GENETIC CODE, a way of recording hereditary information in molecules nucleic acids in the form of a sequence of nucleotides forming these acids. A certain sequence of nucleotides in DNA and RNA corresponds to a certain sequence of amino acids in the polypeptide chains of proteins. It is customary to write the code using capital letters of the Russian or Latin alphabet. Each nucleotide is designated by the letter that begins the name of the nitrogenous base that is part of its molecule: A (A) - adenine, G (G) - guanine, C (C) - cytosine, T (T) - thymine; in RNA, instead of thymine, uracil is U (U). Each is encoded by a combination of three nucleotides - a triplet, or codon. Brief transfer path genetic information summarized in the so-called. the central dogma of molecular biology: DNA ` RNA f protein.
AT special occasions information can be transferred from RNA to DNA, but never from protein to genes.
Realization of genetic information is carried out in two stages. In the cell nucleus, information, or matrix, RNA (transcription) is synthesized on DNA. In this case, the nucleotide sequence of DNA is "rewritten" (recoded) into the nucleotide sequence of mRNA. Then mRNA passes into the cytoplasm, attaches to the ribosome, and on it, as on a matrix, a polypeptide protein chain is synthesized (translation). Amino acids with the help of transfer RNA are attached to the chain under construction in a sequence determined by the order of nucleotides in mRNA.
From the four "letters" you can make 64 different three-letter "words" (codons). Of the 64 codons, 61 encode certain amino acids, and three are responsible for the completion of the synthesis of the polypeptide chain. Since there are 61 codons for 20 amino acids that make up proteins, some amino acids are encoded by more than one codon (the so-called code degeneracy). Such redundancy increases the reliability of the code and the entire mechanism of protein biosynthesis. Another property of the code is its specificity (unambiguity): one codon encodes only one amino acid.
In addition, the code does not overlap - the information is read in one direction sequentially, triplet by triplet. Most amazing property code - its universality: it is the same for all living beings - from bacteria to humans (the exception is the genetic code of mitochondria). Scientists see this as confirmation of the concept of the origin of all organisms from one common ancestor.
The decoding of the genetic code, i.e., the determination of the "meaning" of each codon and the rules by which information is read, was carried out in 1961–1965. and is considered one of the most striking achievements of molecular biology.
The genetic code is a system for recording hereditary information in nucleic acid molecules, based on a certain alternation of nucleotide sequences in DNA or RNA that form codons corresponding to amino acids in a protein.
The genetic code has several properties.
Tripletity.
Degeneracy or redundancy.
Unambiguity.
Polarity.
Non-overlapping.
Compactness.
Versatility.
It should be noted that some authors also offer other properties of the code related to the chemical features of the nucleotides included in the code or to the frequency of occurrence of individual amino acids in the proteins of the body, etc. However, these properties follow from the above, so we will consider them there.
a. Tripletity. The genetic code is like a lot of complicated organized system has the smallest structural and smallest functional unit. A triplet is the smallest structural unit of the genetic code. It consists of three nucleotides. A codon is the smallest functional unit of the genetic code. As a rule, mRNA triplets are called codons. In the genetic code, a codon performs several functions. First, its main function is that it codes for one amino acid. Second, a codon may not code for an amino acid, but in this case it has a different function (see below). As can be seen from the definition, a triplet is a concept that characterizes elementary structural unit genetic code (three nucleotides). codon characterizes elementary semantic unit genome - three nucleotides determine the attachment to the polypeptide chain of one amino acid.
The elementary structural unit was first deciphered theoretically, and then its existence was confirmed experimentally. Indeed, 20 amino acids cannot be encoded by one or two nucleotides. the latter are only 4. Three out of four nucleotides give 4 3 = 64 variants, which more than covers the number of amino acids present in living organisms (see Table 1).
The combinations of nucleotides presented in Table 64 have two features. First, of the 64 variants of triplets, only 61 are codons and encode any amino acid, they are called sense codons. Three triplets do not encode
amino acids a are stop signals marking the end of translation. There are three such triplets UAA, UAG, UGA, they are also called "meaningless" (nonsense codons). As a result of a mutation, which is associated with the replacement of one nucleotide in a triplet with another, a meaningless codon can arise from a sense codon. This type of mutation is called nonsense mutation. If such a stop signal is formed inside the gene (in its informational part), then during protein synthesis in this place the process will be constantly interrupted - only the first (before the stop signal) part of the protein will be synthesized. A person with such a pathology will experience a lack of protein and will experience symptoms associated with this lack. For example, this kind of mutation was found in the gene encoding the hemoglobin beta chain. A shortened inactive hemoglobin chain is synthesized, which is rapidly destroyed. As a result, a hemoglobin molecule devoid of a beta chain is formed. It is clear that such a molecule is unlikely to fully fulfill its duties. There is a serious disease that develops according to the type of hemolytic anemia (beta-zero thalassemia, from the Greek word "Talas" - the Mediterranean Sea, where this disease was first discovered).
The mechanism of action of stop codons is different from the mechanism of action of sense codons. This follows from the fact that for all the codons encoding amino acids, the corresponding tRNAs were found. No tRNAs were found for nonsense codons. Therefore, tRNA does not take part in the process of stopping protein synthesis.
codonAUG (sometimes GUG in bacteria) not only encodes the amino acid methionine and valine, but is alsobroadcast initiator .
b. Degeneracy or redundancy.
61 of the 64 triplets code for 20 amino acids. Such a threefold excess of the number of triplets over the number of amino acids suggests that two coding options can be used in the transfer of information. Firstly, not all 64 codons can be involved in encoding 20 amino acids, but only 20, and secondly, amino acids can be encoded by several codons. Studies have shown that nature used the latter option.
His preference is clear. If only 20 out of 64 triplet variants were involved in coding amino acids, then 44 triplets (out of 64) would remain non-coding, i.e. meaningless (nonsense codons). Earlier, we pointed out how dangerous for the life of the cell is the transformation of a coding triplet into a nonsense codon as a result of mutation - this significantly disrupts the normal operation of RNA polymerase, ultimately leading to the development of diseases. There are currently three nonsense codons in our genome, and now imagine what would happen if the number of nonsense codons increased by about 15 times. It is clear that in such a situation the transition of normal codons to nonsense codons will be immeasurably higher.
A code in which one amino acid is encoded by several triplets is called degenerate or redundant. Almost every amino acid has several codons. So, the amino acid leucine can be encoded by six triplets - UUA, UUG, CUU, CUC, CUA, CUG. Valine is encoded by four triplets, phenylalanine by two and only tryptophan and methionine encoded by one codon. The property that is associated with the recording of the same information with different characters is called degeneracy.
The number of codons assigned to one amino acid correlates well with the frequency of occurrence of the amino acid in proteins.
And this is most likely not accidental. The higher the frequency of occurrence of an amino acid in a protein, the more often the codon of this amino acid is represented in the genome, the higher the probability of its damage by mutagenic factors. Therefore, it is clear that a mutated codon is more likely to code for the same amino acid if it is highly degenerate. From these positions, the degeneracy of the genetic code is a mechanism that protects the human genome from damage.
It should be noted that the term degeneracy is used in molecular genetics in another sense as well. Since the main part of the information in the codon falls on the first two nucleotides, the base in the third position of the codon turns out to be of little importance. This phenomenon is called “degeneracy of the third base”. The latter feature minimizes the effect of mutations. For example, it is known that the main function of red blood cells is to carry oxygen from the lungs to tissues and carbon dioxide from tissues to lungs. This function is carried out by the respiratory pigment - hemoglobin, which fills the entire cytoplasm of the erythrocyte. It consists of a protein part - globin, which is encoded by the corresponding gene. In addition to protein, hemoglobin contains heme, which contains iron. Mutations in globin genes result in various options hemoglobins. Most often, mutations are associated with substitution of one nucleotide for another and the appearance of a new codon in the gene, which can code for a new amino acid in the hemoglobin polypeptide chain. In a triplet, as a result of a mutation, any nucleotide can be replaced - the first, second or third. Several hundred mutations are known to affect the integrity of globin genes. Near 400 of which are associated with the replacement of single nucleotides in the gene and the corresponding amino acid substitution in the polypeptide. Of these, only 100 substitutions lead to instability of hemoglobin and various kinds of diseases from mild to very severe. 300 (approximately 64%) substitution mutations do not affect hemoglobin function and do not lead to pathology. One of the reasons for this is the “degeneracy of the third base” mentioned above, when the replacement of the third nucleotide in the triplet encoding serine, leucine, proline, arginine, and some other amino acids leads to the appearance of a synonym codon encoding the same amino acid. Phenotypically, such a mutation will not manifest itself. In contrast, any replacement of the first or second nucleotide in a triplet in 100% of cases leads to the appearance of a new hemoglobin variant. But even in this case, there may not be severe phenotypic disorders. The reason for this is the replacement of an amino acid in hemoglobin with another one similar to the first in terms of physicochemical properties. For example, if an amino acid with hydrophilic properties is replaced by another amino acid, but with the same properties.
Hemoglobin consists of an iron porphyrin group of heme (oxygen and carbon dioxide molecules are attached to it) and a protein - globin. Adult hemoglobin (HbA) contains two identical - chains and two -chains. Molecule -chain contains 141 amino acid residues, - chain - 146, - and -chains differ in many amino acid residues. The amino acid sequence of each globin chain is encoded by its own gene. The gene encoding - the chain is located on the short arm of chromosome 16, -gene - in the short arm of chromosome 11. Change in the gene encoding - hemoglobin chain of the first or second nucleotide almost always leads to the appearance of new amino acids in the protein, disruption of hemoglobin functions and severe consequences for the patient. For example, replacing “C” in one of the CAU (histidine) triplets with “U” will lead to the appearance of a new UAU triplet encoding another amino acid - tyrosine. Phenotypically, this will manifest itself in a serious illness .. A similar replacement in position 63 -chain of the histidine polypeptide to tyrosine will destabilize hemoglobin. The disease methemoglobinemia develops. Change, as a result of mutation, of glutamic acid to valine in the 6th position chain is the cause of a severe disease - sickle cell anemia. Let's not continue the sad list. We only note that when replacing the first two nucleotides, an amino acid may appear similar in physicochemical properties to the previous one. Thus, the replacement of the 2nd nucleotide in one of the triplets encoding glutamic acid (GAA) in -chain on “Y” leads to the appearance of a new triplet (GUA) encoding valine, and the replacement of the first nucleotide with “A” forms an AAA triplet encoding the amino acid lysine. Glutamic acid and lysine are similar in physicochemical properties - they are both hydrophilic. Valine is a hydrophobic amino acid. Therefore, the replacement of hydrophilic glutamic acid with hydrophobic valine significantly changes the properties of hemoglobin, which ultimately leads to the development of sickle cell anemia, while the replacement of hydrophilic glutamic acid with hydrophilic lysine changes the function of hemoglobin to a lesser extent - patients develop a mild form of anemia. As a result of the replacement of the third base, the new triplet can encode the same amino acids as the previous one. For example, if uracil was replaced by cytosine in the CAH triplet and a CAC triplet arose, then practically no phenotypic changes in a person will be detected. This is understandable, because Both triplets code for the same amino acid, histidine.
In conclusion, it is appropriate to emphasize that the degeneracy of the genetic code and the degeneracy of the third base from a general biological position are protective mechanisms that are incorporated in evolution in the unique structure of DNA and RNA.
in. Unambiguity.
Each triplet (except for meaningless ones) encodes only one amino acid. Thus, in the direction of codon - amino acid, the genetic code is unambiguous, in the direction of amino acid - codon - it is ambiguous (degenerate).
unambiguous
codon amino acid
degenerate
And in this case, the need for unambiguity in the genetic code is obvious. In another variant, during the translation of the same codon, different amino acids would be inserted into the protein chain and, as a result, proteins with different primary structures and different functions would be formed. The cell's metabolism would switch to the "one gene - several polypeptides" mode of operation. It is clear that in such a situation the regulatory function of genes would be completely lost.
g. Polarity
Reading information from DNA and from mRNA occurs only in one direction. Polarity is essential for defining higher order structures (secondary, tertiary, etc.). Earlier we talked about the fact that structures of a lower order determine structures of a higher order. The tertiary structure and structures of a higher order in proteins are formed immediately as soon as the synthesized RNA chain moves away from the DNA molecule or the polypeptide chain moves away from the ribosome. While the free end of the RNA or polypeptide acquires a tertiary structure, the other end of the chain still continues to be synthesized on DNA (if RNA is transcribed) or ribosome (if polypeptide is transcribed).
Therefore, the unidirectional process of reading information (in the synthesis of RNA and protein) is essential not only for determining the sequence of nucleotides or amino acids in the synthesized substance, but for the rigid determination of secondary, tertiary, etc. structures.
e. Non-overlapping.
The code may or may not overlap. In most organisms, the code is non-overlapping. An overlapping code has been found in some phages.
The essence of a non-overlapping code is that the nucleotide of one codon cannot be the nucleotide of another codon at the same time. If the code were overlapping, then the sequence of seven nucleotides (GCUGCUG) could encode not two amino acids (alanine-alanine) (Fig. 33, A) as in the case of a non-overlapping code, but three (if one nucleotide is common) (Fig. 33, B) or five (if two nucleotides are common) (see Fig. 33, C). In the last two cases, a mutation of any nucleotide would lead to a violation in the sequence of two, three, etc. amino acids.
However, it has been found that a mutation of one nucleotide always disrupts the inclusion of one amino acid in a polypeptide. This is a significant argument in favor of the fact that the code is non-overlapping.
Let us explain this in Figure 34. Bold lines show triplets encoding amino acids in the case of non-overlapping and overlapping codes. Experiments have unambiguously shown that the genetic code is non-overlapping. Without going into the details of the experiment, we note that if we replace the third nucleotide in the nucleotide sequence (see Fig. 34)At (marked with an asterisk) to some other then:
1. With a non-overlapping code, the protein controlled by this sequence would have a replacement for one (first) amino acid (marked with asterisks).
2. With an overlapping code in option A, a replacement would occur in two (first and second) amino acids (marked with asterisks). Under option B, the substitution would affect three amino acids (marked with asterisks).
However, numerous experiments have shown that when one nucleotide in DNA is broken, the protein always affects only one amino acid, which is typical for a non-overlapping code.
ГЦУГЦУГ ГЦУГЦУГ ГЦУГЦУГ
HCC HCC HCC UHC CUG HCC CUG UGC HCC CUG
*** *** *** *** *** ***
Alanine - Alanine Ala - Cys - Lei Ala - Lei - Lei - Ala - Lei
A B C
non-overlapping code overlapping code
Rice. 34. Scheme explaining the presence of a non-overlapping code in the genome (explanation in the text).
The non-overlapping of the genetic code is associated with another property - the reading of information begins from a certain point - the initiation signal. Such an initiation signal in mRNA is the codon encoding AUG methionine.
It should be noted that a person still has a small number of genes that deviate from general rule and overlap.
e. Compactness.
There are no punctuation marks between codons. In other words, the triplets are not separated from each other, for example, by one meaningless nucleotide. The absence of "punctuation marks" in the genetic code has been proven in experiments.
well. Versatility.
The code is the same for all organisms living on Earth. Direct evidence of the universality of the genetic code was obtained by comparing DNA sequences with corresponding protein sequences. It turned out that the same sets of code values are used in all bacterial and eukaryotic genomes. There are exceptions, but not many.
The first exceptions to the universality of the genetic code were found in the mitochondria of some animal species. This concerned the terminator codon UGA, which read the same as the UGG codon encoding the amino acid tryptophan. Other rarer deviations from universality have also been found.
The genetic code of DNA consists of 64 triplets of nucleotides. These triplets are called codons. Each codon codes for one of the 20 amino acids used in protein synthesis. This gives some redundancy in the code: most amino acids are encoded by more than one codon.
One codon performs two interrelated functions: it signals the beginning of translation and encodes the incorporation of the amino acid methionine (Met) into the growing polypeptide chain. The DNA code system is designed so that the genetic code can be expressed either as RNA codons or as DNA codons. RNA codons occur in RNA (mRNA) and these codons are able to read information during the synthesis of polypeptides (a process called translation). But each mRNA molecule acquires a nucleotide sequence in transcription from the corresponding gene.
All but two amino acids (Met and Trp) can be coded for by 2 to 6 different codons. However, the genome of most organisms shows that certain codons are favored over others. In humans, for example, alanine is encoded by GCC four times more often than in GCG. This probably indicates a greater translation efficiency of the translation apparatus (eg, the ribosome) for some codons.
The genetic code is almost universal. The same codons are assigned to the same stretch of amino acids and the same start and stop signals are overwhelmingly the same in animals, plants, and microorganisms. However, some exceptions have been found. Most of these involve assigning one or two of the three stop codons to an amino acid.
Chemical composition and structural organization of the DNA molecule.
Nucleic acid molecules are very long chains consisting of many hundreds and even millions of nucleotides. Any nucleic acid contains only four types of nucleotides. The functions of nucleic acid molecules depend on their structure, their constituent nucleotides, their number in the chain, and the sequence of the compound in the molecule.
Each nucleotide is made up of three components: a nitrogenous base, a carbohydrate, and phosphoric acid. AT compound 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), a book with a volume of about 820,000 pages would be required. The alternation of four types of nucleotides can form an infinite number of variants of DNA molecules. These features of the structure of DNA molecules allow them to store a huge amount of information about all the signs of organisms.
In 1953, the American biologist J. Watson and the English physicist F. Crick created a model for the structure of the DNA molecule. Scientists have found that each DNA molecule consists of two strands interconnected and spirally twisted. It looks like a double helix. In each chain, four types of nucleotides alternate in a specific sequence.
Nucleotide DNA composition differs from different types bacteria, fungi, plants, animals. But it does not change with age, it depends little on changes. environment. Nucleotides are paired, that is, the number of adenine nucleotides in any DNA molecule is equal to the number of thymidine nucleotides (A-T), and the number of cytosine nucleotides is equal to the number of guanine nucleotides (C-G). This is due to the fact that the connection of two chains to each other in a DNA molecule obeys a certain rule, namely: adenine of one chain is always connected by two hydrogen bonds only with Thymine of the other chain, and guanine by three hydrogen bonds with cytosine, that is, the nucleotide chains of one molecule DNA is complementary, complement each other.
Nucleic acid molecules - DNA and RNA are made up of nucleotides. The composition of DNA nucleotides includes a nitrogenous base (A, T, G, C), a deoxyribose carbohydrate and a residue of a phosphoric acid molecule. The DNA molecule is a double helix, consisting of two strands connected by hydrogen bonds according to the principle of complementarity. The function of DNA is to store hereditary information.
Properties and functions of DNA.
DNA is a carrier of genetic information, written in the form of a sequence of nucleotides using the genetic code. DNA molecules are associated with two fundamental properties of living organisms - heredity and variability. During a process called DNA replication, two copies of the original chain are formed, which are inherited by daughter cells when they divide, so that the resulting cells are genetically identical to the original.
Genetic information is realized during gene expression in the processes of transcription (synthesis of RNA molecules on a DNA template) and translation (synthesis of proteins on an RNA template).
The nucleotide sequence "encodes" information about various types RNA: information, or matrix (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized from DNA during the transcription process. Their role in protein biosynthesis (translation process) is different. Messenger RNA contains information about the sequence of amino acids in a protein, ribosomal RNA serves as the basis for ribosomes (complex nucleoprotein complexes, the main function of which is to assemble a protein from individual amino acids based on mRNA), transfer RNA deliver amino acids to the protein assembly site - to the active center of the ribosome, " creeping" along the mRNA.
Genetic code, its properties.
Genetic code- a method inherent in all living organisms to encode the amino acid sequence of proteins using a sequence of nucleotides. PROPERTIES:
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 are observed following Features structural organization of chromosomes:
There are mitotic and interphase forms of the structural organization of chromosomes, mutually passing into each other in the mitotic cycle - these are functional and physiological transformations
8. Packing levels of hereditary material in eukaryotes .
Structural and functional levels of organization of the hereditary material of eukaryotes
Heredity and variability provide:
1) individual (discrete) inheritance and changes in individual characteristics;
2) reproduction in individuals of each generation of the entire complex of morphological and functional characteristics of organisms of a particular biological species;
3) redistribution in species with sexual reproduction in the process of reproduction of hereditary inclinations, as a result of which the offspring has a combination of characters that is different from their combination in the parents. Patterns of inheritance and variability of traits and their combinations follow from the principles of the structural and functional organization of genetic material.
There are three levels of organization of the hereditary material of eukaryotic organisms: gene, chromosomal and genomic (genotype level).
The elementary structure of the gene level is the gene. The transfer of genes from parents to offspring is necessary for the development of certain traits in him. Although several forms of biological variability are known, only a disruption in the structure of genes changes the meaning of hereditary information, in accordance with which specific traits and properties are formed. Due to the presence of the gene level, individual, separate (discrete) and independent inheritance and changes in individual traits are possible.
The genes of eukaryotic cells are distributed in groups along the chromosomes. These are the structures of the cell nucleus, which are characterized by individuality and the ability to reproduce themselves with the preservation of individual structural features in a number of generations. The presence of chromosomes determines the allocation of the chromosomal level of organization of hereditary material. The placement of genes in chromosomes affects the relative inheritance of traits, makes it possible to influence the function of a gene from its immediate genetic environment - neighboring genes. The chromosomal organization of hereditary material serves necessary condition redistribution of the hereditary inclinations of parents in the offspring during sexual reproduction.
Despite the distribution over different chromosomes, the entire set of genes functionally behaves as a whole, forming a single system representing the genomic (genotypic) level of organization of hereditary material. At this level, there is a wide interaction and mutual influence of hereditary inclinations, localized both in one and in different chromosomes. The result is the mutual correspondence of the genetic information of different hereditary inclinations and, consequently, the development of traits balanced in time, place and intensity in the process of ontogenesis. The functional activity of genes, the mode of replication and mutational changes in the hereditary material also depend on the characteristics of the genotype of the organism or the cell as a whole. This is evidenced, for example, by the relativity of the property of dominance.
Eu - and heterochromatin.
Some chromosomes appear condensed and intensely colored during cell division. Such differences were called heteropyknosis. The term " heterochromatin". There are euchromatin - the main part of mitotic chromosomes, which undergoes a normal cycle of compactization 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 threads 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 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 follow from the features of this coding.
Each amino acid of a protein is associated with three successive nucleic acid nucleotides - triplet, or codon. Each of the nucleotides can contain one of four nitrogenous bases. In RNA it is adenine(A) uracil(U) guanine(G) cytosine(C). By combining nitrogenous bases in different ways (in this case nucleotides containing them) you can get many different triplets: AAA, GAU, UCC, GCA, AUC, etc. The total number of possible combinations is 64, i.e. 4 3 .
The proteins of living organisms contain about 20 amino acids. If nature "conceived" to encode each amino acid not with three, but with two nucleotides, then the variety of such pairs would not be enough, since there would be only 16 of them, i.e. 4 2 .
Thus, the main property of the genetic code is its triplet. Each amino acid is encoded by a triplet of nucleotides.
Since there are significantly more possible different triplets than amino acids used in biological molecules, such a property as 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.
Correspondence between amino acids and codons is reflected in the form of tables. For example, these:
In relation to nucleotides, the genetic code has the following property: uniqueness(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 is about the fact that several triplets can encode 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 that 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 is understood as the reading of triplets immediately one after another. 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 start at the second or third nucleotide of the preceding codon. In other words, the code does not overlap.
The genetic code has the property universality. 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 of mitochondria and chloroplasts code for amino acids other than their usual ones. This may indicate that at the dawn of the development 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, sometimes occurring in DNA, usually result in the replacement of one nitrogenous base with another. This changes the triplet. For example, it was AAA, 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 property of the redundancy of the genetic code, can correspond to one amino acid. Given that mutations are more often harmful, the noise immunity property is useful.
- 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 are as follows:
1. The genetic code is 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 is three. (In this case, the number of possible nucleotide triplets is 4 3 = 64).
2. The redundancy (degeneracy) of 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 64 triplets). The exceptions are methionine and tryptophan, which are encoded by only one triplet. In addition, some triplets perform specific functions. So, in an mRNA molecule, three of them - 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. Simultaneously with redundancy, the code has the property of unambiguity, which means that each codon corresponds to only one specific amino acid.
4. The code is collinear, i.e. The sequence of nucleotides in a gene exactly matches the sequence of amino acids in a protein.
5. The genetic code is non-overlapping and compact, that is, it 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 triple by triplet up to stop signals (terminating codons). For example, in mRNA next sequence nitrogenous bases AUGGUGTSUUAAAUGUG will be read only by such triplets: AUG, GUG, CUU, AAU, GUG, and not AUG, UGG, GGU, GUG, etc. or AUG, GGU, UGC, CUU, etc. or some other or in a way (for example, codon AUG, punctuation mark G, codon UHC, punctuation mark Y, etc.).
6. The genetic code is universal, that is, the nuclear genes of all organisms encode information about proteins in the same way, regardless of the level of organization and the systematic position of these organisms.
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