Nucleic acids are natural biopolymers that are products of the polycondensation reaction of nucleotides.
Based on the type of nucleotides, there are two types of nucleic acids: ribonucleic acids (RNA) and deoxyribonucleic acids (DNA).
Ribonucleic acids are nucleic acids obtained from the polycondensation of ribonucleotides.
Deoxyribonucleic acids are polycondensation products of deoxyribonucleotides.
Deoxyribonucleotides (DNA nucleotides) contain deoxyribose residues.
Ribonucleotides (RNA nucleotides) are nucleotides whose molecules include ribose residues.
RNA and DNA are called nucleic (nuclear) because they were found in the nuclei of cells (especially DNA), but they can also be found in other organoids (plastids, mitochondria, cell center, etc.).
Brief characteristics of nucleotides, nucleosides, nitrogenous cyclic bases that make up nucleic acids
If a nucleotide (any one) is subjected to complete hydrolysis, then a cyclic nitrogenous base, pentose and phosphoric (ortho) acid can be obtained. Partial hydrolysis of the nucleotide produces phosphoric acid and a nucleoside. If a nucleoside is subjected to hydrolysis, a cyclic nitrogenous base and a pentose (ribose or deoxyribose) can be obtained.
So, when hydrolyzing a DNA nucleotide, one can obtain DEK nucleoside and phosphoric acid (incomplete hydrolysis), or a nitrogenous base (cyclic), deoxyribose (pentose) and phosphoric acid.
The products of partial hydrolysis of RNA nucleotide are phosphoric acid and RNA nucleoside, and the products of complete hydrolysis are cyclic nitrogenous base, ribose (pentose) and phosphoric acid.
If it is necessary to obtain an RNA nucleotide, then first an RNA nucleoside is obtained from a natural cyclic nitrogenous base and ribose, which can be used to synthesize RNA nucleotide by reacting it with phosphoric acid. DNA nucleotide can be synthesized in a similar way, only deoxyribose must be used instead of ribose.
When studying the composition of nucleic acids, a number of natural cyclic bases were discovered, the most important of which are adenine and guanine (they belong to purine bases, contain two interconnected cycles and are derivatives of the cyclic substance purine; residues of these bases are included in both DNA and RNA).
In addition to adenine and guanine, cytosine, thymine and uracil residues were found in the nucleic acids (these nitrogenous bases are classified as pyrimidine bases, since they are derivatives of pyrimidine). They contain one cycle, reminiscent in structure of a benzene ring, but part of the carbon in it is replaced by nitrogen atoms). Cytosine residues are found in both DNA and RNA, while thymine residues are found only in DNA, and uracil residues are found only in RNA.
Empirical formulas (not for memorization): adenine - C 5 H 5 N 5; guanine - C 5 H 5 H 5 O; cytosine - C 4 H 5 N 3 O; uracil - C 4 H 4 N 2 O 2; thymine - C 5 H 6 N 2 O 2.
Nitrogenous bases contain =NH, -IN 2 groups, carbonyl groups, nitrogen atoms, which leads to the formation of hydrogen bonds, which play a large role in the formation of the structures of nucleic acids, their stability and diverse properties.
The student needs to understand and be able to diagram nucleosides and nucleotides. Below are some of these schemes. It is also important to understand the nomenclature (names) of nucleosides and nucleotides. Their names are based on the name of the nitrogenous base, which is an adjective to the word nucleoside or nucleotide, while the name indicates the type of nucleotide (nucleoside) at the pentose residue, for example, adenyl DNA nucleotide; this means that this substance consists of an adenosine residue, deoxyribose and phosphoric acid, connected by oxygen bridges.
Examples of RNA nucleoside schemes:
1) “cytosine residue - ribose residue” is a cytosyl RNA nucleoside;
2) “uracil residue - ribose residue” is a uracil RNA nucleoside.
Example of a diagram and name of a DNA nucleoside:
"adenine residue - deoxyribose residue" is an adenyl DNA nucleoside.
An example of the diagram and name of a DNA nucleotide:
“thymine residue - deoxyribose residue - phosphoric acid residue” - thymidyl DNA nucleotide.
An example of the diagram and name of an RNA nucleotide:
“uracil residue - ribose residue - phosphoric acid residue” is a uracil RNA nucleotide.
Brief description of the structure of DNA molecules
DNA has a very complex structure, which was discovered in the works of a number of scientists, including J. Watson and F. Crick (1953). There are a number of structures, some of which will be discussed below.
1. Like proteins, DNA is characterized by a primary structure, characterized by the sequence of arrangement of nucleotide residues. Adenyl, guanyl, cytosyl and thymidyl DNA nucleotides are involved in the formation of DNA.
So, the primary structure of DNA is a sequence of nucleotide residues connected by phosphoric acid residues, while the latter connects nucleoside residues through the 3-5th carbon atoms of deoxyribose with oxygen bridges. The nitrogenous base residue is bonded to the first carbon atom of deoxyribose, and the own phosphoric acid residue of a given DNA nucleotide is connected through an oxygen bridge to the third carbon atom of deoxyribose, and this phosphoric acid residue, during polycondensation, interacts with the “OH” group bonded to the fifth carbohydrate atom of another nucleotide . Schematically, the primary structure of DNA (without taking into account its structural features) can be depicted by a sequence of capital initial letters from the names of nucleotides, for example:
A-C-C-G-T-T……,
where A is the residue from adenyl, G - guanyl, T - thymidyl, C - cytosyl DNA nucleotide. There are an infinite number of varieties of sequence combinations of nucleotide residues, therefore there are a lot of varieties of DNA molecules, so many that each individual of a particular species has its own DNA, characteristic only for a given organism.
2. The secondary structure of DNA is that it forms a double strand, that is, two polynucleotide chains combine to form a single molecule. Such a union is possible due to the fact that nitrogenous bases (and, consequently, nucleotide residues) have complementarity - complementarity due to the formation of hydrogen bonds between the residues of nitrogenous bases (or the possibility thereof). It has been established that adenine is complementary to thymine, since two hydrogen bonds are formed between them: the first between the group - NH 2 (from adenine) and the oxygen atom of the =C=O group (from thymine), and the second between the nitrogen atom of the six-membered ring of adenine and the atom of the group =NH (in the thymine molecule).
Note. RNA contains uracil residues instead of thymine residues, and this base is complementary to adenine for the same reason that thymine is complementary to adenine; this is important to know and take into account when considering the processes of RNA synthesis.
Guanine is complementary to cytosine, since three hydrogen bonds occur between these bases (or their residues): the first is between the oxygen atom of the carbonyl group (=C=O) of the six-membered ring of guanine and the hydrogen atom of the group - NH 2 of cytosine; the second is carried out by the hydrogen atom of the =NH group of the six-membered guanine ring and the nitrogen atom in the cytosine ring; the third bond is realized by the hydrogen atom of the amino group (-NH 2) of guanine and the oxygen atom of the carbonyl group of cytosine (the characterization of the essence of the complementarity principle for nucleotide residues is given for illustration, and not for memorization).
3. The tertiary structure of DNA is that two double polynucleotide chains are folded into a single alpha helix, with the beginning of the first double polynucleotide chain directed towards the end of the second polynucleotide chain according to the “head-tail” principle.
The stability of the tertiary structure of DNA is associated with the ability of hydrogen bonds to occur between individual sections of polynucleotide chains and other types of bonds.
4. The quaternary structure of DNA is the spatial arrangement of the alpha helix. DNA, like all nucleic acids, forms complex proteins - nucleoproteins, which (for DNA) form special cell organelles - chromosomes.
Brief description of the ecological and biological role of DNA
DNA, along with proteins, is an integral part of living matter; Without these compounds, life as a property of matter is practically impossible, which characterizes the ecological and biological role of DNA. The following biological and ecological functions of DNA can be mentioned.
1. DNA is a “receptacle” for the characteristics of a given organism, therefore, due to the transfer of DNA to generative (reproductive) cells, hereditary characteristics are transmitted from parents to descendants.
2. RNA synthesis occurs on DNA, due to which information about the structure and properties of proteins is transferred to the organelles on which protein biosynthesis occurs, which leads to the synthesis of proteins with certain properties and to the implementation of specific characteristics inherent in a given organism.
3. Individual sections of DNA “know” information about certain specific characteristics of the organism and are called “genes” (genes are the material basis of heredity).
(The definition of a gene is ambiguous; there are different points of view on this issue, however, without complicating the picture, you can use the following definition:
A gene is a certain section of a DNA molecule with various types of its structure, consisting of a certain number of nucleotide residues, responsible for the transfer and implementation of this specific trait from one organism to another.)
4. DNA together with proteins form chromosomes - special cell organelles that clearly manifest themselves during the process of “indirect” division (mitosis). Thanks to the presence of chromosomes, there is an even distribution of nuclear matter, namely DNA, between daughter cells, which is important for the equivalence of future offspring and their survival in environmental conditions.
5. On the original DNA molecules, synthesis (self-reproduction) of new DNA molecules occurs during the period of interphase (the period of time between divisions) or during the preparation of cells for division (if the newly formed cells are subsequently not capable of division, which is typical for sperm, red blood cells, etc. .d.).
Brief description of DNA synthesis processes in organisms
DNA synthesis or replication (reduplication) is one of the most important ecological and biological processes on which the existence of living beings depends, and which is negatively affected by various processes occurring in the environment. Replication is a classic example of template synthesis (synthesis on a specific basis), widely found in nature. Let's look at some features of replication.
Before replication begins, the structure of the mother DNA molecule changes: the quaternary structure of its molecule is disrupted, the double helix unwinds (the tertiary structure is destroyed), and then each of the double polynucleotide chains begins to separate into single polynucleotide chains (the secondary structure of the DNA molecule is disrupted), i.e. In one molecule, the rudiments of four single polynucleotide chains arise. On each of the primordia of a single polynucleotide chain, new double polynucleotide chains are formed as a result of a polycondensation reaction under the influence of enzymes and due to the energy of ATP hydrolysis.
The template in this synthesis is a single polynucleotide chain, on which, according to the principle of complementarity, a new polynucleotide chain is formed, connected to the parent chain by hydrogen bonds.
As a result of this process, four double polynucleotide chains ultimately arise, i.e., the secondary structure of the DNA molecule is formed. From the four double polynucleotide chains that arise, two alpha helices are formed, giving rise to two molecules of equal DNA, which represent copies of the molecule. Due to this process, the amount of DNA in the cell doubles, which is a prerequisite for the implementation of normal division, which occurs either in the form of mitosis (indirect division) or in the form of amitosis (direct division).
Brief characteristics of RNA (structure of molecules, classification, ecological and biological role)
Ribonucleic acids (RNA) are the products of the polycondensation reaction of RNA nucleotides.
RNAs are diverse, have a specific classification, but have a common primary structure for all RNAs, which is that they are all a sequence of RNA nucleotide residues in a single polynucleotide chain; these residues are linked to each other by a phosphoric acid residue through the 3-5th carbon atoms of ribose of different nucleosides. RNA contains residues of four types of RNA nucleotides: adenyl, guanyl, cytoside and uracil (the last nucleotide is similar to thymidyl for DNA).
Based on their structure and functions, there are three types of RNA: 1) messenger or mRNA; 2) transport or tRNAs; 3) ribosomal or rRNA. Let us briefly characterize these types of RNA.
1. Messenger RNA (mRNA) is RNA, the main function of which is to transfer information about the structure, and therefore the properties of proteins, to the cell organelle where protein molecules are synthesized. mRNA is a matrix for the synthesis of a protein molecule, which is its second function. mRNA is a polynucleotide chain of a certain length, corresponding to the length of the gene, which encodes information about the structure of the protein, and, consequently, the trait of the organism. mRNA is always much shorter (in length) than DNA. There are an infinite number of varieties of mRNA, since there are many individual organisms, and, accordingly, the characteristics corresponding to them.
2. Transfer RNAs (tRNAs) are relatively small molecules of a specific structure, there are relatively few of them - 64. Their main function is the transport of molecules of natural alpha amino acids to the site of synthesis of protein molecules (ribosomes). TRNAs activate amino acids and transport them to the site of protein synthesis. They have a specific cross-shaped shape, and at the top of the “cross” there is an anticodon, which attaches the tRNA to the codon on the mRNA. At the opposite pole of the tRNA molecule there is an “acceptor” (capturing) section of the molecule to which this alpha amino acid is attached. There are 64 varieties of tRNA because there are 64 codons of alpha amino acids, with the help of which the polypeptide chain of a protein molecule, the beginning, completion and pauses in the synthesis of a protein molecule are encoded.
3. Ribosomal RNA (rRNA) is RNA that forms ribosomes together with protein molecules; rRNA, along with other RNAs, contribute to the processes of protein biosynthesis; in addition, they perform a construction function, being one of the substances from which ribosomes are formed. There is a wide variety of rRNA molecules.
Differences between RNA and DNA:
1) DNA contains thymine, and RNA (instead of thymine) contains uracil;
2) DNA is predominantly found in the nucleus (but can be found in mitochondria, plastids, and the cell center), and RNA is found in the nucleus, cytoplasm, and ribosomes;
3) the elementary units (nucleotide residues) of DNA include a deoxyribose residue, and the composition of RNA nucleotides includes a ribose residue (which is responsible for the differences in the names of these groups of nucleic acids);
4) DNA is the product of the polycondensation reaction of DNA nucleotides, and RNA is the product of RNA nucleotides;
5) the degree of polycondensation (n) in DNA is significantly higher than in RNA;
6) a DNA molecule consists of groups of nucleotide residues of a certain sequence, forming a “gene”, which contains a certain characteristic of an organism and controls its transmission to descendants; there are no such sections in RNA, i.e. RNA is not a collection of “genes”;
7) DNA is a single group of compounds that has an infinitely large number of varieties, and RNA is divided into three groups of compounds, of which there are infinitely many varieties of mRNA, 64 varieties of tRNA and a large number of varieties of rRNA;
8) DNA molecules have a very complex structure, while the structures of RNA are simpler (for example, one DNA molecule consists of four linear chains, and RNA - of one, etc.);
9) RNA and DNA have different functions in the body.
mRNA synthesis (transcription)
RNA synthesis, like DNA synthesis, is a template synthesis. RNA is synthesized under the influence of enzymes on the surface of despiralized DNA in its individual sections. The body uses proteins such as RNA polymerase as enzymes. RNA synthesis begins with the process of despiralization of the corresponding section of DNA; at this section, the “assembly” (transcription) of the RNA polynucleotide chain occurs according to the principle of complementarity. RNA synthesis is an endothermic process and its implementation requires energy released during the breakdown of ATP to ADP and phosphoric acid.
- nucleic acids - Polynucleotides, phosphorus-containing biopolymers that are universally distributed in living nature. They were first discovered by F. Miescher in 1868 in cells rich in nuclear material (leukocytes, salmon sperm). The term "N. To." proposed in 1889. Biological encyclopedic dictionary
- NUCLEIC ACIDS - NUCLEIC ACIDS are chemical macromolecules present in all living organisms and in viruses. There are two types of nucleic acids: DNA (deoxyribonucleic acid) stores the GENETIC CODE... Scientific and technical dictionary
- Nucleic acids - Polynucleotides, the most important biologically active biopolymers, which have a universal distribution in living nature. Contained in every cell of all organisms. N.K. were discovered in 1868 by a Swiss scientist... Great Soviet Encyclopedia
- nucleic acids - (polynucleotides), biopolymers contained in all living cells and viruses. They were first discovered and isolated from cells by the Swiss biochemist I.F. Misher in 1868. But only in mid. Biology. Modern encyclopedia
- NUCLEIC ACIDS - NUCLEIC ACIDS (polynucleotides) are high molecular weight organic compounds formed by nucleotide residues. Depending on which carbohydrate is included in the nucleic acid - deoxyribose or ribose... Large encyclopedic dictionary
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Nucleic acids and their role in cell life
1. What is the role of the nucleus in a cell?
2. What cell organelles are the transmission of hereditary characteristics associated with?
3. What substances are called nucleic acids?
Nucleic acids and their types.
Nucleic acids are biopolymers consisting of monomers - nucleotides.
Each nucleotide consists of a phosphate group, a five-carbon sugar (pentose) and a nitrogenous base (Figure 17).
The phosphoric acid residue attached to the fifth C atom in the pentose can be joined by a covalent bond to the hydroxyl group near the third C atom of another nucleotide.
Please note: the ends of the chain of nucleotides linked to form a nucleic acid are different.
At one end there is a phosphate bound to the fifth pentose atom, and this end is called the 5"-end (read "five-stroke"). At the other end there remains an OH group not bound to the phosphate near the third pentose atom (3"-end). Thanks to the polymerization reaction of nucleotides, nucleic acids are formed (Fig. 18).
Depending on the type of pentose, two types of nucleic acids are distinguished - deoxyribonucleic acids (abbreviated DNA) and ribonucleic acid (RNA). The name of the acids is due to the fact that the DNA molecule contains deoxyribose, and the RNA molecule contains ribose.
Structure of DNA.
The DNA molecule has a complex structure. It consists of two helically twisted chains, which are connected to each other along their entire length by hydrogen bonds. This structure, unique to DNA molecules, is called a double helix.
The nucleotides that make up DNA contain deoxyribose, a phosphoric acid residue and one of four nitrogenous bases: adenine, guanine, cytosine and thymine. They determine the names of the corresponding nucleotides; adenyl (A), guanyl (G), cytidyl (C) and tishidyl (T) (Fig. 18).
Each DNA strand represents a polynucleotide, which can consist of several tens of thousands or even millions of nucleotides. Nucleotides that are part of one chain are connected sequentially due to the formation of covalent bonds between the deoxyribose of one and the phosphoric acid residue of another nucleotide. The nitrogenous bases, which are located on one side of the resulting backbone of one DNA strand, form hydrogen bonds with the nitrogenous bases of the second strand. Thus, in a helical double-stranded DNA molecule, the nitrogenous bases are located inside the helix. The structure of the helix is such that the polynucleotide chains included in its composition can be separated only after unwinding of the helix (Fig. 18).
In a DNA double helix, the nitrogenous bases of one strand are arranged in a strictly defined order opposite the nitrogenous bases of the other. There are always two hydrogen bonds between adenine and thymine, and three hydrogen bonds between guanine and cytosine. In this regard, an important pattern is revealed: thymine of another chain is always located opposite the adenine of one chain, cytosine is always located opposite guanine, and vice versa. Thus, the nucleotide pairs adenine and thymine, as well as guanine and cytosine, strictly correspond to each other and are complementary (spatial mutual correspondence), or complementary (from the Latin complementum - addition).
Consequently, in any organism the number of adenyl nucleotides is equal to the number of thymidyl nucleotides, and the number of guanyl nucleotides is equal to the number of cytidyl nucleotides. And knowing the sequence of nucleotides in one DNA chain according to the principle of complementarity, you can determine the nucleotides of another chain.
The structure of each DNA molecule is strictly individual and specific, since it represents a code form for recording biological information (genetic code). In other words, with the help of four types of nucleotides, all important information about the organism is recorded in DNA, which is passed on to subsequent generations.
DNA molecules are mainly found in the nuclei of cells, but small amounts are found in mitochondria and plastids.
Structure of RNA.
The RNA molecule, unlike the DNA molecule, is a polymer consisting of one chain of much smaller dimensions.
RNA monomers are nucleotides consisting of ribose, a phosphoric acid residue, and one of four nitrogenous bases. Three nitrogenous bases - adenine, guanine and cytosine - are the same as those of DNA, and the fourth is uracil (Fig. 19).
The formation of an RNA polymer occurs in the same way as in DNA, through covalent bonds between ribose and the phosphoric acid residue of neighboring nucleotides. An RNA molecule can contain from 75 to 10,000 nucleotides.
Types of RNA.
There are three main types of RNA, differing in structure, molecular size, location in the cell and functions performed.
Ribosomal RNA (rRNA) is synthesized primarily in the nucleolus and accounts for approximately 85% of all RNA in a cell. They are part of ribosomes and participate in the formation of the active center of the ribosome, where the process of protein biosynthesis occurs.
Transfer RNAs (tRNAs) are formed in the nucleus on DNA and then move into the cytoplasm. They make up about 10% of cellular RNA and are the smallest RNAs, consisting of 70-100 nucleotides. Each tRNA attaches a specific amino acid and transports it to the site of polypeptide assembly in the ribosome.
All known tRNAs, due to complementary interactions, form a secondary structure shaped like a clover leaf. A tRNA molecule has two active sites: a triplet anticodon at one end and an acceptor end at the other (Fig. 20).
Each amino acid corresponds to a combination of three nucleotides - a triplet. Amino acid-coding triplets - DNA codons - are transmitted in the form of information from mRNA triplets (codons). At the top of the clover leaf there is a triplet of nucleotides that is complementary to the corresponding mRNA codon. This triplet is different for tRNAs carrying different amino acids, and encodes exactly the amino acid that is carried by this tRNA. It is called anticodon.
The acceptor end is the “landing pad” for the amino acid.
Messenger RNAs (mRNAs) make up about 5% of all cellular RNA. They are synthesized on a section of one of the chains of the DNA molecule and transmit information about the structure of the protein from the cell nucleus to the ribosomes, where this information is implemented. Depending on the volume of the copied information an mRNA molecule can have different lengths.
Thus, different types of RNA represent a single functional system aimed at implementing hereditary information through protein synthesis.
RNA molecules are found in the nucleus, cytoplasm, ribosomes, mitochondria and plastids of the cell.
All types of RNA, with the exception of genetic RNA viruses, are not capable of self-duplication and self-assembly.
Nucleic acid. Nucleotide. Deoxyribonucleic acid, or DNA. Ribonucleic acid, or RNA. Nitrogen bases: adenine, guanine, cytosine, thymine, uracil. Complementarity. Transfer RNA (tRNA). Ribosomal RNA (rRNA). Messenger RNA (mRNA).
1. What is the structure of a nucleotide?
2. What is the structure of the DNA molecule?
3. What is the principle of complementarity?
4. What is common and what differences are there in the structure of molecules
5. DNA and RNA?
6. What types of RNA molecules do you know? What is their function?
7. A fragment of one DNA strand has the following composition: A-A-G-G-C-C-C-T-T-. Using the principle of complementarity, complete the second chain.
In the DNA molecule, thymines account for 24% of the total number of nitrogenous bases. Determine the number of other nitrogenous bases in this molecule.
The 1962 Nobel Prize was awarded to two scientists - J. Watson and F. Crick, who in 1953 proposed a model of the structure of the DNA molecule. It was confirmed experimentally. This discovery was of great importance for the development of genetics, molecular biology and other sciences. Viruses, unlike other organisms, contain single-stranded DNA and double-stranded RNA.
Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology 10th grade
Submitted by readers from the website
Of the two types of nucleic acids - DNA and RNA - deoxyribonucleic acid acts as a substance in which all the basic hereditary information of a cell is encoded, and which is capable of self-reproduction, and ribonucleic acids act as intermediaries between DNA and protein. Such functions of nucleic acids are closely related to the features of their individual structure.
DNA and RNA are polymer macromolecules whose monomers are nucleotides. Each nucleotide is formed from three parts - a monosaccharide, a phosphoric acid residue and a nitrogenous base. The nitrogenous base is connected to the sugar by a b-N-glycosidic bond (Fig. 1.1).
The sugar included in the nucleotide (pentose) can be present in one of two forms: b-D-ribose and b-D-2-deoxyribose. The difference between them is that the ribose hydroxyl at the 2'-carbon atom of pentose is replaced by a hydrogen atom in deoxyribose. Nucleotides containing ribose are called ribonucleotides and form RNA monomers, while nucleotides containing deoxyribose are called deoxyribonucleotides and form DNA.
Nitrogen bases are derivatives of one of two compounds - purina or pyrimidine. Nucleic acids are dominated by two purine bases - adenine (A) and guanine (G) and three pyrimidine bases - cytosine (C), thymine (T) and uracil (U). In ribonucleotides and, accordingly, in RNA there are bases A, G, C, U, and in deoxyribonucleotides and in DNA - A, G, C, T.
Rice. 1.1. Structure of nucleoside and nucleotide: numbers indicate
arrangement of atoms in the pentose residue
The nomenclature of nucleosides and nucleotides is widely used in biochemistry and molecular biology and is presented in table. 1.1.
Table 1.1. Nomenclature of nucleotides and nucleosides
Long polynucleotide chains of DNA and RNA are formed when nucleotides are connected to each other using phosphodiester bridges. Each phosphate connects a hydroxyl at the 3'-carbon pentose atom of one nucleotide to an OH group at the 5'-carbon pentose atom of an adjacent nucleotide (Figure 1.2).
During acid hydrolysis of nucleic acids, individual components of nucleotides are formed, and during enzymatic hydrolysis using nucleases Certain bonds in the phosphodiester bridge are cleaved and the 3' and 5' ends of the molecule are exposed (Fig. 1.2).
This gives grounds to consider the nucleic acid chain to be polar, and it becomes possible to determine the direction of reading the nucleotide sequence in it. It should be noted that most enzymes involved in the synthesis and hydrolysis of nucleic acids work in the direction from the 5’ to the 3’ end (5’ → 3’) of the nucleic acid chain. According to the accepted convention, the sequence of nucleotides in nucleic acid chains is also read in the direction 5’ → 3’ (Fig. 1.2).
Features of DNA structure. According to the three-dimensional model proposed by Watson and Crick in 1953, the DNA molecule consists of two polynucleotide chains that form a right-handed helix about the same axis. The direction of the chains in the molecule is mutually opposite, it has an almost constant diameter and other parameters that do not depend on the nucleotide composition, unlike proteins in which the sequence of amino acid residues determines the secondary and tertiary structure of the molecule.
The sugar-phosphate backbone is located along the periphery of the helix, and the nitrogenous bases are located inside, and their planes are perpendicular to the axis of the helix. Specific hydrogen bonds are formed between bases located opposite each other in opposite chains: adenine always binds to thymine, and guanine to cytosine. Moreover, in the AT pair, the bases are connected by two hydrogen bonds: one of them is formed between the amino and keto groups, and the other - between the two nitrogen atoms of purine and pyrimidine, respectively. There are three hydrogen bonds in the GC pair: two of them are formed between the amino and keto groups of the corresponding bases, and the third is between the nitrogen atom of pyrimidine and the hydrogen (substituent at the nitrogen atom) of purine.
Thus, larger purines always pair with smaller pyrimidines. This leads to the fact that the distances between the C1'-atoms of deoxyribose in the two chains are the same for AT and GC pairs and equal to 1.085 nm. These two types of nucleotide pairs, AT and GC, are called complementary in pairs. Pairing between two purines, two pyrimidines, or non-complementary bases (A+C or G+T) is sterically hindered because suitable hydrogen bonds cannot form and, therefore, the geometry of the helix is disrupted.
The geometry of the double helix is such that neighboring nucleotides in the chain are located at a distance of 0.34 nm from each other. There are 10 nucleotide pairs per turn of the helix, and the helix pitch is 3.4 nm (10 * 0.34 nm). The diameter of the double helix is approximately 2.0 nm. Due to the fact that the sugar-phosphate backbone is located further from the axis of the helix than the nitrogenous bases, the double helix has grooves - large and small (Fig. 1.3).
The DNA molecule is capable of taking on different conformations. A-, B- and Z-forms were discovered. B-DNA is the common form in which DNA is found in a cell, in which the planes of the base rings are perpendicular to the axis of the double helix. In A-form DNA, the planes of the base pairs are rotated approximately 20° from the normal to the axis of the right-handed double helix. The Z form of DNA is a left-handed helix with 12 base pairs per turn. The biological functions of the A- and Z-forms of DNA are not fully understood.
The stability of the double helix is due to hydrogen bonds between complementary nucleotides in antiparallel chains, stacking interactions (interplanar van der Waals contacts between atoms and overlapping p-orbitals of atoms of contacting bases), as well as hydrophobic interactions. The latter are expressed in the fact that non-polar nitrogenous bases face the inside of the helix and are protected from direct contact with the polar solvent, and vice versa, charged sugar phosphate groups face outward and are in contact with the solvent.
Since two DNA strands are connected only by non-covalent bonds, a DNA molecule easily breaks down into individual chains when heated or in alkaline solutions ( denaturation). However, with slow cooling ( annealing) the chains are able to associate again, and hydrogen bonds are restored between complementary bases ( renaturation). These properties of DNA are of great importance for genetic engineering methodology (Chapter 20).
The size of DNA molecules is expressed in the number of nucleotide pairs, with one thousand nucleotide pairs (kb) or 1 kilobase (kb) taken as a unit. Molecular mass of one kb. B-form DNA is ~6.6*10 5 Da and its length is 340 nm. The complete genome of E. coli (~ 4*10 6 bp) is represented by one circular DNA molecule (nucleoid) and has a length of 1.4 mm.
Features of the structure and function of RNA. RNA molecules are polynucleotides consisting of a single chain, including 70-10,000 nucleotides (sometimes more), represented by the following types: mRNA (template or information), tRNA (transport), rRNA (ribosomal) and only in eukaryotic cells - hnRNA ( heterogeneous nuclear), as well as snRNA (small nuclear). The listed types of RNA perform specific functions; in addition, in some viral particles, RNA is the carrier of genetic information.
Messenger RNA is a transcript of a specific fragment semantic chain DNA is synthesized during transcriptions. mRNA is a program (matrix) by which a polypeptide molecule is built. Every three consecutive nucleotides in mRNA perform a function codon, determining the position of the corresponding amino acid in the peptide. Thus, mRNA serves as an intermediary between DNA and protein.
Transfer RNA is also involved in the process of protein synthesis. Its function is to deliver amino acids to the site of synthesis and determine the position of the amino acid in the peptide. For this purpose, tRNA contains a specific triplet nucleotides, called "anticodon" and the entire molecule is characterized by a unique structure. The structural representation of a tRNA molecule is called a “clover leaf” (Fig. 1.4).
The tRNA molecule is short and consists of 74-90 nucleotides. Like any nucleic acid chain, it has 2 ends: a phosphorylated 5' end and a 3' end, which always contains 3 nucleotides -CCA and a terminal 3'OH group. An amino acid is attached to the 3' end of tRNA and is called the acceptor end. Several unusually modified nucleotides not found in other nucleic acids were found in tRNA.
Despite the fact that the tRNA molecule is single-stranded, it contains individual duplex regions that form the so-called. stems or branches, where Watson-Crick pairs are formed between asymmetric sections of the chain (Fig. 1.4). All known tRNAs form a “cloverleaf” with four stems (acceptor, D, anticodon and T). The stems are shaped like a right-handed double helix, known as A-form DNA. TRNA loops are single-stranded regions. Some tRNAs have additional loops and/or stems (for example, the yeast phenylalanine tRNA variable loop).
Recognition of the corresponding site in the mRNA by the tRNA molecule is carried out using an anticodon located in the anticodon loop in Fig. 1.4). In this case, hydrogen bonds are formed between the bases of the codon and anticodon, provided that the sequences forming them are complementary and the polynucleotide chains are antiparallel (Fig. 1.5).
Molecules of different tRNAs differ from each other in their nucleotide sequence, but their tertiary structure is very similar. The molecule is laid out in such a way that it resembles the letter G in shape. The acceptor and T-stems are arranged in space in a special way and form one continuous spiral - the “crossbar” of the letter G; The anticodon and D stems form a “pedicle”. The correct spatial arrangement of tRNA molecules is of great importance for their functioning.
Quantitatively, ribosomal RNA predominates in the cell, but its diversity compared to other types of RNA is the least: rRNA accounts for up to 80% of the mass of cellular RNA, and is represented by three to four species. At the same time, the mass of almost 100 types of tRNA is about 15%, and the share of several thousand different mRNAs is less than 5% of the mass of cellular RNA.
In E. coli cells, 3 types of rRNA were found: 5 S, 16 S and 23 S, and in eukaryotic cells 18 S-, 5.8 S-, 28 S- and 5 S-rRNA function. These types of rRNA are part of ribosomes and make up approximately 65% of their mass. As part of ribosomes, rRNAs are tightly packed and are capable of folding to form stems with paired bases, similar to those in tRNA. It is believed that rRNAs are involved in the binding of the ribosome to tRNA. In particular, it has been shown that 5 S-rRNA interacts with the T-arm of tRNA.
In addition to the listed types of RNA, heterogeneous nuclear RNAs and small nuclear RNAs are found in the nuclei of eukaryotes. hnRNA accounts for less than 2% of the total amount of cellular RNA. These molecules are capable of rapid transformations - for most of them the half-life does not exceed 10 minutes. One of the few identified functions of hnRNA is its role as a precursor to mRNA. snRNA
are associated with a number of proteins and form the so-called small nuclear ribonucleoprotein particles(snRNP) carrying out splicing RNA (Chapter 3).
Evidence for the genetic role of DNA
The name “nucleic acids” comes from the Latin word “nucleus”, i.e. core. They were first discovered in 1868 by I.F. Miescher in the nuclei of leukocytes.
Experiments in the 1940s and 1950s convincingly proved that it is nucleic acids (and not proteins, as many assumed) that are the carriers of hereditary information in all organisms. These experiments revealed the biological nature of the phenomena transformations and transductions, at the level of microorganisms, mechanisms of interaction between organisms and cells.
Transformation(from Latin transformation - transformation, change) - a change in the hereditary properties of a bacterial cell as a result of the penetration of foreign DNA into it. First discovered in 1928 by F. Griffiths. Griffiths discovered that when mice were injected simultaneously with two strains of pneumococci (the R-strain, non-virulent, and the S-strain, virulent, but killed by heat), after a few days they died and virulent pneumococci of the S-strain were found in their blood (Fig. 7.1 .).
THIS. Avery, together with his colleagues (1944), established that DNA molecules are the factor that transforms non-pathogenic bacteria into pathogenic ones.
With the discovery and study of transformation, it became clear that DNA is the material carrier of hereditary information. Transformation is also possible in cells of higher organisms.
Transduction (from Latin transductio - movement) - the transfer by a bacteriophage of DNA fragments from one bacterial cell to another, which leads to a change in the hereditary properties of the cell. The introduced information during the process of DNA replication is transmitted through a series of cell generations of the bacterium.
The phenomenon of transduction is a confirmation of the genetic role of DNA, and is also used to study the structure of chromosomes, the structure of genes, and is one of the methods of genetic engineering.
Fig.7. 1. Schematic representation of Griffiths’ experiment: a – a mouse injected with a culture of a pathogenic encapsulated strain of S-pneumococci dies; b – a mouse injected with a culture of a non-pathogenic non-capsular R-mutant does not die; c – a mouse that received an injection of a heat-killed S-strain culture does not die; A mouse that received a mixture of a live R-mutant culture and a heat-killed S-strain culture by injection dies.
One more proof that nucleic acids, and not proteins, are the material substrate of genetic information were the experiments of H. Frenkel-Konrath (1950) with the tobacco mosaic virus (TMV).
Scheme of experiments by H. Frenkel-Konrath
Thus, with the discovery of the chemical nature of transformation and transduction factors in bacteria and the mechanisms of interaction between the virus and the cell, the role of nucleic acids in the transmission of hereditary information was proven.
Nucleic acid structure
Nucleic acids are polymers whose monomers are nucleotides. The nucleotide includes a nitrogenous base, a pentose carbohydrate and a phosphoric acid residue (Fig. 7.2.).
| Nitrogen base |
| pentose |
| 2" |
| 4" |
| 5" |
| 1 " |
| 3" |
Fig.7.2. Nucleotide structure
The nitrogenous bases of nucleotides are divided into two types: pyrimidine(consist of one 6-membered ring) and purine(consist of two fused 5- and 6-membered rings). Each carbon atom of the base rings has its own specific number, but with a prime index (′). In a nucleotide, the nitrogenous base is always attached to the first atom of the pentose carbohydrate.
It is the nitrogenous bases that determine the unique structure of DNA and RNA molecules. There are 5 main types of nitrogenous bases in nucleic acids (purine - adenine and guanine, pyrimidine - thymine, cytosine, uracil) and more than 50 rare (atypical) bases. Basic nitrogenous bases are designated by initial letters A, G, T, C, U. Nucleotides are named according to the nitrogenous bases they contain (Table 7.1.).
Table 7.1. Types of nitrogenous bases, nucleosides and nucleotides of RNA and DNA
| Names of nitrogenous bases | Nucleosides | Nucleotides | Abbreviated designations nucleotides | ||||
| Full | Abbreviated in Russian. and English.. | ||||||
| RNA | |||||||
| Purine: | |||||||
| Adenine | (A; A) | Adenosine | Adenylic acid (adenosine-5"-phosphate) | AMF | |||
| Guanine | (G;G) | Guanosine | Guanilic acid (guanosine 5"-phosphate") | GMF | |||
| Pyrimidine: | |||||||
| Cytosine | (C; C) | Cytidine | Cytidylic acid (cytidine 5"-phosphate) | CMF | |||
| Uracil | (U; U) | Uridine | Uridylic acid (uridine-5"-phosphate) | UMF | |||
| DNA | |||||||
| Purine: | |||||||
| Adenine | (A, A) | Deoxy-adenosine | Deoxyadenylic acid (deoxyadenosine-5-phosphate) | dAMP | |||
| Guanine | (G;G) | Deoxy-guanosine | Deoxyguanylic acid (deoxyguanosine-5-phosphate) | dGMP | |||
| Pyrimidine: | |||||||
| Cytosine | (C; C) | Deoxycytidine | Deoxycytidylic acid (deoxycytidine-5"-phosphate) | dCMF | |||
| Timin | (T; T) | Thymidine | Thymidylic acid (thymidine-5"-phosphate) | TMF | |||
The formation of a linear polynucleotide chain occurs through the formation phosphodiester bond pentoses of one nucleotide with a phosphate of another. The pentose phosphate backbone consists of (5′-3′) bonds. The terminal nucleotide at one end of the chain always has a free 5′ group, and at the other - a 3′ group.
Fig.7.3. Formation of polypeptide chains of DNA and RNA molecules
There are two types of nucleic acids found in nature: DNA and RNA. In prokaryotic and eukaryotic organisms, genetic functions are performed by both types of nucleic acids. Viruses always contain only one type of nucleic acid.
The main differences between DNA and RNA are presented in Table 7.2.
Table 7.2. Characteristics of nucleic acids
| Characteristic | DNA | RNA |
| Structure | double helix | different for different RNAs |
| Number of circuits | two | one |
| Nitrogen bases in nucleotides | adenine (A), guanine (G), cytosine (C), thymine (T) | adenine (A), guanine (G), cytosine (C), uracil (U) |
| Monosaccharides in nucleotides | deoxyribose | ribose |
| Synthesis method | Doubling according to the principle of complementarity. Each new double helix contains one old and one new synthesized strand | Template synthesis based on the principle of complementarity on one of the DNA strands |
| Functions | Preservation and transmission of genetic information over generations | Participates in protein synthesis; m-RNA (matrix) – transmits information about the structure of the protein from DNA to the place of its synthesis; r-RNA (ribosomal) - part of the structure of ribosomes on which protein is synthesized; t-RNA (transport) – transports amino acid molecules to ribosomes. |
DNA
nitrogenous base:
adenine, guanine, thymine , cytosine
carbohydrate: deoxyribose C 5 H 10 O 4
phosphoric acid residue
RNA
nitrogenous base:
adenine, guanine, thymine, uracil
carbohydrate: ribose C 5 H 10 O 5
phosphoric acid residue
Deoxyribonucleic acid (DNA)
In 1951, E. Chargaf formulated rules of DNA nucleotide composition:
1. Cells of different tissues of the body have the same nucleotide composition of DNA.
2. Organisms of the same species have different nucleotide compositions.
3. In a DNA molecule A=T and G=C, in turn A+G = T+C. For each type of organism, the ratio A + G / T + C is specific (in humans this ratio is 1.52).
These rules became the key to unlocking the macromolecular structure of DNA.
The structure of the DNA molecule was first deciphered by J. Watson and F. Crick in 1953. According to their model, DNA consists of two polynucleotide chains, helically twisted relative to each other.
The monomers of these chains are nucleotides. Nucleotides are joined in a chain by forming phosphodiester (covalent) bonds between the deoxyribose of one nucleotide and the phosphoric acid residue of another, neighboring nucleotide (Fig. 7.4.).
Two polynucleotide chains are combined into a DNA molecule using hydrogen bonds between the nitrogenous bases of nucleotides of different chains. Nitrogenous bases are connected according to the principle of complementarity. (adenine connects to thymine using two hydrogen bonds, and guanine connects to cytosine using three)
Fig.7.4. Principle of complementarity
The principle of complementarity is one of the fundamental laws of living nature, which determines the mechanism of transmission of hereditary information.
Polynucleotide chains of one molecule are antiparallel, i.e. Opposite the 3′ end of one chain is the 5′ end of the other chain.
Although there are only 4 types of different nucleotides in a DNA molecule, due to their different sequences and huge numbers in the polypeptide chain, an incredible variety of DNA molecules is achieved.
A violation in the sequence of nucleotides in a DNA chain leads to hereditary changes in the human body - mutations. DNA is accurately reproduced during cell division, which ensures the transmission of hereditary characteristics and properties over a number of generations and cells.
The discovery of the DNA double helix was one of the most remarkable events in the history of biology. Only five years later the first experimental confirmation of the DNA model was obtained in the works of M. Meselson and F. Stahl. After these discoveries, the time came for unprecedented progress in understanding the greatest secret of nature - the implementation of hereditary information. The era of molecular biology has begun.
Species specificity of DNA
Representatives of different species differ in the ratio of (A + T) and (G + C). In animals, the A+T pair predominates; in microorganisms, the ratio (A+T) and (G+C) is the same. This is the species specificity of DNA. This indicator is used as one of the genetic criteria for determining the species.
Structural levels of DNA
DNA is divided into primary, secondary and tertiary structure.
Primary structure is a sequence of nucleotides in a polynucleotide chain.
Secondary structure is a double helix of polynucleotide chains connected by hydrogen bonds.
There are several types of DNA helices. Under normal physiological conditions, the most common right-handed helix is the B-form. This is the standard Watson–Crick structure. The helix diameter is 2 nm, the helix pitch is 3.4 nm, each turn of the helix contains 10 base pairs.
Along with the B-form, DNA sections were found that have a different configuration, both right-handed (A- and C-forms) and left-handed (Z-form).
A-form - a full turn of the helix is 2.8 2.8 nm, one turn has 11 pairs of nitrogenous bases. DNA in this form acts as a template during replication.
The C form has 9 base pairs per turn of the helix. The Ζ form is a left-handed helix that has 12 base pairs per turn. The letter Z indicates the zigzag shape of the sugar-phosphate backbone of DNA. In a cell, DNA is usually in the B-form, but individual sections can be in the A-Z - or even another configuration due to DNA supercoiling. The conformation of DNA molecules depends on conditions and is one of the levers of influence on the functioning of genes.
Tertiary structure – This three-dimensional the DNA superhelix is characteristic of eukaryotic chromosomes and is caused by the interaction of DNA with nuclear proteins. In most prokaryotes, some viruses, as well as in the mitochondria and chloroplasts of eukaryotes, DNA is not associated with proteins.
The main properties of DNA are its ability to replicate and repair
DNA replication
Replication (autoreproduction, autosynthesis, reduplication) is the doubling of DNA molecules with the participation of special enzymes. It occurs before each nuclear division in the S-period of interphase. Reduplication ensures the accurate transmission of genetic information contained in DNA molecules from generation to generation.
The giant DNA molecules of eukaryotes have many replication sites - replicons, while the relatively small circular DNA molecules of prokaryotes each represent one replicon. The polyreplicative nature of the huge DNA molecules of eukaryotes allows replication without simultaneous decoiling of the entire molecule. Otherwise, in general terms, the replication processes of prokaryotes and eukaryotes are very similar.
The process of DNA replication in a replicon occurs in 3 stages, which involve several different enzymes.
First stage. DNA replication begins from a local site where the DNA double helix (under the action of the enzymes DNA helicase, DNA topoisomerase, etc.) unwinds, hydrogen bonds are broken and the chains diverge. As a result, a structure called replication fork(Fig. 7.5).
Fig.7.5. DNA replication scheme
At the second stage A typical matrix synthesis occurs. Free nucleotides are added to the formed free bonds on the mother DNA strands according to the principle of complementarity (A-T, G-C). This process occurs along the entire DNA molecule. For each daughter DNA molecule, one strand comes from the mother molecule, and the other is newly synthesized. This replication model is called semi-conservative. This stage is carried out by the enzyme DNA polymerase (several varieties are known).
Synthesis occurs differently on the two mother threads. Since synthesis is possible only in the 5′ - 3′ direction, fast synthesis occurs on one strand, and slow synthesis occurs on the other strand, in short fragments of 1000-2000 nucleotides. They are called in honor of R. Okazaki, who discovered them. fragments of Okazaki. Okazaki fragments are formed on the basis of RNA primers (RNA primers), which are synthesized using a special enzyme RNA primase. After performing its function, the RNA primer is removed, and the DNA ligase joins the Okazaki fragments and restores the primary structure of the DNA.
At the third stage The helix is twisted and the secondary structure of DNA is restored with the help of DNA gyrase.
Most enzymes involved in DNA replication operate in a multienzyme complex associated with DNA. This allows replication to occur at a tremendous speed (in prokaryotes - about 3000 nucleotide pairs (bp) per second, in eukaryotes - 100-300 bp per second).
The two new DNA molecules are exact copies of the original molecule (Fig. 7.6)
Fig.7.6. A – DNA replication; B- DNA synthesis
If during replication an erroneous nucleotide appears in the growing DNA chain, then in this situation the self-correction mechanism is activated. DNA self-correction involves correcting errors that occur during the synthesis of nucleic acid using the enzyme DNA polymerase (or a closely related enzyme, reducing endonuclease).
DNA repair
Reparation (from Latin reparation - restoration)– the process of restoring the primary structure of DNA damaged as a result of exposure to mutagenic factors.
Cells have various “repair” systems that repair DNA damage caused by radiation or chemical factors. Three main types of reparation are usually considered:
· photorepair (photoreactivation);
· excision repair;
· post-replicative repair.
The best studied is the repair of damage caused by ultraviolet rays. When exposed to ultraviolet light, dimers appear between adjacent pyrimidine bases of the same DNA strand. Most often, a T-T dimer, i.e. instead of hydrogen bonds between T and A of two nucleotide chains, T-T bonds are formed within one chain (Fig. 7.7).
Photorepair occurs when exposed to visible light. At the same time, the enzyme DNA photoligase divides the dimer into monomers and again restores the T-A hydrogen bonds between complementary chains
Excision and post-replicative reparation does not depend on light, and therefore it is called dark repair .
Excision repairconsists of recognizing DNA damage, cutting out (excision) of the damaged area, and synthesizing and inserting a new fragment.
It occurs in 4 stages:
1. Endonuclease recognizes the damaged area and breaks the DNA strand next to it.
2. Exonuclease “cuts out” the damaged area
3. DNA polymerase, based on the intact chain, which serves as a template, synthesizes a new fragment according to the principle of complementarity.
4. Ligase connects the free ends of the old part of the chain with the ends of the newly synthesized fragment.
Figure 7.7. Reparative processes. A. Excision repair (using the example of Escherichia coli). B. Post-replicative repair. In the presented example, a break in one DNA molecule is closed by SOS repair, and a mutation occurs (M). There may be a break in the second DNA molecule; is also filled by SOS repair or closed by recombination with subsequent repair synthesis, in which the intact DNA strand serves as the template. (According to Böhme, Adler, with modifications.)
Post-replicative repair turns on in cases where damage to DNA that occurred before its replication is not eliminated.
If the dimers are not eliminated, then the corresponding bases will not be able to act as a template and gaps (breaks) will appear in these places in the newly synthesized DNA. By exchanging fragments (recombination) between two double strands of DNA, replication products can form one normal double strand (post-replicative repair).
If the DNA damage is so close to each other that the gaps overlap, then another “repair” system is activated to fill the gaps - SOS reparation , capable of synthesizing a new DNA strand on a defective template. With this replication system, errors often occur and mutations .
Cell reparative systems play an important role in maintaining genetic homeostasis, structural and functional stability of living systems .
Ribonucleic acids
Ribonucleic acid is a biopolymer that consists primarily of one polynucleotide chain. The structure of nucleotides in RNA is similar to that of DNA, but there are the following differences :
1. Instead of deoxyribose, RNA nucleotides contain ribose;
2. Instead of the nitrogenous base thymine - uracil.
There are several types of RNA in a cell, which differ in molecular size, structure, location in the cell and functions.
Messenger RNA – mRNA (mRNA) It is synthesized on a section of one of the chains of the DNA molecule and transmits information about the structure of the protein from the cell nucleus to the ribosomes. It consists of 300-3000 (other authors give 300-30000) nucleotides and makes up 3-5% of the total RNA of the cell.
Like a DNA molecule, it has secondary and tertiary structures, which are formed through hydrogen bonds, hydrophobic, and electrostatic interactions.
Ribosomal RNA (rRNA) makes up 80-85% of the total RNA of the cell. Contains 3000-5000 nucleotides. Part of ribosomes. It is believed that rRNA provides a certain spatial arrangement of mRNA and tRNA during protein synthesis. Information about the structure of rRNA is contained in the region of secondary chromosome constriction.
Transfer RNA (tRNA) consists of 70-80 nucleotides and makes up 10-15% of the total RNA of the cell. The function of tRNA is the transfer of amino acids from the cytoplasm to the site of protein synthesis in ribosomes. tRNA molecules have a characteristic secondary structure called cloverleaf (Fig. 7.8).
The three-dimensional model of tRNA has a compact L-like shape. There are four loops in tRNA: an acceptor loop (serves as a site for amino acid attachment), an anticodon loop (recognizes codons in mRNA) and two side loops.
Fig.7.8. Structure of tRNA
Heterogeneous nuclear RNA– hya-RNA. It is the precursor of mRNA in eukaryotes and is converted into mRNA as a result of processing. Typically, hn-RNA is much longer than i-RNA.
Small nuclear RNA – snRNA. Takes part in the process of transformation of hRNA.
RNA primer – a tiny RNA (usually 10 nucleotides) involved in the process of DNA replication.
Biological role of RNA consists of preserving, implementing, transmitting hereditary information and ensuring the biosynthesis of proteins.
Adenosine triphosphoric acid (ATP)
ATP is a mononucleotide consisting of the nitrogenous base adenine, ribose monosaccharide and three phosphoric acid residues (Fig. 7.9). Phosphoric acid residues are connected to each other by high-energy bonds. When energy is needed, ATP is broken down to form adenosine diphosphoric acid (ADP) and a phosphorus residue. This releases energy.
ATP + H 2 O = ADP + H 3 PO 4 + 40 kJ
ADP can also break down to form AMP (adenosine monophosphoric acid) and a phosphoric acid residue.
ADP + H 2 O = AMP + H 3 PO 4 + 40 kJ
Fig.7.9. Scheme of the structure of ATP and its conversion to ADP
The reverse reactions of the conversion of AMP into ADP and ADP into ATP occur with the absorption of energy in the process of energy metabolism and photosynthesis.
ATP is a universal source of energy for all processes in the life of living organisms.