Gene code. Basic properties of the genetic code and their significance

The genetic code is a special encryption of hereditary information using 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 shape 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 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 basic amino acids. With transcript genetic code difficulties arose because there are much fewer nucleotides than amino acids. In addressing this problem, it was proposed that amino acids are encoded by different combinations of three nucleotides (called a codon or triplet).

In addition, it was necessary to explain exactly how 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 alternating “meaningless” sections, i.e. so-called “commas” and “paragraphs” are formed in the code;

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

Currently, the theory of code continuity is mainly used.

Genetic code and its properties

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

2) The genetic code is redundant - its triplets. One amino acid can be encoded by several codons, since, according to mathematical calculations, there are three times more codons than amino acids. Some codons perform specific 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 - each codon can correspond to only one amino acid.

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

5) The code is written continuously and compactly; there are no “meaningless” nucleotides in it. It begins with a specific triplet, which is replaced by the next one without a break and ends with a stop 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 during the generation of a new organism from bone matter. Random changes and evolutionary processes make any code variants possible, i.e. amino acids can be rearranged in any order. Why did this particular type of code survive during 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.

Genetic functions of DNA are that it ensures the storage, transmission and implementation of hereditary information, which is 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 is equated in importance to the discovery of 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, it was given full characteristics genetic code.

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. They usually have the same bases 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".

Ministry of Education and Science Russian Federation Federal agency by education

State educational institution higher vocational education"Altai State technical university them. I.I. Polzunov"

Department of Natural Sciences and System Analysis

Abstract on the topic "Genetic code"

1. The concept of genetic code

3. Genetic information

References

1. The concept of genetic code

Genetic code - characteristic of living organisms unified system recording hereditary information in nucleic acid molecules in the form of a sequence of nucleotides. Each nucleotide is designated by a capital letter, which begins the name of the nitrogenous base included in its composition: - A (A) adenine; - G (G) guanine; - C (C) cytosine; - T (T) thymine (in DNA) or U (U) uracil (in mRNA).

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

The first of them occurs in the core; it consists in the synthesis of mRNA molecules at the corresponding sections of DNA. In this case, the DNA nucleotide sequence is “rewritten” into the RNA nucleotide sequence. The second stage takes place in the cytoplasm, on ribosomes; in this case, the sequence of nucleotides of the mRNA is translated into the sequence of amino acids in the protein: this stage occurs with the participation of transfer RNA (tRNA) and the corresponding enzymes.

2. Properties of the genetic code

1. Triplety

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

A triplet or codon is a sequence of three nucleotides encoding one amino acid.

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

2. Degeneracy.

All amino acids, with the exception of methionine and tryptophan, are encoded by more than one triplet: 2 amino acids of 1 triplet = 2 9 amino acids of 2 triplets = 18 1 amino acid 3 triplets = 3 5 amino acids of 4 triplets = 20 3 amino acids of 6 triplets = 18 Total 61 triplets encode 20 amino acids.

3. Presence of intergenic punctuation marks.

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

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

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

Conventionally, the AUG codon, the first after the leader sequence, also belongs to punctuation marks. It functions as a capital letter. In this position it encodes formylmethionine (in prokaryotes).

4. Unambiguity.

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

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

5. Compactness, or absence of intragenic punctuation marks.

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

In 1961 Seymour Benzer and Francis Crick experimentally proved the triplet nature of the code and its compactness.

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

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

3. Genetic information

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

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

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

The implementation of genetic information occurs in the process of synthesis of protein molecules using three RNAs: messenger RNA (mRNA), transport RNA (tRNA) and ribosomal RNA (rRNA). The process of information transfer occurs: - through a direct communication channel: DNA - RNA - protein; and - through the feedback channel: environment - protein - DNA.

Living organisms are capable of receiving, storing and transmitting information. Moreover, living organisms have an inherent desire to use the information received about themselves and the world around them as efficiently as possible. Hereditary information embedded in genes and necessary for a living organism to exist, develop and reproduce is transmitted from each individual to his descendants. This information determines the direction of development of the organism, and in the process of its interaction with the environment, the reaction to its individual can be distorted, thereby ensuring the evolution of the development of descendants. In the process of evolution of a living organism, it arises and is remembered. new information, including for him the value of information increases.

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

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

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

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

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

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

4. Decoding the human genetic code

In May 2006, scientists working to decipher the human genome published a complete genetic map of chromosome 1, which was the last human chromosome not fully sequenced.

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

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 these are adenine (A), uracil (U), guanine (G), cytosine (C). By combining nitrogenous bases (in this case, the nucleotides containing them) in different ways, you can get many different triplets: AAA, GAU, UCC, GCA, AUC, etc. Total quantity possible combinations - 64, i.e. 43.

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. 42.

Thus, the main property of the genetic code is its triplicity. Each amino acid is encoded by a triple 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 the same amino acid. Considering that mutations are often harmful, the property of noise immunity is useful.

The genetic, or biological, code is one of the universal properties of living nature, proving the unity of its origin. Genetic code is a method of encoding the sequence of amino acids of a polypeptide using a sequence of nucleic acid nucleotides (messenger RNA or a complementary DNA section on which mRNA is synthesized).

There are other definitions.

Genetic code- this is the correspondence of each amino acid (part of living proteins) to a specific sequence of three nucleotides. Genetic code is the relationship between nucleic acid bases and protein amino acids.

IN scientific literature The genetic code does not mean the sequence of nucleotides in the DNA of an organism that determines its individuality.

It is incorrect to assume that one organism or species has one code, and another has another. The genetic code is how amino acids are encoded by nucleotides (i.e. principle, mechanism); it is universal for all living things, the same for all organisms.

Therefore, it is incorrect to say, for example, “The genetic code of a person” or “The genetic code of an organism,” which is often used in pseudo-scientific literature and films.

In these cases, we usually mean the genome of a person, an organism, etc.

The diversity of living organisms and the characteristics of their life activity is primarily due to the diversity of proteins.

The specific structure of a protein is determined by the order and quantity of the various amino acids that make up its composition. The amino acid sequence of the peptide is encoded in DNA using a biological code. From the point of view of the diversity of the set of monomers, DNA is a more primitive molecule than a peptide. DNA is various options alternating only four nucleotides. This for a long time prevented researchers from considering DNA as a material of heredity.

How are amino acids coded by nucleotides?

1) Nucleic acids(DNA and RNA) are polymers made up of nucleotides.

Each nucleotide can contain one of four nitrogenous bases: adenine (A, en: A), guanine (G, G), cytosine (C, en: C), thymine (T, en: T). In the case of RNA, thymine is replaced by uracil (U, U).

When considering the genetic code, only nitrogenous bases are taken into account.

Then the DNA chain can be represented as their linear sequence. For example:

Complimentary this code the mRNA section will be like this:

2) Proteins (polypeptides) are polymers consisting of amino acids.

In living organisms, 20 amino acids are used to build polypeptides (a few more are very rare). To designate them, you can also use one letter (although more often they use three - an abbreviation for the name of the amino acid).

The amino acids in a polypeptide are also connected linearly by a peptide bond. For example, suppose there is a section of a protein with the following sequence of amino acids (each amino acid is designated by one letter):

3) If the task is to encode each amino acid using nucleotides, then it comes down to how to encode 20 letters using 4 letters.

This can be done by matching letters of a 20-letter alphabet with words made up of several letters of a 4-letter alphabet.

If one amino acid is encoded by one nucleotide, then only four amino acids can be encoded.

If each amino acid is associated with two consecutive nucleotides in the RNA chain, then sixteen amino acids can be encoded.

Indeed, if there are four letters (A, U, G, C), then the number of their different pair combinations will be 16: (AU, UA), (AG, GA), (AC, CA), (UG, GU), ( UC, CU), (GC, CG), (AA, UU, GG, CC).

[Brackets are used for ease of perception.] This means that only 16 different amino acids can be encoded with such a code (a two-letter word): each will have its own word (two consecutive nucleotides).

From mathematics, the formula to determine the number of combinations looks like this: ab = n.

Here n is the number of different combinations, a is the number of letters of the alphabet (or the base of the number system), b is the number of letters in the word (or digits in the number). If we substitute the 4-letter alphabet and words consisting of two letters into this formula, we get 42 = 16.

If three consecutive nucleotides are used as the code word for each amino acid, then 43 = 64 different amino acids can be encoded, since 64 different combinations can be composed of four letters taken in groups of three (for example, AUG, GAA, CAU, GGU, etc.)

d.). This is already more than enough to encode 20 amino acids.

Exactly three letter code used in genetic code. Three consecutive nucleotides coding for one amino acid are called triplet(or codon).

Each amino acid is associated with a specific triplet of nucleotides.

In addition, since the combinations of triplets overlap the number of amino acids in excess, many amino acids are encoded by several triplets.

Three triplets do not code for any of the amino acids (UAA, UAG, UGA).

They mark the end of the broadcast and are called stop codons(or nonsense codons).

The AUG triplet encodes not only the amino acid methionine, but also initiates translation (plays the role of a start codon).

Below are tables of amino acid correspondence to nucleoitide triplets.

Using the first table, it is convenient to determine the corresponding amino acid from a given triplet. For the second - for a given amino acid, the triplets corresponding to it.

Let's consider an example of the implementation of a genetic code. Let there be an mRNA with the following content:

Let's split the nucleotide sequence into triplets:

Let us associate each triplet with the amino acid of the polypeptide it encodes:

Methionine - Aspartic acid - Serine - Threonine - Tryptophan - Leucine - Leucine - Lysine - Asparagine - Glutamine

The last triplet is a stop codon.

Properties of the genetic code

The properties of the genetic code are largely a consequence of the way amino acids are encoded.

The first and obvious property is triplicity.

It refers to the fact that the unit of code is a sequence of three nucleotides.

An important property of the genetic code is its non-overlapping. A nucleotide included in one triplet cannot be included in another.

That is, the sequence AGUGAA can only be read as AGU-GAA, but not, for example, like this: AGU-GUG-GAA. That is, if a GU pair is included in one triplet, it cannot already be integral part another.

Under unambiguity The genetic code understands that each triplet corresponds to only one amino acid.

For example, the AGU triplet codes for the amino acid serine and nothing else.

Genetic code

This triplet uniquely corresponds to only one amino acid.

On the other hand, several triplets can correspond to one amino acid. For example, the same serine, in addition to AGU, corresponds to the AGC codon. This property called degeneracy genetic code.

Degeneracy allows many mutations to remain harmless, since often replacing one nucleotide in DNA does not lead to a change in the value of the triplet. If you look closely at the table of amino acid correspondence to triplets, you can see that if an amino acid is encoded by several triplets, they often differ in the last nucleotide, i.e. it can be anything.

Some other properties of the genetic code are also noted (continuity, noise immunity, universality, etc.).

Resilience as the adaptation of plants to living conditions. Basic reactions of plants to the action of unfavorable factors.

Plant resistance is the ability to withstand the effects of extreme environmental factors (soil and air drought).

The uniqueness of the genetic code is manifested in the fact that

This property was developed during the process of evolution and was genetically fixed. In areas with unfavorable conditions stable decorative forms and local varieties of cultivated plants – drought-resistant – were formed. A particular level of resistance inherent in plants is revealed only under the influence of extreme environmental factors.

As a result of the onset of such a factor, the irritation phase begins - a sharp deviation from the norm of a number of physiological parameters and their rapid return to normal. Then there is a change in metabolic rate and damage to intracellular structures. At the same time, all synthetic ones are suppressed, all hydrolytic ones are activated, and the overall energy supply of the body decreases. If the effect of the factor does not exceed the threshold value, the adaptation phase begins.

An adapted plant reacts less to repeated or increasing exposure to an extreme factor. At the organismal level, interaction between organs is added to the adaptation mechanisms. Weakening the movement of water flows, mineral and organic compounds exacerbates competition between organs, their growth stops.

Biostability in plants defined. the maximum value of the extreme factor at which plants still form viable seeds. Agronomic stability is determined by the degree of yield reduction. Plants are characterized by their resistance to a specific type of extreme factor - wintering, gas-resistant, salt-resistant, drought-resistant.

Type roundworms, unlike flat ones, they have a primary body cavity - a schizocoel, formed due to the destruction of the parenchyma that fills the gaps between the body wall and internal organs– its function is transport.

It maintains homeostasis. The body shape is round in diameter. The integument is cuticulated. The muscles are represented by a layer of longitudinal muscles. The intestine is through and consists of 3 sections: anterior, middle and posterior. The mouth opening is located on the ventral surface of the anterior end of the body. The pharynx has a characteristic triangular lumen. Excretory system represented by protonephridia or special skin - hypodermal glands. Most species are dioecious and reproduce only sexually.

Development is direct, less often with metamorphosis. They have a constant cellular composition of the body and lack the ability to regenerate. The anterior intestine consists of the oral cavity, pharynx, and esophagus.

They do not have a middle or posterior section. The excretory system consists of 1-2 giant cells of the hypodermis. Longitudinal excretory canals lie in the lateral ridges of the hypodermis.

Properties of the genetic code. Evidence of triplet code. Decoding codons. Stop codons. The concept of genetic suppression.

The idea that a gene encodes information in the primary structure of a protein was concretized by F.

Crick in his sequence hypothesis, according to which the sequence of gene elements determines the sequence of amino acid residues in the polypeptide chain. The validity of the sequence hypothesis is proven by the colinearity of the structures of the gene and the polypeptide it encodes. The most significant development in 1953 was the idea that. That the code is most likely triplet.

; DNA base pairs: A-T, T-A, G-C, C-G - can only encode 4 amino acids if each pair corresponds to one amino acid. As you know, proteins contain 20 basic amino acids. If we assume that each amino acid has 2 base pairs, then 16 amino acids (4*4) can be encoded - this is again not enough.

If the code is triplet, then 64 codons (4*4*4) can be made from 4 base pairs, which is more than enough to encode 20 amino acids. Crick and his colleagues assumed that the code was triplet; there were no “commas” between the codons, i.e., separating marks; The code within a gene is read from a fixed point in one direction. In the summer of 1961, Kirenberg and Mattei reported the decoding of the first codon and suggested a method for establishing the composition of codons in a cell-free protein synthesis system.

Thus, the codon for phenylalanine was transcribed as UUU in mRNA. Further, as a result of the application of methods developed by Korana, Nirenberg and Leder in 1965.

a code dictionary was compiled in his modern form. Thus, the occurrence of mutations in T4 phages caused by the loss or addition of bases was evidence of the triplet nature of the code (property 1). These deletions and additions, leading to frame shifts when “reading” the code, were eliminated only by restoring the correctness of the code; this prevented the appearance of mutants. These experiments also showed that triplets do not overlap, that is, each base can belong to only one triplet (property 2).

Most amino acids have several codons. Code in which the number of amino acids less number Codons are called degenerate (property 3), i.e.

e. a given amino acid can be encoded by more than one triplet. In addition, three codons do not code for any amino acid at all (“nonsense codons”) and act as a “stop signal.” A stop codon is the end point of a functional unit of DNA, the cistron. Stop codons are the same in all species and are represented as UAA, UAG, UGA. A notable feature of the code is that it is universal (property 4).

In all living organisms, the same triplets code for the same amino acids.

The existence of three types of mutant codon terminators and their suppression have been demonstrated in E. coli and yeast. Detection of suppressor genes that “interpret” nonsense alleles different genes, indicates that the translation of the genetic code may change.

Mutations affecting the anticodon of tRNAs change their codon specificity and create the possibility of suppression of mutations at the translational level. Suppression at the translational level can occur due to mutations in the genes encoding certain ribosomal proteins. As a result of these mutations, the ribosome “makes mistakes,” for example, in reading nonsense codons and “interprets” them using some non-mutant tRNAs. Along with genotypic suppression acting at the translation level, phenotypic suppression of nonsense alleles is also possible: when the temperature decreases, when cells are exposed to aminoglycoside antibiotics that bind to ribosomes, for example streptomycin.

22. Reproduction higher plants: vegetative and asexual. Sporulation, spore structure, equal and heterosporous. Reproduction as a property of living matter, i.e. the ability of an individual to give rise to its own kind, existed in the early stages of evolution.

Forms of reproduction can be divided into 2 types: asexual and sexual. Asexual reproduction itself is carried out without the participation of germ cells, with the help of specialized cells - spores. They are formed in the organs of asexual reproduction - sporangia as a result of mitotic division.

During its germination, the spore reproduces a new individual, similar to the mother, with the exception of spores of seed plants, in which the spore has lost the function of reproduction and dispersal. Spores can also be formed by reduction division, with single-celled spores spilling out.

Reproduction of plants using vegetative (part of a shoot, leaf, root) or division of unicellular algae in half is called vegetative (bulb, cuttings).

Sexual reproduction is carried out by special sex cells - gametes.

Gametes are formed as a result of meiosis, there are female and male. As a result of their fusion, a zygote appears, from which a new organism subsequently develops.

Plants differ in the types of gametes. In some unicellular organisms it functions as a gamete at certain times. Organisms of different sexes (gametes) merge - this sexual process is called hologamia. If male and female gametes are morphologically similar and mobile, these are isogametes.

And the sexual process - isogamous. If female gametes are somewhat larger and less mobile than male ones, then these are heterogametes, and the process is heterogamy. Oogamy - female gametes are very large and immobile, male gametes are small and mobile.

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Genetic code - correspondence between DNA triplets and protein amino acids

The need to encode the structure of proteins in the linear sequence of nucleotides of mRNA and DNA is dictated by the fact that during translation:

• there is no correspondence between the number of monomers in the mRNA matrix and the product - the synthesized protein;
• there is no structural similarity between RNA and protein monomers.

This eliminates the complementary interaction between the matrix and the product - the principle by which the construction of new DNA and RNA molecules is carried out during replication and transcription.

From this it becomes clear that there must be a “dictionary” that allows one to find out which sequence of mRNA nucleotides ensures the inclusion of amino acids in a protein in a given sequence. This “dictionary” is called the genetic, biological, nucleotide, or amino acid code. It allows you to encrypt the amino acids that make up proteins using a specific sequence of nucleotides in DNA and mRNA. It is characterized by certain properties.

Tripletity. One of the main questions in determining the properties of the code was the question of the number of nucleotides, which should determine the inclusion of one amino acid in the protein.

It was found that the coding elements in the encryption of an amino acid sequence are indeed triples of nucleotides, or triplets, which were named "codons".

The meaning of codons.

It was possible to establish that out of 64 codons, the inclusion of amino acids in the synthesized polypeptide chain encodes 61 triplets, and the remaining 3 - UAA, UAG, UGA - do not encode the inclusion of amino acids in the protein and were originally called meaningless, or non-sense codons. However, it was later shown that these triplets signal the completion of translation, and therefore they came to be called termination or stop codons.

The codons of mRNA and triplets of nucleotides in the coding strand of DNA with the direction from the 5′ to the 3′ end have the same sequence of nitrogenous bases, except that in DNA instead of uracil (U), characteristic of mRNA, there is thymine (T).

Specificity.

Each codon corresponds to only one specific amino acid. In this sense, the genetic code is strictly unambiguous.

Table 4-3.

Unambiguousness is one of the properties of the genetic code, manifested in the fact that...

Main components of the protein synthesizing system

Required Components	Functions
1. Amino acids	Substrates for protein synthesis
2. tRNA	tRNAs act as adapters. Their acceptor end interacts with amino acids, and their anticodon interacts with the codon of the mRNA.
3. Aminoacyl-tRNA synthetase	Each aa-tRNA synthetase catalyzes the specific binding of one of 20 amino acids to the corresponding tRNA
4.mRNA	The matrix contains a linear sequence of codons that determine the primary structure of proteins
5. Ribosomes	Ribonucleoprotein subcellular structures that are the site of protein synthesis
6.	Energy sources
7. Protein factors of initiation, elongation, termination	Specific extraribosomal proteins required for the translation process (12 initiation factors: elF; 2 elongation factors: eEFl, eEF2, and termination factors: eRF)
8. Magnesium ions	Cofactor that stabilizes ribosome structure

Notes: elF( eukaryotic initiation factors) — initiation factors; eEF ( eukaryotic elongation factors) — elongation factors; eRF ( eukaryotic releasing factors) are termination factors.

Degeneracy. There are 61 triplets in mRNA and DNA, each of which encodes the inclusion of one of 20 amino acids in the protein.

It follows from this that in information molecules the inclusion of the same amino acid in a protein is determined by several codons. This property of the biological code is called degeneracy.

In humans, only 2 amino acids are encoded with one codon - Met and Tri, while Leu, Ser and Apr - with six codons, and Ala, Val, Gly, Pro, Tre - with four codons (Table

Redundancy of coding sequences is the most valuable property of a code, since it increases the resistance of the information flow to the adverse effects of external and internal environment. When determining the nature of the amino acid to be included in a protein, the third nucleotide in a codon is not as important as the first two. As can be seen from table. 4-4, for many amino acids, replacing a nucleotide in the third position of a codon does not affect its meaning.

Linearity of information recording.

During translation, mRNA codons are “read” from a fixed starting point sequentially and do not overlap. The information record does not contain signals indicating the end of one codon and the beginning of the next. The AUG codon is the initiation codon and is read both at the beginning and in other parts of the mRNA as Met. The triplets following it are read sequentially without any gaps until the stop codon, at which the synthesis of the polypeptide chain is completed.

Versatility.

Until recently, it was believed that the code was absolutely universal, i.e. the meaning of code words is the same for all studied organisms: viruses, bacteria, plants, amphibians, mammals, including humans.

However, one exception later became known; it turned out that mitochondrial mRNA contains 4 triplets that have a different meaning than in nuclear-origin mRNA. Thus, in mitochondrial mRNA, the triplet UGA encodes Tri, AUA encodes Met, and ACA and AGG are read as additional stop codons.

Colinearity of gene and product.

In prokaryotes, a linear correspondence between the sequence of gene codons and the amino acid sequence in protein product, or as they say, there is colinearity between the gene and the product.

Table 4-4.

Genetic code

First base	Second base
	U	WITH	A	G
U	UUU Hairdryer	UCU Cep	UAU Shooting Range	UGU Cis
UUC Hairdryer	UCC Ser	iASTir	UGC Cis
UUA Lei	UCA Cep	UAA*	UGA*
UUG Lei	UCG Ser	UAG*	UGG April
WITH	CUU Lei	CCU Pro	CAU Gis	CGU April
CUC Lei	SSS Pro	SAS Gis	CGC April
CUA Lei	SSA Pro	SAA Gln	CGA April
CUG Lei	CCG Pro	CAG Gln	CGG April
A	AUU Ile	ACU Tpe	AAU Asn	AGU Ser
AUC Ile	ACC Tre	AAS Asn	AGG Gray
AUA Meth	ASA Tre	AAA Liz	AGA April
AUG Met	ACG Tre	AAG Liz	AGG April
G	GUU Ban	GCU Ala	GAU Asp	GGU Gli
GUC Val	GCC Ala	GAC Asp	GGC Gli
GUA Val	GSA Ala	GAA Glu	GGA Gli
GUG Val	GСG Ala	GAG Glu	GGG Glee

Notes: U - uracil; C - cytosine; A - adenine; G - guanine; *—termination codon.

In eukaryotes, base sequences in a gene that are colinear with the amino acid sequence in the protein are interrupted by nitrones.

Therefore, in eukaryotic cells, the amino acid sequence of a protein is colinear with the sequence of exons in a gene or mature mRNA after post-transcriptional removal of introns.

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

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

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

The DNA nucleotide sequence contains code “words” for each amino acid of the future protein molecule - the genetic code. It consists in a certain sequence of arrangement of nucleotides in a DNA molecule.

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

Currently, the DNA code has been completely deciphered, and we can talk about certain properties characteristic of this unique biological system, which ensures the translation of information from the “language” of DNA into the “language” of protein.

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

Amino acid	RNA coding triplets
Alanin	GCU GCC GCA GCH
Arginine	TsGU TsGTs TsGA TsGG AGA AGG
Asparagine	AAU AAC
Aspartic acid	GAU GAC
Valin	GUU GUTS GUA GUG
Histidine	TsAU TsATs
Glycine	GGU GGC GGA YYY
Glutamine	CAA CAG
Glutamic acid	GAA GAG
Isoleucine	AUU AUC AUA
Leucine	TSUU TSUTS TSUA TSUG UUA UUG
Lysine	AAA AAG
Methionine	AUG
Proline	TsTsU TsTs TsTsTsG
Serin	UCU UCC UCA UCG ASU AGC
Tyrosine	UAU UAC
Threonine	ACU ACC ACA ACG
Tryptophan	UGG
Phenylalanine	UUU UUC
Cysteine	UGU UGC
STOP	UGA UAG UAA

Properties of the genetic code

Three consecutive nucleotides (nitrogen bases) encode the “name” of one amino acid, that is, each of the 20 amino acids is encrypted with a significant code unit - a combination of three nucleotides called triplet or codon

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

• Uniqueness (discreteness)

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

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

• Non-overlapping

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

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

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

From the history of the discovery of the genetic code

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

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

To encode 20 amino acids, as well as a “stop” signal, indicating the end of the protein sequence, three consecutive nucleotides are sufficient, when the number of possible combinations is 64 (4 3 = 64).