Matrix type of reactions- a specific feature of the chemistry of living systems. They are the basis of the fundamental property of all living things - its ability to reproduce its own kind.

Matrix synthesis reactions

1. DNA replication - replication (from lat. replicatio - renewal) - the process of synthesis of a daughter molecule of deoxyribonucleic acid on the matrix of the parent DNA molecule. During the subsequent division of the mother cell, each daughter cell receives one copy of a DNA molecule that is identical to the DNA of the original mother cell. This process ensures the accurate transmission of genetic information from generation to generation. DNA replication is carried out by a complex enzyme complex, consisting of 15-20 different proteins, called replisome . The material for synthesis is free nucleotides present in the cytoplasm of cells. The biological meaning of replication lies in the exact transfer of hereditary information from the parent molecule to the daughter ones, which normally occurs during the division of somatic cells.

The DNA molecule consists of two complementary strands. These chains are held together by weak hydrogen bonds that can be broken by enzymes. The DNA molecule is capable of self-doubling (replication), and a new half of it is synthesized on each old half of the molecule.
In addition, an mRNA molecule can be synthesized on a DNA molecule, which then transfers the information received from DNA to the site of protein synthesis.

Information transfer and protein synthesis follow a matrix principle, comparable to the work of a printing press in a printing house. Information from DNA is copied over and over again. If errors occur during copying, they will be repeated in all subsequent copies.

True, some errors in copying information by a DNA molecule can be corrected - the process of eliminating errors is called reparations. The first of the reactions in the process of information transfer is the replication of the DNA molecule and the synthesis of new DNA strands.

2. Transcription (from Latin transcriptio - rewriting) - the process of RNA synthesis using DNA as a template, occurring 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. RNA polymerase moves along the DNA molecule in the direction 3 " → 5". Transcription consists of steps initiation, elongation and termination . The unit of transcription is the operon, a fragment of the DNA molecule consisting of promoter, transcribed moiety, and terminator . i-RNA consists of one strand and is synthesized on DNA in accordance with the rule of complementarity with the participation of an enzyme that activates the beginning and end of the synthesis of the i-RNA molecule.

The finished mRNA molecule enters the cytoplasm on the ribosomes, where the synthesis of polypeptide chains takes place.

3. Broadcast (from lat. translation- transfer, movement) - the process of protein synthesis from amino acids on the matrix of informational (matrix) RNA (mRNA, mRNA) carried out by the ribosome. In other words, this is the process of translating the information contained in the nucleotide sequence of i-RNA into the sequence of amino acids in the polypeptide.

4. reverse transcription is the process of forming double-stranded DNA based on information from single-stranded RNA. This process is called reverse transcription, since the transfer of genetic information occurs in the “reverse” direction relative to transcription. The idea of reverse transcription was initially very unpopular, as it went against the central dogma of molecular biology, which assumed that DNA is transcribed into RNA and then translated into proteins.

However, in 1970, Temin and Baltimore independently discovered an enzyme called reverse transcriptase (revertase) , and the possibility of reverse transcription was finally confirmed. In 1975, Temin and Baltimore were awarded the Nobel Prize in Physiology or Medicine. Some viruses (such as the human immunodeficiency virus that causes HIV infection) have the ability to transcribe RNA into DNA. HIV has an RNA genome that integrates into DNA. As a result, the DNA of the virus can be combined with the genome of the host cell. The main enzyme responsible for the synthesis of DNA from RNA is called revertase. One of the functions of reversease is to create complementary DNA (cDNA) from the viral genome. The associated enzyme ribonuclease cleaves RNA, and reversetase synthesizes cDNA from the DNA double helix. cDNA is integrated into the host cell genome by integrase. The result is synthesis of viral proteins by the host cell that form new viruses. In the case of HIV, apoptosis (cell death) of T-lymphocytes is also programmed. In other cases, the cell may remain a distributor of viruses.

The sequence of matrix reactions in protein biosynthesis can be represented as a diagram.

In this way, protein biosynthesis- this is one of the types of plastic exchange, during which the hereditary information encoded in the DNA genes is realized in a certain sequence of amino acids in protein molecules.

Protein molecules are essentially polypeptide chains made up of individual amino acids. But amino acids are not active enough to connect with each other on their own. Therefore, before they combine with each other and form a protein molecule, amino acids must activate . This activation occurs under the action of special enzymes.

As a result of activation, the amino acid becomes more labile and, under the action of the same enzyme, binds to t- RNA. Each amino acid corresponds to a strictly specific t- RNA, which finds "its" amino acid and endures it into the ribosome.

Therefore, the ribosome receives various activated amino acids linked to their t- RNA. The ribosome is like conveyor to assemble a protein chain from various amino acids entering it.

Simultaneously with t-RNA, on which its own amino acid "sits", " signal» from the DNA that is contained in the nucleus. In accordance with this signal, one or another protein is synthesized in the ribosome.

The directing influence of DNA on protein synthesis is not carried out directly, but with the help of a special intermediary - matrix or messenger RNA (mRNA or i-RNA), which synthesized into the nucleus It is not influenced by DNA, so its composition reflects the composition of DNA. The RNA molecule is, as it were, a cast from the form of DNA. The synthesized mRNA enters the ribosome and, as it were, transfers it to this structure plan- in what order should the activated amino acids entering the ribosome be combined with each other in order to synthesize a certain protein. Otherwise, genetic information encoded in DNA is transferred to mRNA and then to protein.

The mRNA molecule enters the ribosome and flashes her. That segment of it that is currently in the ribosome is determined codon (triplet), interacts in a completely specific way with a structure suitable for it triplet (anticodon) in the transfer RNA that brought the amino acid into the ribosome.

Transfer RNA with its amino acid approaches a certain codon of mRNA and connects with him; to the next, neighboring site of i-RNA joins another tRNA with a different amino acid and so on until the entire i-RNA chain is read, until all the amino acids are strung in the appropriate order, forming a protein molecule. And t-RNA, which delivered the amino acid to a specific site of the polypeptide chain, freed from its amino acid and exits the ribosome.

Then again in the cytoplasm, the desired amino acid can join it, and it will again transfer it to the ribosome. In the process of protein synthesis, not one, but several ribosomes, polyribosomes, are simultaneously involved.

The main stages of the transfer of genetic information:

1. Synthesis on DNA as on an mRNA template (transcription)
2. Synthesis of the polypeptide chain in ribosomes according to the program contained in i-RNA (translation) .

The stages are universal for all living beings, but the temporal and spatial relationships of these processes differ in pro- and eukaryotes.

At prokaryotes transcription and translation can occur simultaneously because DNA is located in the cytoplasm. At eukaryote transcription and translation are strictly separated in space and time: the synthesis of various RNAs occurs in the nucleus, after which the RNA molecules must leave the nucleus, passing through the nuclear membrane. The RNA is then transported in the cytoplasm to the site of protein synthesis.

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 nucleotide units form a characteristic double chain, assembled in the form of a helix.

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 the 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. When solving this problem, it was suggested that amino acids are encoded by 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 triplet code - 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.

The genetic code of different organisms has some common properties:
1) Tripletity. To record any information, including hereditary information, a certain cipher is used, the element of which is a letter or symbol. The combination of such symbols makes up the alphabet. Individual messages are written as a combination of characters called code groups, or codons. An alphabet consisting of only two characters is known - this is Morse code. There are 4 letters in DNA - the first letters of the names of nitrogenous bases (A, G, T, C), which means that the genetic alphabet consists of only 4 characters. What is a code group, or, in a word, a genetic code? 20 basic amino acids are known, the content of which must be written in the genetic code, i.e. 4 letters must give 20 code words. Let's say the word consists of one character, then we will get only 4 code groups. If the word consists of two characters, then there will be only 16 such groups, and this is clearly not enough to encode 20 amino acids. Therefore, there must be at least 3 nucleotides in the code word, which will give 64 (43) combinations. This number of triplet combinations is quite enough to encode all amino acids. Thus, the codon of the genetic code is a triplet of nucleotides.
2) Degeneracy (redundancy) - a property of the genetic code consisting, on the one hand, in the fact that it contains redundant triplets, i.e., synonyms, and on the other, "meaningless" triplets. Since the code includes 64 combinations, and only 20 amino acids are encoded, some amino acids are encoded by several triplets (arginine, serine, leucine - six; valine, proline, alanine, glycine, threonine - four; isoleucine - three; phenylalanine, tyrosine, histidine, lysine , asparagine, glutamine, cysteine, aspartic and glutamic acids - two; methionine and tryptophan - one triplet). Some code groups (UAA, UAG, UGA) do not carry a semantic load at all, i.e. they are "meaningless" triplets. "Senseless", or nonsense, codons act as chain terminators - punctuation marks in the genetic text - serve as a signal for the end of protein chain synthesis. Such code redundancy is of great importance for increasing the reliability of the transmission of genetic information.
3) Non-overlapping. Code triplets never overlap, i.e. they are always broadcast together. When reading information from a DNA molecule, it is impossible to use the nitrogenous base of one triplet in combination with the bases of another triplet.
4) Uniqueness. There are no cases where the same triplet would correspond to more than one acid.
5) The absence of separating characters within the gene. The genetic code is read from a certain place without commas.
6) Versatility. In different types of living organisms (viruses, bacteria, plants, fungi and animals), the same triplets encode the same amino acids.
7) Species specificity. The number and sequence of nitrogenous bases in the DNA chain are different in different organisms.

Gene- a structural and functional unit of heredity that controls the development of a particular trait or property. Parents pass on a set of genes to their offspring during reproduction. A great contribution to the study of the gene was made by Russian scientists: Simashkevich E.A., Gavrilova Yu.A., Bogomazova O.V. (2011)

Currently, in molecular biology, it has been established that genes are sections of DNA that carry any integral information - about the structure of one protein molecule or one RNA molecule. These and other functional molecules determine the development, growth and functioning of the body.

At the same time, each gene is characterized by a number of specific regulatory DNA sequences, such as promoters, which are directly involved in regulating the expression of the gene. Regulatory sequences can be located either in the immediate vicinity of the open reading frame encoding the protein, or the beginning of the RNA sequence, as is the case with promoters (the so-called cis cis-regulatory elements), and at a distance of many millions of base pairs (nucleotides), as in the case of enhancers, insulators and suppressors (sometimes classified as trans-regulatory elements trans-regulatory elements). Thus, the concept of a gene is not limited to the coding region of DNA, but is a broader concept that includes regulatory sequences.

Originally the term gene appeared as a theoretical unit for the transmission of discrete hereditary information. The history of biology remembers disputes about which molecules can be carriers of hereditary information. Most researchers believed that only proteins can be such carriers, since their structure (20 amino acids) allows you to create more options than the structure of DNA, which is composed of only four types of nucleotides. Later, it was experimentally proved that it is DNA that includes hereditary information, which was expressed as the central dogma of molecular biology.

Genes can undergo mutations - random or purposeful changes in the sequence of nucleotides in the DNA chain. Mutations can lead to a change in sequence, and therefore a change in the biological characteristics of a protein or RNA, which, in turn, can result in a general or local altered or abnormal functioning of the organism. Such mutations in some cases are pathogenic, since their result is a disease, or lethal at the embryonic level. However, not all changes in the nucleotide sequence lead to a change in the protein structure (due to the effect of the degeneracy of the genetic code) or to a significant change in the sequence and are not pathogenic. In particular, the human genome is characterized by single nucleotide polymorphisms and copy number variations. copy number variations), such as deletions and duplications, which make up about 1% of the entire human nucleotide sequence. Single nucleotide polymorphisms, in particular, define different alleles of the same gene.

The monomers that make up each of the DNA chains are complex organic compounds that include nitrogenous bases: adenine (A) or thymine (T) or cytosine (C) or guanine (G), a five-atom sugar-pentose-deoxyribose, named after which and received the name of DNA itself, as well as the residue of phosphoric acid. These compounds are called nucleotides.

Gene Properties

discreteness - immiscibility of genes;
stability - the ability to maintain a structure;
lability - the ability to repeatedly mutate;
multiple allelism - many genes exist in a population in a variety of molecular forms;
allelism - in the genotype of diploid organisms, only two forms of the gene;
specificity - each gene encodes its own trait;
pleiotropy - multiple effect of a gene;
expressivity - the degree of expression of a gene in a trait;
penetrance - the frequency of manifestation of a gene in the phenotype;
amplification - an increase in the number of copies of a gene.

Classification

Structural genes are unique components of the genome, representing a single sequence encoding a specific protein or some types of RNA. (See also the article housekeeping genes).
Functional genes - regulate the work of structural genes.

Genetic code- a method inherent in all living organisms to encode the amino acid sequence of proteins using a sequence of nucleotides.

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, C and T. 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 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 template) and translation of the genetic code into an amino acid sequence (synthesis of a polypeptide chain on mRNA). 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

Tripletity- a significant unit of the code is a combination of three nucleotides (triplet, or codon).
Continuity- there are no punctuation marks between the triplets, that is, the information is read continuously.
non-overlapping- the same nucleotide cannot simultaneously be part of two or more triplets (not observed for some overlapping genes of viruses, mitochondria and bacteria that encode several frameshift proteins).
Unambiguity (specificity)- a certain codon corresponds to only one amino acid (however, the UGA codon in Euplotes crassus codes for two amino acids - cysteine and selenocysteine)
Degeneracy (redundancy) Several codons can correspond to the same amino acid.
Versatility- the genetic code works in the same way in organisms of different levels of complexity - from viruses to humans (genetic engineering methods are based on this; there are a number of exceptions, shown in the table in the "Variations of the standard genetic code" section below).
Noise immunity- mutations of nucleotide substitutions that do not lead to a change in the class of the encoded amino acid are called conservative; nucleotide substitution mutations that lead to a change in the class of the encoded amino acid are called radical.

Protein biosynthesis and its steps

Protein biosynthesis- a complex multi-stage process of synthesis of a polypeptide chain from amino acid residues, occurring on the ribosomes of cells of living organisms with the participation of mRNA and tRNA molecules.

Protein biosynthesis can be divided into stages of transcription, processing and translation. During transcription, the genetic information encrypted in DNA molecules is read and this information is written into mRNA molecules. During a series of successive stages of processing, some fragments that are unnecessary in subsequent stages are removed from mRNA, and the nucleotide sequences are edited. After the code is transported from the nucleus to the ribosomes, the actual synthesis of protein molecules occurs by attaching individual amino acid residues to the growing polypeptide chain.

Between transcription and translation, the mRNA molecule undergoes a series of successive changes that ensure the maturation of a functioning template for the synthesis of the polypeptide chain. A cap is attached to the 5' end, and a poly-A tail is attached to the 3' end, which increases the lifespan of the mRNA. With the advent of processing in a eukaryotic cell, it became possible to combine gene exons to obtain a greater variety of proteins encoded by a single sequence of DNA nucleotides - alternative splicing.

Translation consists in the synthesis of a polypeptide chain in accordance with the information encoded in messenger RNA. The amino acid sequence is arranged using transport RNA (tRNA), which form complexes with amino acids - aminoacyl-tRNA. Each amino acid has its own tRNA, which has a corresponding anticodon that “matches” the mRNA codon. During translation, the ribosome moves along the mRNA, as the polypeptide chain builds up. Energy for protein synthesis is provided by ATP.

The finished protein molecule is then cleaved from the ribosome and transported to the right place in the cell. Some proteins require additional post-translational modification to reach their active state.

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

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, mRNA synthesis 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 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- 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 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 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 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 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 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 be stored as informosomes for a long time. 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) repair of 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) eliminate the corresponding damage, restoring the original structure of nucleotides. This is how, for example, O6-methylguanine-DNA methyltransferase acts, which removes a methyl group from a nitrogenous base to one of its own cysteine residues.