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. IN 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 infinite set 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 in the 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 another chain, and guanine - by three hydrogen bonds with cytosine, that is, the nucleotide chains of one DNA molecule are 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 sequence of nucleotides "encodes" information about various types of RNA: information, or template (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:
- Tripletity- a significant unit of the code is a combination of three nucleotides (triplet, or codon).
- Continuity- there are no punctuation marks between the triplets, that is, the information is read continuously.
- non-overlapping- the same nucleotide cannot be part of two or more triplets at the same time (not observed for some overlapping genes of viruses, mitochondria and bacteria that encode several frameshift proteins).
- Unambiguity (specificity)- a certain codon corresponds to only one amino acid (however, the UGA codon in Euplotes crassus codes for two amino acids - cysteine and selenocysteine)
- Degeneracy (redundancy) Several codons can correspond to the same amino acid.
- Versatility- the genetic code works in the same way in organisms of different levels of complexity - from viruses to humans (genetic engineering methods are based on this; there are a number of exceptions, shown in the table in the "Variations of the standard genetic code" section below).
- Noise immunity- mutations of nucleotide substitutions that do not lead to a change in the class of the encoded amino acid are called conservative; nucleotide substitution mutations that lead to a change in the class of the encoded amino acid are called radical.
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 cell nuclei containing DNA, which contains the hereditary Information of the organism, are capable of self-reproduction, have 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:
Distinguish between mitotic and interphase forms Structural organization 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 morphofunctional characteristics 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 the hereditary material serves as a necessary condition for the redistribution of the hereditary inclinations of the 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. goes through a full cycle of 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 dyes and using chromosome rearrangements that break heterochromatic regions, many small regions in Drosophila have been characterized where the affinity for color is different from neighboring regions.
10. Morphological features of the 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 - a specially organized section of the chromosome, 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 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 same nucleotides are used, except for the nucleotide containing thymine, which is replaced by a similar nucleotide containing uracil, which is denoted by the letter ( in Russian-language literature). In DNA and RNA molecules, nucleotides line up in chains and, thus, sequences of genetic letters are obtained.
The proteins of almost all living organisms are built from only 20 types of amino acids. These amino acids are called canonical. Each protein is a chain or several chains of amino acids connected in a strictly defined sequence. This sequence determines the structure of the protein, and therefore all its biological properties.
However, in the early 1960s, new data revealed the failure of the “comma-free code” hypothesis. Then experiments showed that codons, considered by Crick to be meaningless, can provoke protein synthesis in a test tube, and by 1965 the meaning of all 64 triplets was established. It turned out that some codons are simply redundant, that is, a number of amino acids are encoded by two, four or even six triplets.
Properties
Correspondence tables of mRNA codons and amino acids
Genetic code common to most pro- and eukaryotes. The table lists all 64 codons and lists the corresponding amino acids. The base order is from the 5" to the 3" end of the mRNA.
| 1st base |
2nd base | 3rd base |
|||||||
|---|---|---|---|---|---|---|---|---|---|
| U | C | A | G | ||||||
| U | UUU | (Phe/F) Phenylalanine | UCU | (Ser/S) Serine | UAU | (Tyr/Y) Tyrosine | UGU | (Cys/C) Cysteine | U |
| UUC | UCC | UAC | UGC | C | |||||
| UUA | (Leu/L) Leucine | UCA | UAA | Stop ( Ocher) | UGA | Stop ( Opal) | A | ||
| UUG | UCG | UAG | Stop ( Amber) | UGG | (Trp/W) Tryptophan | G | |||
| C | CUU | CCU | (Pro/P) Proline | CAU | (His/H) Histidine | CGU | (Arg/R) Arginine | U | |
| CUC | CCC | CAC | CGC | C | |||||
| CUA | CCA | CAA | (Gln/Q) Glutamine | CGA | A | ||||
| CUG | CCG | CAG | CGG | G | |||||
| A | AUU | (Ile/I) Isoleucine | ACU | (Thr/T) Threonine | AAU | (Asn/N) Asparagine | AGU | (Ser/S) Serine | U |
| AUC | ACC | AAC | AGC | C | |||||
| AUA | ACA | AAA | (Lys/K) Lysine | AGA | (Arg/R) Arginine | A | |||
| AUG | (Met/M) Methionine | ACG | AAG | AGG | G | ||||
| G | GUU | (Val/V) Valine | GCU | (Ala/A) Alanine | GAU | (Asp/D) Aspartic acid | GGU | (Gly/G) Glycine | U |
| GUC | GCC | GAC | GGC | C | |||||
| GUA | GCA | GAA | (Glu/E) Glutamic acid | GGA | A | ||||
| GUG | GCG | GAG | GGG | G | |||||
| Ala/A | GCU, GCC, GCA, GCG | Leu/L | UUA, UUG, CUU, CUC, CUA, CUG |
|---|---|---|---|
| Arg/R | CGU, CGC, CGA, CGG, AGA, AGG | Lys/K | AAA, AAG |
| Asn/N | AAU, AAC | Met/M | AUG |
| Asp/D | GAU, GAC | Phe/F | UUU, UUC |
| Cys/C | UGU, UGC | Pro/P | CCU, CCC, CCA, CCG |
| Gln/Q | CAA, CAG | Ser/S | UCU, UCC, UCA, UCG, AGU, AGC |
| Glu/E | GAA, GAG | Thr/T | ACU, ACC, ACA, ACG |
| Gly/G | GGU, GGC, GGA, GGG | Trp/W | UGG |
| His/H | CAU, CAC | Tyr/Y | UAU, UAC |
| Ile/I | AUU, AUC, AUA | Val/V | GUU, GUC, GUU, GUG |
| START | AUG | STOP | UAG, UGA, UAA |
Variations on the Standard Genetic Code
The first example of a deviation from the standard genetic code was discovered in 1979 during the study of human mitochondrial genes. Since that time, several such variants have been found, including a variety of alternative mitochondrial codes, such as reading the stop codon UGA as the codon defining tryptophan in mycoplasmas. In bacteria and archaea, GUG and UUG are often used as start codons. In some cases, genes start coding for a protein at a start codon that is different from the one normally used by the species.
In some proteins, non-standard amino acids such as selenocysteine and pyrrolysine are inserted by the stop codon-reading ribosome, which depends on the sequences in the mRNA. Selenocysteine is now regarded as the 21st, and pyrrolysine the 22nd of the amino acids that make up proteins.
Despite these exceptions, the genetic code of all living organisms has common features: codons consist of three nucleotides, where the first two are defining, codons are translated by tRNA and ribosomes into a sequence of amino acids.
| Example | codon | Usual value | Reads like: |
|---|---|---|---|
| Some types of yeast of the genus Candida | CUG | Leucine | Serene |
| Mitochondria, in particular Saccharomyces cerevisiae | CU(U, C, A, G) | Leucine | Serene |
| Mitochondria of higher plants | CGG | Arginine | tryptophan |
| Mitochondria (in all studied organisms without exception) | UGA | Stop | tryptophan |
| Nuclear genome of ciliates Euplotes | UGA | Stop | Cysteine or selenocysteine |
| Mammalian mitochondria, Drosophila, S.cerevisiae and many simple | AUA | Isoleucine | Methionine = Start |
| prokaryotes | GUG | Valine | Start |
| Eukaryotes (rare) | CUG | Leucine | Start |
| Eukaryotes (rare) | GUG | Valine | Start |
| Prokaryotes (rare) | UUG | Leucine | Start |
| Eukaryotes (rare) | ACG | Threonine | Start |
| Mammalian mitochondria | AGC, AGU | Serene | Stop |
| Drosophila mitochondria | AGA | Arginine | Stop |
| Mammalian mitochondria | AG(A,G) | Arginine | Stop |
Evolution
It is believed that the triplet code was formed quite early in the course of the evolution of life. But the existence of differences in some organisms that appeared at different evolutionary stages indicates that it was not always so.
According to some models, at first the code existed in a primitive form, when a small number of codons denoted a relatively small number of amino acids. More accurate meaning of codons and more amino acids could be introduced later. At first, only the first two of the three bases could be used for recognition [which depends on the structure of the tRNA].
- Lewin b. Genes. M. : 1987. C. 62.
see also
Notes
- Sanger F. (1952). “The arrangement of amino acids in proteins”. Adv. Protein Chem. 7 : 1-67. PMID.
- Ichas M. biological code. - M.: Mir, 1971.
- Watson J. D., Crick F. H. (April 1953). “Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid”. Nature. 171 : 737-738. PMID. reference)
- Watson J. D., Crick F. H. (May 1953). “Genetical implications of the structure of deoxyribonucleic acid”. Nature. 171 : 964-967. PMID. Uses deprecated |month= parameter (help)
- Crick F. H. (April 1966). “The genetic code - yesterday, today, and tomorrow”. Cold Spring Harb. Symp. quant. Biol.: 1-9. PMID. Uses deprecated |month= parameter (help)
- Gamow G. (February 1954). “Possible relation between deoxyribonucleic acid and protein structures”. Nature. 173 : 318. DOI: 10.1038/173318a0 . PMID. Uses deprecated |month= parameter (help)
- Gamow G., Rich A., Ycas M. (1956). “The problem of information transfer from the nucleic acids to proteins”. Adv. Bio.l Med. Phys. 4 : 23-68. PMID.
- Gamow G, Ycas M. (1955). “Statistical correlation of protein and ribonucleic acid composition” . Proc. Natl. Acad. sci. U.S.A. 41 : 1011-1019. PMID.
- Crick F. H., Griffith J. S., Orgel L. E. (1957).
Ministry of Education and Science Russian Federation Federal Agency for Education
State educational institution higher vocational education"Altai State Technical University them. I.I. Polzunov"
Department of Natural Science and System Analysis
Essay on the topic "Genetic code"
1. The concept of the genetic code
3. Genetic information
Bibliography
1. The concept of the genetic code
The genetic code is a unified system for recording hereditary information in nucleic acid molecules in the form of a sequence of nucleotides, characteristic of living organisms. Each nucleotide is indicated by a capital letter, which begins the name of the nitrogenous base that is part of it: - A (A) adenine; - G (G) guanine; - C (C) cytosine; - T (T) thymine (in DNA) or U (U) uracil (in mRNA).
The implementation of the genetic code in the cell occurs in two stages: transcription and translation.
The first of these takes place in the nucleus; it consists in the synthesis of mRNA molecules on the corresponding sections of DNA. In this case, the DNA nucleotide sequence is "rewritten" into the RNA nucleotide sequence. The second stage takes place in the cytoplasm, on ribosomes; in this case, the nucleotide sequence of the i-RNA is translated into the sequence of amino acids in the protein: this stage proceeds with the participation of transfer RNA (t-RNA) and the corresponding enzymes.
2. Properties of the genetic code
1. Tripletity
Each amino acid is encoded by a sequence of 3 nucleotides.
A triplet or codon is a sequence of three nucleotides that codes for one amino acid.
The code cannot be monopleth, since 4 (the number of different nucleotides in DNA) is less than 20. The code cannot be doublet, because 16 (the number of combinations and permutations of 4 nucleotides by 2) is less than 20. The code can be triplet, because 64 (the number of combinations and permutations from 4 to 3) is greater than 20.
2. Degeneracy.
All amino acids, with the exception of methionine and tryptophan, are encoded by more than one triplet: 2 amino acids 1 triplet = 2 9 amino acids 2 triplets each = 18 1 amino acid 3 triplets = 3 5 amino acids 4 triplets each = 20 3 amino acids 6 triplets each = 18 Total 61 triplet codes for 20 amino acids.
3. The presence of intergenic punctuation marks.
A gene is a section of DNA that codes for one polypeptide chain or one molecule of tRNA, rRNA, or sRNA.
The tRNA, rRNA, and sRNA genes do not code for proteins.
At the end of each gene encoding a polypeptide, there is at least one of 3 termination codons, or stop signals: UAA, UAG, UGA. They terminate the broadcast.
Conventionally, the AUG codon also belongs to punctuation marks - the first after the leader sequence. It performs the function of a capital letter. In this position, it codes for formylmethionine (in prokaryotes).
4. Uniqueness.
Each triplet encodes only one amino acid or is a translation terminator.
The exception is the AUG codon. In prokaryotes in the first position ( capital letter) it codes for formylmethionine, and in any other it codes for methionine.
5. Compactness, or the absence of intragenic punctuation marks.
Within a gene, each nucleotide is part of a significant codon.
In 1961 Seymour Benzer and Francis Crick experimentally proved that the code is triplet and compact.
The essence of the experiment: "+" mutation - the insertion of one nucleotide. "-" mutation - loss of one nucleotide. A single "+" or "-" mutation at the beginning of a gene corrupts the entire gene. A double "+" or "-" mutation also spoils the entire gene. A triple "+" or "-" mutation at the beginning of the gene spoils only part of it. A quadruple "+" or "-" mutation again spoils the entire gene.
The experiment proves that the code is triplet and there are no punctuation marks inside the gene. The experiment was carried out on two adjacent phage genes and showed, in addition, the presence of punctuation marks between the genes.
3. Genetic information
Genetic information is a program of the properties of an organism, received from ancestors and embedded in hereditary structures in the form of a genetic code.
It is assumed that the formation of genetic information proceeded according to the scheme: geochemical processes - mineral formation - evolutionary catalysis (autocatalysis).
It is possible that the first primitive genes were microcrystalline crystals of clay, and each new layer of clay lines up in accordance with the structural features of the previous one, as if receiving information about the structure from it.
Realization of genetic information occurs in the process of synthesis of protein molecules with the help of three RNAs: informational (mRNA), transport (tRNA) and ribosomal (rRNA). The process of information transfer goes: - through the channel of direct communication: DNA - RNA - protein; and - via the feedback channel: environment - protein - DNA.
Living organisms are able to receive, store and transmit information. Moreover, living organisms tend to use the information received about themselves and the world around them as efficiently as possible. Hereditary information embedded in genes and necessary for a living organism for existence, development and reproduction is transmitted from each individual to his descendants. This information determines the direction of development of the organism, and in the process of its interaction with the environment, the reaction to its individual can be distorted, thereby ensuring the evolution of the development of descendants. In the process of evolution of a living organism, new information arises and is remembered, including the value of information for it increases.
In the course of 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 warehouse, predisposition to diseases and genetic defects of the body.
Many scientists, rightly emphasizing the role of information in the formation and evolution of living things, noted this circumstance as one of the main criteria of life. So, V.I. Karagodin believes: "The living is such a form of existence of information and the structures encoded by it, which ensures the reproduction of this information in suitable environmental conditions." The connection of information with life is also noted by A.A. Lyapunov: "Life is a highly ordered state of matter that uses information encoded by the states of individual molecules to develop persistent reactions." Our well-known astrophysicist N.S. Kardashev also emphasizes the information component of life: “Life arises due to the possibility of synthesizing a special kind of molecules that are able to remember and use at first the simplest information about environment and their own structure, which they use for self-preservation, for reproduction and, which is especially important for us, to obtain even more information. "The ecologist S.S. Chetverikov on population genetics, in which it was shown that not individual traits and individuals are subjected to selection, but the genotype of the entire population, but it is carried out through the phenotypic traits of individual individuals. This leads to the spread of beneficial changes in the entire population. Thus, the mechanism of evolution is realized as through random mutations 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 result, to a decrease in hereditary variability. This process is sometimes called genetic drift. And on the other hand, to changes in the concentration of various mutations and a decrease in the diversity of genotypes contained in the population, which can lead to changes in the direction and intensity of the selection action.
4. Deciphering 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 incompletely sequenced human chromosome.
A preliminary human genetic map was published in 2003, marking the formal end of the Human Genome Project. Within its framework, genome fragments containing 99% of human genes were sequenced. The accuracy of gene identification was 99.99%. However, at the end of the project, only four of the 24 chromosomes had been fully sequenced. The fact is that in addition to genes, chromosomes contain fragments that do not encode any traits and are not involved in protein synthesis. The role that these fragments play in the life of the organism is still unknown, but more and more researchers are inclined to believe that their study requires the closest attention.
In the body's metabolism leading role
belongs to proteins and nucleic acids.
Protein substances form the basis of all vital cell structures, have an unusually high reactivity, and are endowed with catalytic functions.
Nucleic acids are part of the most important body cells - nuclei, as well as cytoplasm, ribosomes, mitochondria, etc. Nucleic acids play an important, primary role in heredity, body variability, and protein synthesis.
Plan synthesis protein is stored in the cell nucleus, and direct synthesis occurs outside the nucleus, so it is necessary delivery service encoded plan from the nucleus to the site of synthesis. This delivery service is performed by RNA molecules.
The process starts at core cells: part of the DNA "ladder" unwinds and opens. Due to this, the RNA letters form bonds with the open DNA letters of one of the DNA strands. The enzyme transfers the letters of the RNA to connect them into a thread. So the letters of DNA are "rewritten" into the letters of RNA. The newly formed RNA chain is separated, and the DNA "ladder" twists again. The process of reading information from DNA and synthesizing its RNA template is called transcription , and the synthesized RNA is called informational or i-RNA .
After further modifications, this kind of encoded mRNA is ready. i-RNA comes out of the nucleus and goes to the site of protein synthesis, where the letters i-RNA are deciphered. Each set of three letters of i-RNA forms a "letter" that stands for one specific amino acid.
Another type of RNA looks for this amino acid, captures it with the help of an enzyme, and delivers it to the site of protein synthesis. This RNA is called transfer RNA, or tRNA. As the mRNA message is read and translated, the chain of amino acids grows. This chain twists and folds into a unique shape, creating one kind of protein. Even the process of protein folding is remarkable: to use a computer to calculate all options it would take 1027 (!) years to fold a medium-sized protein consisting of 100 amino acids. And for the formation of a chain of 20 amino acids in the body, it takes no more than one second, and this process occurs continuously in all cells of the body.
Genes, genetic code and its properties.
About 7 billion people live on Earth. Except for 25-30 million pairs of identical twins, then genetically all people are different : each is unique, has unique hereditary characteristics, character traits, abilities, temperament.
Such differences are explained differences in genotypes- sets of genes of an organism; each one is unique. The genetic traits of a particular organism are embodied in proteins - consequently, the structure of the protein of one person differs, although quite a bit, from the protein of another person.
It does not mean that humans do not have exactly the same proteins. Proteins that perform the same functions may be the same or very slightly differ by one or two amino acids from each other. But does not exist on the Earth of people (with the exception of identical twins), in which all proteins would be are the same .
Information about the primary structure of a protein encoded as a sequence of nucleotides in a section of a DNA molecule, gene - a unit of hereditary information of an organism. Each DNA molecule contains many genes. The totality of all the genes of an organism makes up its genotype . In this way,
A gene is a unit of hereditary information of an organism, which corresponds to a separate section of DNA
Hereditary information is encoded using genetic code , which is universal for all organisms and differs only in the alternation of nucleotides that form genes and code for proteins of specific organisms.
Genetic code consists of triplets (triplets) of DNA nucleotides, combined in different sequences (AAT, HCA, ACG, THC, etc.), each of which encodes a specific amino acid (which will be built into the polypeptide chain).
Actually code
counts sequence of nucleotides in an i-RNA molecule
, because it removes information from DNA (the process transcriptions
) and translates it into a sequence of amino acids in the molecules of synthesized proteins (process broadcasts
).
The composition of mRNA includes nucleotides A-C-G-U, the triplets of which are called codons
: the CHT DNA triplet on mRNA will become the HCA triplet, and the AAG DNA triplet will become the UUC triplet. Exactly i-RNA codons
reflects the genetic code in the record.
In this way, genetic code - a unified system for recording hereditary information in nucleic acid molecules in the form of a sequence of nucleotides . The genetic code is based on the use of an alphabet consisting of only four nucleotide letters that differ in nitrogenous bases: A, T, G, C.
The main properties of the genetic code:
1. Genetic code triplet. A triplet (codon) is a sequence of three nucleotides that codes for one amino acid. Since proteins contain 20 amino acids, it is obvious that each of them cannot be encoded by one nucleotide ( since there are only four types of nucleotides in DNA, in this case 16 amino acids remain uncoded). Two nucleotides for coding amino acids are also not enough, since in this case only 16 amino acids can be encoded. This means that the smallest number of nucleotides encoding one amino acid must be at least three. In this case, the number of possible nucleotide triplets is 43 = 64.
2. Redundancy (degeneracy) The code is a consequence of its triplet nature and means that one amino acid can be encoded by several triplets (since there are 20 amino acids, and there are 64 triplets), with the exception of methionine and tryptophan, which are encoded by only one triplet. In addition, some triplets perform specific functions: in the mRNA molecule, the triplets UAA, UAG, UGA - are terminating codons, i.e. stop-signals that stop the synthesis of the polypeptide chain. The triplet corresponding to methionine (AUG), standing at the beginning of the DNA chain, does not encode an amino acid, but performs the function of initiating (exciting) reading.
3. Unambiguity code - along with redundancy, the code has the property uniqueness : each codon matches only one specific amino acid.
4. Collinearity code, i.e. sequence of nucleotides in a gene exactly corresponds to the sequence of amino acids in the protein.
5. Genetic code non-overlapping and compact , i.e. does not contain "punctuation marks". This means that the reading process does not allow for the possibility of overlapping columns (triplets), and, starting at a certain codon, the reading goes continuously triplet by triplet until stop-signals ( termination codons).
6. Genetic code universal , i.e., the nuclear genes of all organisms encode information about proteins in the same way, regardless of the level of organization and systematic position these organisms.
Exist genetic code tables for decryption codons i-RNA and building chains of protein molecules.
Matrix synthesis reactions.
In living systems, there are reactions unknown in inanimate nature - matrix synthesis reactions.
The term "matrix" in technology they denote the form used for casting coins, medals, typographic type: the hardened metal exactly reproduces all the details of the form used for casting. Matrix synthesis resembles a casting on a matrix: new molecules are synthesized in strict accordance with the plan laid down in the structure of already existing molecules.
The matrix principle lies at the core the most important synthetic reactions of the cell, such as the synthesis of nucleic acids and proteins. In these reactions, an exact, strictly specific sequence of monomeric units in the synthesized polymers is provided.
This is where directional pulling monomers to a specific location cells - into molecules that serve as a matrix where the reaction takes place. If such reactions occurred as a result of a random collision of molecules, they would proceed infinitely slowly. The synthesis of complex molecules based on the matrix principle is carried out quickly and accurately. The role of the matrix macromolecules of nucleic acids play in matrix reactions DNA or RNA .
monomeric molecules, from which the polymer is synthesized - nucleotides or amino acids - in accordance with the principle of complementarity are arranged and fixed on the matrix in a strictly defined, predetermined order.
Then comes "crosslinking" of monomer units into a polymer chain, and the finished polymer is dropped from the matrix.
Thereafter matrix ready to the assembly of a new polymer molecule. It is clear that just as only one coin, one letter can be cast on a given mold, so only one polymer can be "assembled" on a given matrix molecule.
Matrix type of reactions- a specific feature of the chemistry of living systems. They are the foundation fundamental property of all living things - its ability to reproduce its own kind.
Matrix synthesis reactions
1. DNA replication - replication (from lat. replicatio - renewal) - the process of synthesis of a daughter molecule of deoxyribonucleic acid on the matrix of the parent DNA molecule. During the subsequent division of the mother cell, each daughter cell receives one copy of a DNA molecule that is identical to the DNA of the original mother cell. This process ensures the accurate transmission of genetic information from generation to generation. DNA replication is carried out by a complex enzyme complex, consisting of 15-20 different proteins, called replisome . The material for synthesis is free nucleotides present in the cytoplasm of cells. The biological meaning of replication lies in the exact transfer of hereditary information from the parent molecule to the daughter ones, which normally occurs during the division of somatic cells.
The DNA molecule consists of two complementary strands. These chains are held together by weak hydrogen bonds that can be broken by enzymes. The DNA molecule is capable of self-doubling (replication), and a new half of it is synthesized on each old half of the molecule.
In addition, an mRNA molecule can be synthesized on a DNA molecule, which then transfers the information received from DNA to the site of protein synthesis.
Information transfer and protein synthesis follow a matrix principle, comparable to the work of a printing press in a printing house. Information from DNA is copied over and over again. If errors occur during copying, they will be repeated in all subsequent copies.
True, some errors in copying information by a DNA molecule can be corrected - the process of eliminating errors is called reparations. The first of the reactions in the process of information transfer is the replication of the DNA molecule and the synthesis of new DNA strands.
2. Transcription (from Latin transcriptio - rewriting) - the process of RNA synthesis using DNA as a template, occurring in all living cells. In other words, it is the transfer of genetic information from DNA to RNA.
Transcription is catalyzed by the enzyme DNA-dependent RNA polymerase. RNA polymerase moves along the DNA molecule in the direction 3 " → 5". Transcription consists of steps initiation, elongation and termination . The unit of transcription is the operon, a fragment of the DNA molecule consisting of promoter, transcribed moiety, and terminator . i-RNA consists of one strand and is synthesized on DNA in accordance with the rule of complementarity with the participation of an enzyme that activates the beginning and end of the synthesis of the i-RNA molecule.
The finished mRNA molecule enters the cytoplasm on the ribosomes, where the synthesis of polypeptide chains takes place.
3. Broadcast (from lat. translation- transfer, movement) - the process of protein synthesis from amino acids on the matrix of information (matrix) RNA (mRNA, mRNA) carried out by the ribosome. In other words, this is the process of translating the information contained in the nucleotide sequence of i-RNA into the sequence of amino acids in the polypeptide.
4. reverse transcription is the process of forming double-stranded DNA based on information from single-stranded RNA. This process is called reverse transcription, since the transfer of genetic information occurs in the “reverse” direction relative to transcription. The idea of reverse transcription was initially very unpopular, as it contradicted the central dogma molecular biology, which suggested that DNA is transcribed into RNA and then translated into proteins.
However, in 1970, Temin and Baltimore independently discovered an enzyme called reverse transcriptase (revertase)
, and the possibility of reverse transcription was finally confirmed. In 1975, Temin and Baltimore were awarded Nobel Prize in the field of physiology and medicine. Some viruses (such as the human immunodeficiency virus that causes HIV infection) have the ability to transcribe RNA into DNA. HIV has an RNA genome that integrates into DNA. As a result, the DNA of the virus can be combined with the genome of the host cell. The main enzyme responsible for the synthesis of DNA from RNA is called revertase. One of the functions of reversease is to create complementary DNA
(cDNA) from the viral genome. The associated enzyme ribonuclease cleaves RNA, and reversetase synthesizes cDNA from the DNA double helix. cDNA is integrated into the host cell genome by integrase. The result is synthesis of viral proteins by the host cell that form new viruses. In the case of HIV, apoptosis (cell death) of T-lymphocytes is also programmed. In other cases, the cell may remain a distributor of viruses.
The sequence of matrix reactions in protein biosynthesis can be represented as a diagram.
In this way, protein biosynthesis- this is one of the types of plastic exchange, during which the hereditary information encoded in the DNA genes is realized in a certain sequence of amino acids in protein molecules.
Protein molecules are essentially polypeptide chains made up of individual amino acids. But amino acids are not active enough to connect with each other on their own. Therefore, before they combine with each other and form a protein molecule, amino acids must activate . This activation occurs under the action of special enzymes.
As a result of activation, the amino acid becomes more labile and, under the action of the same enzyme, binds to t- RNA. Each amino acid corresponds to a strictly specific t- RNA, which finds "its" amino acid and endures it into the ribosome.
Therefore, the ribosome receives various activated amino acids linked to their T- RNA. The ribosome is like conveyor to assemble a protein chain from various amino acids entering it.
Simultaneously with t-RNA, on which its own amino acid "sits", " signal» from the DNA that is contained in the nucleus. In accordance with this signal, one or another protein is synthesized in the ribosome.
The directing influence of DNA on protein synthesis is not carried out directly, but with the help of a special intermediary - matrix or messenger RNA (mRNA or i-RNA), which synthesized into the nucleus It is not influenced by DNA, so its composition reflects the composition of DNA. The RNA molecule is, as it were, a cast from the form of DNA. The synthesized mRNA enters the ribosome and, as it were, transfers it to this structure plan- in what order should the activated amino acids entering the ribosome be combined with each other in order to synthesize a certain protein. Otherwise, genetic information encoded in DNA is transferred to mRNA and then to protein.
The mRNA molecule enters the ribosome and flashes her. That segment of it that is currently in the ribosome is determined codon (triplet), interacts in a completely specific way with a structure suitable for it triplet (anticodon) in the transfer RNA that brought the amino acid into the ribosome.
Transfer RNA with its amino acid approaches a certain codon of mRNA and connects with him; to the next, neighboring site of i-RNA joins another tRNA with a different amino acid and so on until the entire i-RNA chain is read, until all the amino acids are strung in the appropriate order, forming a protein molecule. And t-RNA, which delivered the amino acid to a specific site of the polypeptide chain, freed from its amino acid and exits the ribosome.
Then again in the cytoplasm, the desired amino acid can join it, and it will again transfer it to the ribosome. In the process of protein synthesis, not one, but several ribosomes, polyribosomes, are simultaneously involved.
The main stages of the transfer of genetic information:
1. Synthesis on DNA as on an mRNA template (transcription)
2. Synthesis of the polypeptide chain in ribosomes according to the program contained in i-RNA (translation)
.
The stages are universal for all living beings, but the temporal and spatial relationships of these processes differ in pro- and eukaryotes.
At prokaryotes transcription and translation can occur simultaneously because DNA is located in the cytoplasm. At eukaryote transcription and translation are strictly separated in space and time: the synthesis of various RNAs occurs in the nucleus, after which the RNA molecules must leave the nucleus, passing through the nuclear membrane. The RNA is then transported in the cytoplasm to the site of protein synthesis.
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 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 vertical racks. These nitrogenous base molecules are called adenine, guanine, thymine, and cytosine, but they are commonly referred to simply as 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. 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.
