Why is the genetic code continuous. Genetic code as a way to record hereditary information

Today it is no secret to anyone that the life program of all living organisms is written on the DNA molecule. The easiest way to think of a DNA molecule is as a long ladder. The vertical uprights of this ladder are made up of molecules of sugar, oxygen, and phosphorus. All the important working information in the molecule is recorded on the rungs of the ladder - they consist of two molecules, each of which is attached to one of the uprights. These molecules, the nitrogenous bases, are called adenine, guanine, thymine, and cytosine, but they are usually referred to simply by the letters A, G, T, and C. The shape of these molecules allows them to form bonds - finished steps - of only a certain type. These are the bonds between the bases A and T and between the bases G and C (the pair formed in this way is called "pair of reasons"). There can be no other types of bonds in the DNA molecule.

Going down the steps along one strand of the DNA molecule, you get the sequence of bases. It is this message in the form of a sequence of bases that determines the flow of chemical reactions in the cell and, consequently, the characteristics of the organism that has this DNA. According to the central dogma of molecular biology, information about proteins is encoded on the DNA molecule, which, in turn, acting as enzymes ( cm. Catalysts and enzymes), regulate everything chemical reactions in living organisms.

A strict correspondence between the sequence of base pairs in a DNA molecule and the sequence of amino acids that make up protein enzymes is called the genetic code. The genetic code was deciphered shortly after the discovery of the double-stranded structure of DNA. It was known that the newly discovered molecule informational, or matrix RNA (mRNA, or mRNA) carries information written on DNA. Biochemists Marshall W. Nirenberg and J. Heinrich Matthaei of the National Institutes of Health in Bethesda, Washington, DC, performed the first experiments that led to the unraveling of the genetic code.

They started by synthesizing artificial mRNA molecules consisting only of the repeating nitrogenous base uracil (which is analogous to thymine, "T", and forms bonds only with adenine, "A", from the DNA molecule). They added these mRNAs to test tubes with a mixture of amino acids, with only one of the amino acids in each tube labeled with a radioactive label. The researchers found that the mRNA artificially synthesized by them initiated protein formation in only one test tube, where the labeled amino acid phenylalanine was located. So they established that the sequence "-U-U-U-" on the mRNA molecule (and, therefore, the equivalent sequence "-A-A-A-" on the DNA molecule) encodes a protein consisting only of the amino acid phenylalanine. This was the first step towards deciphering the genetic code.

Today it is known that three base pairs of a DNA molecule (such a triplet is called codon) code for one amino acid in a protein. By performing experiments similar to the one described above, geneticists eventually deciphered the entire genetic code, in which each of the 64 possible codons corresponds to a specific amino acid.

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 that form 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 offer other properties of the code related to chemical features nucleotides included in the code or with the frequency of occurrence of individual amino acids in body proteins, etc. However, these properties follow from the above, so we will consider them there.

a. Tripletity. The genetic code is like a lot of complicated organized system has the smallest structural and smallest functional unit. A triplet is the smallest structural unit of the genetic code. It consists of three nucleotides. codon is the smallest functional unit genetic code. As a rule, mRNA triplets are called codons. In the genetic code, a codon performs several functions. First, its main function is that it codes for one amino acid. Second, a codon may not code for an amino acid, but in this case it has a different 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 to the polypeptide chain of one amino acid.

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

The combinations of nucleotides presented in Table 64 have two features. First, of the 64 variants of triplets, 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 marking 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 meaningless 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 informational 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 such a pathology will experience a lack of protein and will experience symptoms associated with this lack. For example, this kind of mutation was found in the gene encoding the hemoglobin beta chain. A shortened inactive hemoglobin chain is synthesized, which is rapidly 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. There is a serious disease that develops according to the type of hemolytic anemia (beta-zero thalassemia, from the Greek word "Talas" - the Mediterranean Sea, where this disease was first discovered).

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

codonAUG (sometimes GUG in bacteria) not only encodes the amino acid methionine and valine, but is alsobroadcast initiator .

b. Degeneracy or redundancy.

61 of the 64 triplets code for 20 amino acids. Such a threefold 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. Studies have shown that nature used the latter option.

His preference is clear. If only 20 out of 64 triplet variants were involved in coding amino acids, then 44 triplets (out of 64) would remain non-coding, i.e. meaningless (nonsense codons). Earlier, we pointed out how dangerous for the life of a cell is the transformation of a coding triplet as a result of a mutation into a nonsense codon - this significantly violates normal work RNA polymerase, ultimately leading to the development of diseases. There are currently three nonsense codons in our genome, and 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. So, the amino acid leucine can be encoded by six triplets - UUA, UUG, CUU, CUC, CUA, CUG. Valine is encoded by four triplets, phenylalanine by two and only tryptophan and methionine encoded by one codon. The property that is associated with the recording of the same information with different characters is called degeneracy.

The number of codons assigned to 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 present in the genome, the higher the probability of its damage by mutagenic factors. Therefore, it is clear that a mutated codon is more likely to code for the same amino acid if it is highly degenerate. From these positions, 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 as well. Since the main part of the information in the codon falls on 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 tissues and carbon dioxide from tissues to lungs. This function is carried out 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, hemoglobin contains heme, which contains iron. Mutations in globin genes result in various options hemoglobins. Most often, mutations are associated with substitution of one nucleotide for another and the appearance of a new codon in the gene, which can code for a new amino acid in the hemoglobin polypeptide chain. In a triplet, as a result of a mutation, any nucleotide can be replaced - the first, second or third. Several hundred mutations are known to affect the integrity of globin genes. About 400 of which are associated with the replacement of single nucleotides in the gene and the corresponding amino acid substitution in the polypeptide. Of these, only 100 substitutions 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 “degeneracy of the third base” mentioned above, when the replacement of the third nucleotide in the triplet encoding serine, leucine, proline, arginine, and some other amino acids leads to the appearance of a synonym codon encoding the same amino acid. Phenotypically, such a mutation will not manifest itself. 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 an iron porphyrin group of heme (oxygen and carbon dioxide molecules are attached to it) and a 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. The gene encoding - the chain is located on the short arm of chromosome 16, -gene - in the short arm of chromosome 11. Change in the gene encoding - 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 CAU (histidine) triplets with “U” will lead to the appearance of a new UAU triplet encoding another amino acid - tyrosine. Phenotypically, this will manifest itself in a serious illness .. A similar replacement in position 63 -chain of the histidine polypeptide to tyrosine will destabilize hemoglobin. The disease methemoglobinemia develops. Change, as a result of mutation, of glutamic acid to valine in the 6th position chain is the cause of a severe disease - sickle cell anemia. Let's not continue the sad list. We only note that when replacing the first two nucleotides, an amino acid may appear similar in physicochemical properties to the previous one. Thus, the replacement of the 2nd nucleotide in one of the triplets encoding glutamic acid (GAA) in -chain on “Y” leads to the appearance of a new triplet (GUA) encoding valine, and the replacement of the first nucleotide with “A” forms an AAA triplet 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, the replacement of hydrophilic glutamic acid with hydrophobic valine significantly changes the properties of hemoglobin, which ultimately leads to the development of sickle cell anemia, while the replacement of 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 uracil was replaced by cytosine in the CAH triplet and a CAC triplet arose, then practically no phenotypic changes in a person will be detected. 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 position are protective mechanisms that are incorporated in evolution in the unique structure of DNA and RNA.

in. Unambiguity.

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

unambiguous

codon amino acid

degenerate

And in this case, the need for unambiguity in the genetic code is obvious. In another variant, during the translation of 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. The cell's 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 from mRNA occurs only in one direction. Polarity is essential for defining higher order structures (secondary, tertiary, etc.). Earlier we talked about the fact that structures of a lower order determine structures of a higher order. The tertiary structure and structures of a higher order in proteins are formed immediately as soon as the synthesized RNA chain moves away from the DNA molecule or the polypeptide chain moves away from the ribosome. While the free end of the RNA or polypeptide acquires a tertiary structure, the other end of the chain still continues to be synthesized on DNA (if RNA is transcribed) or ribosome (if polypeptide is transcribed).

Therefore, the unidirectional process of reading information (in 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 rigid determination of secondary, tertiary, etc. structures.

e. Non-overlapping.

The code may or may not overlap. In most organisms, the code is non-overlapping. An overlapping code has been found in some phages.

The essence of a non-overlapping code is that the nucleotide of one codon cannot be the nucleotide of another codon at the same time. 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 one nucleotide is 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 found that a mutation of one nucleotide always disrupts the inclusion of one amino acid in a polypeptide. This is a significant argument in favor of the fact 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 unambiguously shown that the genetic code is non-overlapping. Without going into the details of the experiment, we note that if we replace the third nucleotide in the nucleotide sequence (see Fig. 34)At (marked with an asterisk) to some other then:

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

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

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

ГЦУГЦУГ ГЦУГЦУГ ГЦУГЦУГ

HCC HCC HCC UHC CUG HCC CUG UGC HCU CUG

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

Alanine - Alanine Ala - Cys - Lei Ala - Lei - Lei - Ala - Lei

A B C

non-overlapping code overlapping code

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

The non-overlapping 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 AUG methionine.

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

e. Compactness.

There are no punctuation marks between codons. In other words, the 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 proof of the universality of the genetic code was obtained by comparing DNA sequences with corresponding protein sequences. It turned out that the same sets of code values ​​are used in all bacterial and eukaryotic genomes. 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 read the same as the UGG codon encoding the amino acid tryptophan. Other rarer deviations from universality have also been 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 encoded by more than one codon.
One codon performs two interrelated functions: it signals the beginning of translation and encodes the incorporation of the amino acid methionine (Met) into the growing polypeptide chain. The DNA code system is designed so that the genetic code can be expressed either as RNA codons or as DNA codons. RNA codons occur 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 coded for 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 in GCG. This probably indicates a greater translation efficiency of the translation apparatus (eg, the ribosome) for some codons.

The genetic code is almost universal. The same codons are assigned to the same stretch 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 of these involve assigning one or two of the three stop codons to an amino acid.

In any cell and organism, all features of the anatomical, morphological and functional nature are determined by the structure of the proteins that are included in them. The hereditary property of an organism is the ability to synthesize certain proteins. Amino acids are located in a polypeptide chain, on which biological characteristics depend.
Each cell has its own sequence of nucleotides in the DNA polynucleotide chain. This is the genetic code of DNA. Through it, information about the synthesis of certain proteins is recorded. About what the genetic code is, about its properties and genetic information is described in this article.

A bit of history

The idea that perhaps a genetic code exists was formulated by J. Gamow and A. Down in the middle of the twentieth century. They described that the nucleotide sequence responsible for the synthesis of a particular amino acid contains at least three units. Later proved exact amount of three nucleotides (this is a unit of the genetic code), which is called a triplet or codon. There are sixty-four nucleotides in total, because the acid molecule, where or RNA occurs, consists of residues of four different nucleotides.

What is the genetic code

The method of coding the protein sequence of amino acids due to the sequence of nucleotides is characteristic of all living cells and organisms. That's what the genetic code is.
There are four nucleotides in DNA:

• adenine - A;
• guanine - G;
• cytosine - C;
• thymine - T.

They are indicated by capital letters in Latin or (in Russian-language literature) Russian.
RNA also has four nucleotides, but one of them is different from DNA:

• adenine - A;
• guanine - G;
• cytosine - C;
• uracil - U.

All nucleotides line up in chains, and in DNA a double helix is ​​obtained, and in RNA it is single.
Proteins are built on twenty amino acids, where they, located in a certain sequence, determine its biological properties.

Properties of the genetic code

Tripletity. The unit of the genetic code consists of three letters, it is triplet. This means that the twenty existing amino acids are coded for by three specific nucleotides called codons or trilpets. There are sixty-four combinations that can be created from four nucleotides. This amount is more than enough to encode twenty amino acids.
Degeneracy. Each amino acid corresponds to more than one codon, with the exception of methionine and tryptophan.
Unambiguity. One codon codes for one amino acid. For example, in the gene of a healthy person with information about the beta target of hemoglobin, the triplet of GAG and GAA encodes A in everyone who has sickle cell anemia, one nucleotide is changed.
Collinearity. The amino acid sequence always corresponds to the nucleotide sequence that the gene contains.
The genetic code is continuous and compact, which means that it does not have "punctuation marks". That is, starting at a certain codon, there is a continuous reading. For example, AUGGUGTSUUAAAUGUG will be read as: AUG, GUG, CUU, AAU, GUG. But not AUG, UGG, and so on, or in any other way.
Versatility. It is the same for absolutely all terrestrial organisms, from humans to fish, fungi and bacteria.

Table

Not all available amino acids are present in the presented table. Hydroxyproline, hydroxylysine, phosphoserine, iodo derivatives of tyrosine, cystine, and some others are absent, since they are derivatives of other amino acids encoded by mRNA and formed after protein modification as a result of translation.
From the properties of the genetic code, it is known that one codon is able to code for one amino acid. The exception is the performer additional functions and coding for valine and methionine, the genetic code. RNA, being at the beginning with a codon, attaches a t-RNA that carries formyl methion. Upon completion of the synthesis, it splits off itself and takes the formyl residue with it, transforming into a methionine residue. Thus, the above codons are the initiators of the synthesis of a chain of polypeptides. If they are not at the beginning, then they are no different from others.

genetic information

This concept means a program of properties that is transmitted from ancestors. It is embedded in heredity as a genetic code.
Implemented during protein synthesis genetic code:

• information and RNA;
• ribosomal rRNA.

Information is transmitted by direct communication (DNA-RNA-protein) and reverse (environment-protein-DNA).
Organisms can receive, store, transfer it and use it most effectively.
Being inherited, information determines the development of an organism. But due to interaction with the environment, the reaction of the latter is distorted, due to which evolution and development take place. Thus, new information is laid in the body.

The calculation of the laws of molecular biology and the discovery of the genetic code illustrated the need to combine genetics with Darwin's theory, on the basis of which a synthetic theory of evolution emerged - non-classical biology.
Heredity, variability and Darwin's natural selection are complemented by genetically determined selection. Evolution is realized at the genetic level through random mutations and inheritance of the most valuable traits that are most adapted to the environment.

Deciphering the human code

In the nineties, the Human Genome Project was launched, as a result of which, in the 2000s, fragments of the genome containing 99.99% of human genes were discovered. Fragments that are not involved in protein synthesis and are not encoded remained unknown. Their role is still unknown.

Chromosome 1, last discovered in 2006, is the longest in the genome. More than three hundred and fifty diseases, including cancer, appear as a result of disorders and mutations in it.

The role of such research can hardly be overestimated. When they discovered what the genetic code is, it became known what patterns development occurs, how the morphological structure, the psyche, predisposition to certain diseases, metabolism and vices of individuals are formed.

Chemical composition and structural organization of the DNA molecule.

Nucleic acid molecules are very long chains consisting of many hundreds and even millions of nucleotides. Any nucleic acid contains only four types of nucleotides. The functions of nucleic acid molecules depend on their structure, their constituent nucleotides, their number in the chain, and the sequence of the compound in the molecule.

Each nucleotide is made up of three components: a nitrogenous base, a carbohydrate, and phosphoric acid. AT composition each nucleotide DNA one of the four types of nitrogenous bases (adenine - A, thymine - T, guanine - G or cytosine - C) is included, as well as a deoxyribose carbon and a phosphoric acid residue.

Thus, DNA nucleotides differ only in the type of nitrogenous base.
The DNA molecule consists of a huge number of nucleotides connected in a chain in a certain sequence. Each type of DNA molecule has its own number and sequence of nucleotides.

DNA molecules are very long. For example, to write down the sequence of nucleotides in DNA molecules from one human cell (46 chromosomes), one would need a book of about 820,000 pages. The alternation of four types of nucleotides can form an infinite number of variants of DNA molecules. These features of the structure of DNA molecules allow them to store a huge amount of information about all the signs of organisms.

In 1953, the American biologist J. Watson and the English physicist F. Crick created a model for the structure of the DNA molecule. Scientists have found that each DNA molecule consists of two strands interconnected and spirally twisted. It looks like a double helix. In each chain, four types of nucleotides alternate in a specific sequence.

Nucleotide DNA composition differs from different types bacteria, fungi, plants, animals. But it does not change with age, it depends little on changes. environment. Nucleotides are paired, that is, the number of adenine nucleotides in any DNA molecule is equal to the number of thymidine nucleotides (A-T), and the number of cytosine nucleotides is equal to the number of guanine nucleotides (C-G). This is due to the fact that the connection of two chains to each other in a DNA molecule obeys a certain rule, namely: adenine of one chain is always connected by two hydrogen bonds only with Thymine of the other chain, and guanine by three hydrogen bonds with cytosine, that is, the nucleotide chains of one molecule DNA is complementary, complement each other.

Nucleic acid molecules - DNA and RNA are made up of nucleotides. The composition of DNA nucleotides includes a nitrogenous base (A, T, G, C), a deoxyribose carbohydrate and a residue of a phosphoric acid molecule. The DNA molecule is a double helix, consisting of two strands connected by hydrogen bonds according to the principle of complementarity. The function of DNA is to store hereditary information.

Properties and functions of DNA.

DNA is a carrier of genetic information, written in the form of a sequence of nucleotides using the genetic code. DNA molecules are associated with two fundamental properties of living organisms - heredity and variability. During a process called DNA replication, two copies of the original chain are formed, which are inherited by daughter cells when they divide, so that the resulting cells are genetically identical to the original.

Genetic information is realized during gene expression in the processes of transcription (synthesis of RNA molecules on a DNA template) and translation (synthesis of proteins on an RNA template).

The nucleotide sequence "encodes" information about various types RNA: information, or matrix (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized from DNA during the transcription process. Their role in protein biosynthesis (translation process) is different. Messenger RNA contains information about the sequence of amino acids in a protein, ribosomal RNA serves as the basis for ribosomes (complex nucleoprotein complexes, the main function of which is to assemble a protein from individual amino acids based on mRNA), transfer RNA deliver amino acids to the protein assembly site - to the active center of the ribosome, " creeping" along the mRNA.

Genetic code, its properties.

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

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. 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).
4. 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)
5. Degeneracy (redundancy) Several codons can correspond to the same amino acid.
6. Versatility- the genetic code works the same way in organisms different levels complexity - from viruses to humans (genetic engineering methods are based on this; there are a number of exceptions, shown in the table in the section "Variations of the standard genetic code" below).
7. 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.

5. DNA autoreproduction. Replicon and its functioning .

The process of self-reproduction of nucleic acid molecules, accompanied by the transmission by inheritance (from cell to cell) of exact copies of genetic information; R. carried out with the participation of a set of specific enzymes (helicase<helicase>, which controls the unwinding of the molecule DNA, DNA-polymerase<DNA polymerase> I and III, DNA-ligase<DNA ligase>), passes through a semi-conservative type with the formation of a replication fork<replication fork>; on one of the chains<leading strand> the synthesis of the complementary chain is continuous, and on the other<lagging strand> occurs due to the formation of Dkazaki fragments<Okazaki fragments>; R. - high-precision process, the error rate in which does not exceed 10 -9 ; in eukaryotes R. can occur at several points on the same molecule at once DNA; speed R. eukaryotes have about 100, and bacteria have about 1000 nucleotides per second.

6. Levels of organization of the eukaryotic genome .

In eukaryotic organisms, the transcriptional regulation mechanism is much more complex. As a result of cloning and sequencing of eukaryotic genes, specific sequences involved in transcription and translation have been found.
A eukaryotic cell is characterized by:
1. The presence of introns and exons in the DNA molecule.
2. Maturation of i-RNA - excision of introns and stitching of exons.
3. The presence of regulatory elements that regulate transcription, such as: a) promoters - 3 types, each of which sits a specific polymerase. Pol I replicates ribosomal genes, Pol II replicates protein structural genes, Pol III replicates genes encoding small RNAs. The Pol I and Pol II promoters are upstream of the transcription initiation site, the Pol III promoter is within the framework of the structural gene; b) modulators - DNA sequences that enhance the level of transcription; c) enhancers - sequences that enhance the level of transcription and act regardless of their position relative to the coding part of the gene and the state of the starting point of RNA synthesis; d) terminators - specific sequences that stop both translation and transcription.
These sequences differ from prokaryotic sequences in their primary structure and location relative to the initiation codon, and bacterial RNA polymerase does not "recognize" them. Thus, for the expression of eukaryotic genes in prokaryotic cells, the genes must be under the control of prokaryotic regulatory elements. This circumstance must be taken into account when constructing vectors for expression.

7. Chemical and structural composition of chromosomes .

Chemical chromosome composition - DNA - 40%, Histone proteins - 40%. Non-histone - 20% a little RNA. Lipids, polysaccharides, metal ions.

The chemical composition of a chromosome is a complex of nucleic acids with proteins, carbohydrates, lipids and metals. The regulation of gene activity and their restoration in case of chemical or radiation damage occurs in the chromosome.

STRUCTURAL????

Chromosomes- nucleoprotein structural Elements of the cell nucleus, containing DNA, which contains the hereditary Information of the organism, are capable of self-reproduction, have a structural and functional individuality and retain it in a number of generations.

in the mitotic cycle, the following features of the structural organization of chromosomes are observed:

There are mitotic and interphase forms of the structural organization of chromosomes, mutually passing into each other in the mitotic cycle - these are functional and physiological transformations

8. Packing levels of hereditary material in eukaryotes .

Structural and functional levels of organization of the hereditary material of eukaryotes

Heredity and variability provide:

1) individual (discrete) inheritance and changes in individual characteristics;

2) reproduction in individuals of each generation of the entire complex of morphological and functional characteristics of organisms of a particular biological species;

3) redistribution in species with sexual reproduction in the process of reproduction of hereditary inclinations, as a result of which the offspring has a combination of characters that is different from their combination in the parents. Patterns of inheritance and variability of traits and their combinations follow from the principles of the structural and functional organization of genetic material.

There are three levels of organization of the hereditary material of eukaryotic organisms: gene, chromosomal and genomic (genotype level).

The elementary structure of the gene level is the gene. The transfer of genes from parents to offspring is necessary for the development of certain traits in him. Although several forms of biological variability are known, only a violation of the structure of genes changes the meaning of hereditary information, in accordance with which specific traits and properties are formed. Due to the presence of the gene level, individual, separate (discrete) and independent inheritance and changes in individual traits are possible.

The genes of eukaryotic cells are distributed in groups along the chromosomes. These are the structures of the cell nucleus, which are characterized by individuality and the ability to reproduce themselves with the preservation of individual structural features in a number of generations. The presence of chromosomes determines the allocation of the chromosomal level of organization of hereditary material. The placement of genes in chromosomes affects the relative inheritance of traits, makes it possible to influence the function of a gene from its immediate genetic environment - neighboring genes. The chromosomal organization of hereditary material serves necessary condition redistribution of the hereditary inclinations of parents in the offspring during sexual reproduction.

Despite the distribution over different chromosomes, the entire set of genes functionally behaves as a whole, forming a single system representing the genomic (genotypic) level of organization of hereditary material. At this level, there is a wide interaction and mutual influence of hereditary inclinations, localized both in one and in different chromosomes. The result is the mutual correspondence of the genetic information of different hereditary inclinations and, consequently, the development of traits balanced in time, place and intensity in the process of ontogenesis. The functional activity of genes, the mode of replication and mutational changes in the hereditary material also depend on the characteristics of the genotype of the organism or the cell as a whole. This is evidenced, for example, by the relativity of the property of dominance.

Eu - and heterochromatin.

Some chromosomes appear condensed and intensely colored during cell division. Such differences were called heteropyknosis. The term " heterochromatin". There are euchromatin - the main part of the mitotic chromosomes, which undergoes the usual cycle of compaction decompactization during mitosis, and heterochromatin- regions of chromosomes that are constantly in a compact state.

In most eukaryotic species, the chromosomes contain both eu- and heterochromatic regions, the latter being a significant part of the genome. Heterochromatin located in the centromeric, sometimes in the telomeric regions. Heterochromatic regions were found in the euchromatic arms of chromosomes. They look like intercalations (intercalations) of heterochromatin into euchromatin. Such heterochromatin called intercalary. Compaction of chromatin. Euchromatin and heterochromatin differ in compactization cycles. Euhr. passes full cycle compactization-decompactization from interphase to interphase, hetero. maintains a state of relative compactness. Differential staining. Different sections of heterochromatin are stained with different dyes, some areas - with some one, others - with several. Using various stains and using chromosomal rearrangements that break heterochromatin regions, many small regions in Drosophila have been characterized where the affinity for color is different from neighboring regions.

10. Morphological features metaphase chromosome .

The metaphase chromosome consists of two longitudinal strands of deoxyribonucleoprotein - chromatids, connected to each other in the region of the primary constriction - the centromere. Centromere - in a special way organized site chromosome that is common to both sister chromatids. The centromere divides the body of the chromosome into two arms. Depending on the location of the primary constriction, the following types of chromosomes are distinguished: equal-arm (metacentric), when the centromere is located in the middle, and the arms are approximately equal in length; unequal arms (submetacentric), when the centromere is displaced from the middle of the chromosome, and the arms are of unequal length; rod-shaped (acrocentric), when the centromere is shifted to one end of the chromosome and one arm is very short. There are also point (telocentric) chromosomes, they do not have one arm, but they are not in the human karyotype (chromosomal set). In some chromosomes, there may be secondary constrictions that separate a region called the satellite from the body of the chromosome.

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 collection 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 hand, “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. At various kinds 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 vary from organism to organism.