RNA biology message. Ribonucleic acids (RNA)

TO nucleic acids include high-polymer compounds that decompose during hydrolysis into purine and pyrimidine bases, pentose and phosphoric acid. Nucleic acids contain carbon, hydrogen, phosphorus, oxygen and nitrogen. There are two classes of nucleic acids: ribonucleic acids (RNA) And deoxyribonucleic acids (DNA).

Structure and functions of DNA

DNA- a polymer whose monomers are deoxyribonucleotides. A model of the spatial structure of the DNA molecule in the form of a double helix was proposed in 1953 by J. Watson and F. Crick (to build this model they used the work of M. Wilkins, R. Franklin, E. Chargaff).

DNA molecule formed by two polynucleotide chains, helically twisted around each other and together around an imaginary axis, i.e. is a double helix (with the exception that some DNA-containing viruses have single-stranded DNA). The diameter of the DNA double helix is ​​2 nm, the distance between adjacent nucleotides is 0.34 nm, and there are 10 nucleotide pairs per turn of the helix. The length of the molecule can reach several centimeters. Molecular weight - tens and hundreds of millions. The total length of DNA in the nucleus of a human cell is about 2 m. In eukaryotic cells, DNA forms complexes with proteins and has a specific spatial conformation.

DNA monomer - nucleotide (deoxyribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of nucleic acids belong to the classes of pyrimidines and purines. DNA pyrimidine bases(have one ring in their molecule) - thymine, cytosine. Purine bases(have two rings) - adenine and guanine.

The DNA nucleotide monosaccharide is deoxyribose.

The name of a nucleotide is derived from the name of the corresponding base. Nucleotides and nitrogenous bases are indicated by capital letters.

The polynucleotide chain is formed as a result of nucleotide condensation reactions. In this case, between the 3"-carbon of the deoxyribose residue of one nucleotide and the phosphoric acid residue of another, phosphoester bond(belongs to the category of strong covalent bonds). One end of the polynucleotide chain ends with a 5" carbon (called the 5" end), the other ends with a 3" carbon (3" end).

Opposite one strand of nucleotides is a second strand. The arrangement of nucleotides in these two chains is not random, but strictly defined: thymine is always located opposite the adenine of one chain in the other chain, and cytosine is always located opposite guanine, two hydrogen bonds arise between adenine and thymine, and three hydrogen bonds arise between guanine and cytosine. The pattern according to which the nucleotides of different DNA chains are strictly ordered (adenine - thymine, guanine - cytosine) and selectively connect with each other is called the principle of complementarity. It should be noted that J. Watson and F. Crick came to understand the principle of complementarity after familiarizing themselves with the works of E. Chargaff. E. Chargaff, having studied a huge number of samples of tissues and organs of various organisms, found that in any DNA fragment the content of guanine residues always exactly corresponds to the content of cytosine, and adenine to thymine ( "Chargaff's rule"), but he could not explain this fact.

From the principle of complementarity it follows that the nucleotide sequence of one chain determines the nucleotide sequence of the other.

The DNA strands are antiparallel (multidirectional), i.e. nucleotides of different chains are located in opposite directions, and, therefore, opposite the 3" end of one chain is the 5" end of the other. The DNA molecule is sometimes compared to a spiral staircase. The “railing” of this staircase is a sugar-phosphate backbone (alternating residues of deoxyribose and phosphoric acid); “steps” are complementary nitrogenous bases.

Function of DNA- storage and transmission of hereditary information.

DNA replication (reduplication)

- the process of self-duplication, the main property of the DNA molecule. Replication belongs to the category of matrix synthesis reactions and occurs with the participation of enzymes. Under the action of enzymes, the DNA molecule unwinds, and a new chain is built around each chain, acting as a template, according to the principles of complementarity and antiparallelism. Thus, in each daughter DNA, one strand is the mother strand, and the second is newly synthesized. This synthesis method is called semi-conservative.

The “building material” and source of energy for replication are deoxyribonucleoside triphosphates(ATP, TTP, GTP, CTP) containing three phosphoric acid residues. When deoxyribonucleoside triphosphates are incorporated into a polynucleotide chain, two terminal phosphoric acid residues are cleaved off, and the released energy is used to form a phosphodiester bond between nucleotides.

The following enzymes are involved in replication:

1. helicases (“unwind” DNA);
2. destabilizing proteins;
3. DNA topoisomerases (cut DNA);
4. DNA polymerases (select deoxyribonucleoside triphosphates and complementarily attach them to the DNA template strand);
5. RNA primases (form RNA primers);
6. DNA ligases (link DNA fragments together).

With the help of helicases, DNA is unraveled in certain sections, single-stranded sections of DNA are bound by destabilizing proteins, and a replication fork. With a divergence of 10 nucleotide pairs (one turn of the helix), the DNA molecule must make a full revolution around its axis. To prevent this rotation, DNA topoisomerase cuts one strand of DNA, allowing it to rotate around the second strand.

DNA polymerase can attach a nucleotide only to the 3" carbon of the deoxyribose of the previous nucleotide, therefore this enzyme is able to move along the template DNA in only one direction: from the 3" end to the 5" end of this template DNA. Since in the mother DNA the chains are antiparallel , then on its different chains the assembly of daughter polynucleotide chains occurs differently and in opposite directions. On chain 3"-5", the synthesis of the daughter polynucleotide chain proceeds without interruption; this daughter chain will be called leading. On a 5"-3" chain - intermittently, in fragments ( fragments of Okazaki), which, after completion of replication, are stitched into one strand by DNA ligases; this child chain will be called lagging (lagging behind).

A special feature of DNA polymerase is that it can begin its work only with "seeds" (primer). The role of “primers” is performed by short RNA sequences formed by the enzyme RNA primase and paired with template DNA. RNA primers are removed after completion of the assembly of polynucleotide chains.

Replication proceeds similarly in prokaryotes and eukaryotes. The rate of DNA synthesis in prokaryotes is an order of magnitude higher (1000 nucleotides per second) than in eukaryotes (100 nucleotides per second). Replication begins simultaneously in several parts of the DNA molecule. A fragment of DNA from one origin of replication to another forms a replication unit - replicon.

Replication occurs before cell division. Thanks to this ability of DNA, hereditary information is transferred from the mother cell to the daughter cells.

Reparation (“repair”)

Reparations is the process of eliminating damage to the DNA nucleotide sequence. Carried out by special enzyme systems of the cell ( repair enzymes). In the process of restoring the DNA structure, the following stages can be distinguished: 1) DNA repair nucleases recognize and remove the damaged area, as a result of which a gap is formed in the DNA chain; 2) DNA polymerase fills this gap, copying information from the second (“good”) strand; 3) DNA ligase “crosslinks” nucleotides, completing repair.

Three repair mechanisms have been most studied: 1) photorepair, 2) excisional, or pre-replicative, repair, 3) post-replicative repair.

Changes in the DNA structure occur in the cell constantly under the influence of reactive metabolites, ultraviolet radiation, heavy metals and their salts, etc. Therefore, defects in repair systems increase the rate of mutation processes and cause hereditary diseases (xeroderma pigmentosum, progeria, etc.).

Structure and functions of RNA

- a polymer whose monomers are ribonucleotides. Unlike DNA, RNA is formed not by two, but by one polynucleotide chain (with the exception that some RNA-containing viruses have double-stranded RNA). RNA nucleotides are capable of forming hydrogen bonds with each other. RNA chains are much shorter than DNA chains.

RNA monomer - nucleotide (ribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of RNA also belong to the classes of pyrimidines and purines.

The pyrimidine bases of RNA are uracil and cytosine, and the purine bases are adenine and guanine. The RNA nucleotide monosaccharide is ribose.

Highlight three types of RNA: 1) informational(messenger) RNA - mRNA (mRNA), 2) transport RNA - tRNA, 3) ribosomal RNA - rRNA.

All types of RNA are unbranched polynucleotides, have a specific spatial conformation and take part in the processes of protein synthesis. Information about the structure of all types of RNA is stored in DNA. The process of synthesizing RNA on a DNA template is called transcription.

Transfer RNAs usually contain 76 (from 75 to 95) nucleotides; molecular weight - 25,000-30,000. tRNA accounts for about 10% of the total RNA content in the cell. Functions of tRNA: 1) transport of amino acids to the site of protein synthesis, to ribosomes, 2) translational intermediary. There are about 40 types of tRNA found in a cell, each of them has a unique nucleotide sequence. However, all tRNAs have several intramolecular complementary regions, due to which the tRNAs acquire a clover-leaf-like conformation. Any tRNA has a loop for contact with the ribosome (1), an anticodon loop (2), a loop for contact with the enzyme (3), an acceptor stem (4), and an anticodon (5). The amino acid is added to the 3" end of the acceptor stem. Anticodon- three nucleotides that “identify” the mRNA codon. It should be emphasized that a specific tRNA can transport a strictly defined amino acid corresponding to its anticodon. The specificity of the connection between amino acid and tRNA is achieved due to the properties of the enzyme aminoacyl-tRNA synthetase.

Ribosomal RNA contain 3000-5000 nucleotides; molecular weight - 1,000,000-1,500,000. rRNA accounts for 80-85% of the total RNA content in the cell. In complex with ribosomal proteins, rRNA forms ribosomes - organelles that carry out protein synthesis. In eukaryotic cells, rRNA synthesis occurs in the nucleoli. Functions of rRNA: 1) a necessary structural component of ribosomes and, thus, ensuring the functioning of ribosomes; 2) ensuring the interaction of the ribosome and tRNA; 3) initial binding of the ribosome and the initiator codon of the mRNA and determination of the reading frame, 4) formation of the active center of the ribosome.

Messenger RNAs varied in nucleotide content and molecular weight (from 50,000 to 4,000,000). mRNA accounts for up to 5% of the total RNA content in the cell. Functions of mRNA: 1) transfer of genetic information from DNA to ribosomes, 2) matrix for the synthesis of a protein molecule, 3) determination of the amino acid sequence of the primary structure of a protein molecule.

Structure and functions of ATP

Adenosine triphosphoric acid (ATP)- a universal source and main energy accumulator in living cells. ATP is found in all plant and animal cells. The amount of ATP is on average 0.04% (of the wet weight of the cell), the largest amount of ATP (0.2-0.5%) is found in skeletal muscles.

ATP consists of residues: 1) a nitrogenous base (adenine), 2) a monosaccharide (ribose), 3) three phosphoric acids. Since ATP contains not one, but three phosphoric acid residues, it belongs to ribonucleoside triphosphates.

Most of the work that happens in cells uses the energy of ATP hydrolysis. In this case, when the terminal residue of phosphoric acid is eliminated, ATP transforms into ADP (adenosine diphosphoric acid), and when the second phosphoric acid residue is eliminated, it turns into AMP (adenosine monophosphoric acid). The free energy yield upon elimination of both the terminal and second residues of phosphoric acid is 30.6 kJ. The elimination of the third phosphate group is accompanied by the release of only 13.8 kJ. The bonds between the terminal and second, second and first residues of phosphoric acid are called high-energy (high-energy).

ATP reserves are constantly replenished. In the cells of all organisms, ATP synthesis occurs in the process of phosphorylation, i.e. addition of phosphoric acid to ADP. Phosphorylation occurs with varying intensity during respiration (mitochondria), glycolysis (cytoplasm), and photosynthesis (chloroplasts).

ATP is the main link between processes accompanied by the release and accumulation of energy, and processes occurring with energy expenditure. In addition, ATP, along with other ribonucleoside triphosphates (GTP, CTP, UTP), is a substrate for RNA synthesis.

Go to lectures No. 3“Structure and functions of proteins. Enzymes"

Go to lectures No. 5"Cell theory. Types of cellular organization"

Is the synthesis of a protein molecule based on messenger RNA (translation). However, unlike transcription, a nucleotide sequence cannot be translated into an amino acid sequence directly, since these compounds have different chemical natures. Therefore, translation requires an intermediary in the form of transfer RNA (tRNA), whose function is to translate the genetic code into the “language” of amino acids.

General characteristics of transfer RNA

Transfer RNAs or tRNAs are small molecules that deliver amino acids to the site of protein synthesis (ribosomes). The amount of this type of ribonucleic acid in the cell is approximately 10% of the total RNA pool.

Like other types of tRNA, it consists of a chain of ribonucleoside triphosphates. The length of the nucleotide sequence is 70-90 units, and about 10% of the composition of the molecule is made up of minor components.

Due to the fact that each amino acid has its own transporter in the form of tRNA, the cell synthesizes a large number of varieties of this molecule. Depending on the type of living organism, this indicator varies from 80 to 100.

Functions of tRNA

Transfer RNA is the supplier of substrate for protein synthesis, which occurs in ribosomes. Due to the unique ability to bind to both amino acids and the template sequence, tRNA performs the function of a sense adapter when translating genetic information from RNA form to protein form. The interaction of such an intermediary with the coding matrix, as in transcription, is based on the principle of complementarity of nitrogenous bases.

The main function of tRNA is to accept amino acid units and transport them to the protein synthesis apparatus. Behind this technical process there is a huge biological meaning - the implementation of the genetic code. The implementation of this process is based on the following features:

• all amino acids are encoded by nucleotide triplets;
• for each triplet (or codon) there is an anticodon that is part of the tRNA;
• each tRNA can only bind to a specific amino acid.

Thus, the amino acid sequence of a protein is determined by which tRNAs and in what order will complementarily interact with the messenger RNA during translation. This is possible due to the presence of functional centers in transfer RNA, one of which is responsible for the selective addition of an amino acid, and the other for binding to a codon. Therefore, the functions are closely interrelated.

Structure of transfer RNA

The uniqueness of tRNA is that its molecular structure is not linear. It includes double-stranded helical regions called stems and 3 single-stranded loops. This conformation is shaped like a clover leaf.

The following stems are distinguished in the structure of tRNA:

• acceptor;
• anticodon;
• dihydrouridyl;
• pseudouridyl;
• additional.

The double helices of the stems contain from 5 to 7 Watson-Crickson pairs. At the end of the acceptor stem there is a small chain of unpaired nucleotides, the 3-hydroxyl of which is the site of attachment of the corresponding amino acid molecule.

The structural region for connection with mRNA is one of the tRNA loops. It contains an anticodon, complementary to the sense triplet. It is the anticodon and the acceptor end that provide the adapter function of tRNA.

Tertiary structure of the molecule

The “clover leaf” is the secondary structure of tRNA, but due to folding the molecule acquires an L-shaped conformation, which is held together by additional hydrogen bonds.

The L-form is the tertiary structure of tRNA and consists of two almost perpendicular A-RNA helices, having a length of 7 nm and a thickness of 2 nm. This form of the molecule has only 2 ends, on one of which there is an anticodon, and on the other there is an acceptor center.

Features of binding of tRNA to amino acid

Activation of amino acids (their addition to transfer RNA) is carried out by aminoacyl-tRNA synthetase. This enzyme simultaneously performs 2 important functions:

• catalyzes the formation of a covalent bond between the 3'-hydroxyl group of the acceptor stem and the amino acid;
• provides the principle of selective matching.

Each of them has its own aminoacyl-tRNA synthetase. It can only interact with the appropriate type of transport molecule. This means that the anticodon of the latter must be complementary to the triplet encoding this particular amino acid. For example, leucine synthetase will only bind to leucine-targeted tRNA.

The aminoacyl-tRNA synthetase molecule has three nucleotide-binding pockets, the conformation and charge of which are complementary to the nucleotides of the corresponding anticodon in the tRNA. Thus, the enzyme determines the desired transport molecule. Much less frequently, the recognition fragment is the nucleotide sequence of the acceptor stem.

Structure

Nitrogenous bases in RNA can form hydrogen bonds between cytosine and guanine, adenine and uracil, and between guanine and uracil. However, other interactions are also possible, for example, several adenines can form a loop, or a loop consisting of four nucleotides, in which there is an adenine-guanine base pair. An important structural feature of RNA that distinguishes it from DNA is the presence of a hydroxyl group at the 2" position of ribose, which allows the RNA molecule to exist in the A rather than the B conformation most often observed in DNA. The A form has a deep and narrow major groove and a shallow and wide minor groove. The second consequence of the presence of a 2" hydroxyl group is that conformationally plastic , that is, sections of the RNA molecule that do not take part in the formation of the double helix can chemically attack other phosphate bonds and break them down. The “working” form of a single-stranded RNA molecule, like that of proteins, often has a tertiary structure. The tertiary structure is formed on the basis of elements of the secondary structure formed by hydrogen bonds within a single molecule. There are several types of secondary structure elements - stem-loops, loops and pseudoknots. Due to the large number of possible base pairings, predicting the secondary structure of RNA is a much more difficult task than predicting the secondary structure of proteins, but there are now efficient programs such as mfold.

An example of the dependence of the function of RNA molecules on their secondary structure is the internal ribosome entry sites (IRES). IRES is a structure at the 5" end of messenger RNA that ensures the attachment of a ribosome, bypassing the usual mechanism for initiating protein synthesis, which requires the presence of a special modified base (cap) at the 5" end and protein initiation factors. IRES were initially discovered in viral RNAs, but there is growing evidence that cellular mRNAs also use an IRES-dependent initiation mechanism under stress conditions.

Many types of RNA, such as rRNA and snRNA, function in cells in the form of complexes with proteins that associate with RNA molecules after their synthesis or (in eukaryotes) export from the nucleus to the cytoplasm. Such RNA-protein complexes are called ribonucleoprotein complexes or ribonucleoproteins.

Comparison with DNA

There are three main differences between DNA and RNA:

DNA contains the sugar deoxyribose, RNA contains ribose, which has an additional hydroxyl group compared to deoxyribose. This group increases the likelihood of hydrolysis of the molecule, that is, it reduces the stability of the RNA molecule.

The nucleotide complementary to adenine in RNA is not thymine, as in DNA, but uracil is the unmethylated form of thymine.

DNA exists in the form of a double helix, consisting of two separate molecules. RNA molecules are, on average, much shorter and predominantly single-stranded.

Structural analysis of biologically active RNA molecules, including tRNA, rRNA, snRNA and other molecules that do not code for proteins, showed that they do not consist of one long helix, but of numerous short helices located close to each other and forming something similar to tertiary structure of the protein. As a result, RNA can catalyze chemical reactions, for example, the peptidyl transferase center of the ribosome, involved in the formation of peptide bonds in proteins, consists entirely of RNA.

Messenger (information) RNA - RNA, which serves as an intermediary in the transmission of information encoded in DNA to ribosomes, molecular machines that synthesize proteins in a living organism. The coding sequence of mRNA determines the amino acid sequence of the polypeptide chain of a protein. However, the vast majority of RNAs do not code for protein. These noncoding RNAs can be transcribed from individual genes (eg, ribosomal RNAs) or derived from introns. The classic, well-studied types of non-coding RNAs are transfer RNAs ( tRNA) and rRNA, which are involved in the translation process. There are also classes of RNA responsible for gene regulation, mRNA processing, and other roles. In addition, there are non-coding RNA molecules that can catalyze chemical reactions, such as cutting and ligation of RNA molecules. By analogy with proteins capable of catalyzing chemical reactions - enzymes (enzymes), catalytic RNA molecules are called ribozymes.

Participating in the broadcast

Information about the amino acid sequence of a protein is contained in mRNA. Three consecutive nucleotides (codon) correspond to one amino acid. In eukaryotic cells, transcyributed precursor mRNA or pre-mRNA is processed to form mature mRNA. Processing involves the removal of non-protein-coding sequences (introns). After this, the mRNA is exported from the nucleus into the cytoplasm, where it is attached to ribosomes, which translate the mRNA using tRNAs connected to amino acids.

In anucleated cells (bacteria and archaea), ribosomes can attach to mRNA immediately after transcription of a section of RNA. In both eukaryotes and prokaryotes, the life cycle of mRNA ends with its controlled destruction by enzymes ribonucleases.

Transport (tRNA) are small molecules, consisting of approximately 80 nucleotides, with a conservative tertiary structure. They transfer specific amino acids to the site of peptide bond synthesis in the ribosome. Each tRNA contains a site for amino acid attachment and an anticodon for recognition and attachment to mRNA codons. The anticodon forms hydrogen bonds with the codon, which places the tRNA in a position that facilitates the formation of a peptide bond between the last amino acid of the peptide formed and the amino acid attached to the tRNA.

Ribosomal RNA (rRNA) is the catalytic component of ribosomes. Eukaryotic ribosomes contain four types of rRNA molecules: 18S, 5.8S, 28S and 5S. Three of the four types of rRNA are synthesized in the nucleolus. In the cytoplasm, ribosomal RNAs combine with ribosomal proteins to form a nucleoprotein called a ribosome. The ribosome attaches to the mRNA and synthesizes the protein. rRNA makes up up to 80% of the RNA found in the cytoplasm of a eukaryotic cell.

An unusual type of RNA that acts as tRNA and mRNA (tmRNA) is found in many bacteria and plastids. When the ribosome stops at defective mRNAs without stop codons, tmRNA attaches a small peptide that directs the protein to degradation.

FUNCTIONS OF RNA

There are different types of RNA. Messenger (information) RNA - RNA, which serves as an intermediary in the transmission of information encoded in DNA to ribosomes, molecular machines that synthesize proteins in a living organism. Transfer RNAs (tRNAs) and rRNAs that are involved in the translation process. There are also classes of RNA responsible for gene regulation, mRNA processing, and other roles. In addition, there are non-coding RNA molecules that can catalyze chemical reactions, such as cutting and ligation of RNA molecules.
To summarize all of the above, it can be noted that the functions of RNA:
- storage of hereditary information (for some viruses);
- participation in the process of translation and transcription (protein synthesis);
- regulation of gene activity;
- catalyzing a number of chemical reactions

there are three types of RNA in a cell

Transfer RNA (tRNA). Transfer RNA is mainly found in the cytoplasm of the cell. The function is to transfer amino acids to ribosomes, to the site of protein synthesis. Of the total RNA content of a cell, t-RNA accounts for about 10%.

Ribosomal RNA (r-RNA). Ribosomal RNA constitutes an essential part of the structure of the ribosome. Of the total RNA content in a cell, r-RNA accounts for about 90%.

Messenger RNA (i-RNA), or messenger RNA (m-RNA). Contained in the nucleus and cytoplasm. Its function is to transfer information about the structure of the protein from DNA to the site of protein synthesis in ribosomes. mRNA accounts for approximately 0.5-1% of the total RNA content of the cell.

The cytoplasm of cells contains three main functional types of RNA:

• messenger RNAs (mRNAs), which act as templates for protein synthesis;
• ribosomal RNAs (rRNAs), which act as structural components of ribosomes;
• transfer RNAs (tRNAs) involved in the translation (translation) of mRNA information into the amino acid sequence of a protein molecule.

Nuclear RNA is found in the cell nucleus, accounting for 4 to 10% of total cellular RNA. The bulk of nuclear RNA is represented by high-molecular-weight precursors of ribosomal and transfer RNA. Precursors of high molecular weight rRNAs (28 S, 18 S and 5 S RNAs) are mainly localized in the nucleolus.

RNA is basic genetic material in some animal and plant viruses (genomic RNA). Most RNA viruses are characterized by reverse transcription of their RNA genome, directed by reverse transcriptase.

All ribonucleic acids are ribonucleotide polymers, connected, as in a DNA molecule, by 3",5"-phosphorodiester bonds. Unlike DNA, which has a double-stranded structure, RNA is single-chain linear polymer molecules.

Structure of mRNA. mRNA is the most heterogeneous class of RNA in terms of size and stability. The content of mRNA in cells is 2-6% of the total amount of RNA. mRNAs consist of sections called cistrons that determine the sequence of amino acids in the proteins they encode.

Structure of tRNA . Transfer RNAs act as intermediaries (adapters) during the translation of mRNA. They account for approximately 15% of total cellular RNA. Each of the 20 proteinogenic amino acids has its own tRNA. For some amino acids encoded by two or more codons, there are several tRNAs. tRNAs are relatively small single-stranded molecules consisting of 70-93 nucleotides. Their molecular weight is (2.4-3.1).104 kDa.

Secondary structure of tRNA is formed due to the formation of the maximum number of hydrogen bonds between intramolecular complementary pairs of nitrogenous bases. As a result of the formation of these bonds, the tRNA polynucleotide chain twists to form helical branches ending in loops of unpaired nucleotides. The spatial representation of the secondary structures of all tRNAs has the form clover leaf.

In the “clover leaf” there are four required branches, longer tRNAs also contain short fifth (additional) branch. The adapter function of tRNA is provided by an acceptor branch, to the 3" end of which an amino acid residue is attached by an ester bond, and an anticodon branch opposing the acceptor branch, at the top of which there is a loop containing an anticodon. An anticodon is a specific triplet of nucleotides that is complementary in the antiparallel direction to the mRNA codon, encoding the corresponding amino acid.

The T-branch, carrying a pseudouridine loop (TyC-loop), ensures the interaction of tRNA with ribosomes.

The D-branch, carrying a dehydrouridine loop, ensures the interaction of tRNA with the corresponding aminoacyl-tRNA synthetase.

Secondary structure of tRNA

The functions of the fifth additional branch have so far been little studied; most likely it equalizes the length of different tRNA molecules.

Tertiary structure of tRNA very compact and is formed by bringing together individual branches of a clover leaf through additional hydrogen bonds to form an L-shaped structure "elbow bend". In this case, the acceptor arm that binds the amino acid is located at one end of the molecule, and the anticodon at the other.

Tertiary structure of tRNA (according to A.S. Spirin)

Structure of rRNA and ribosomes . Ribosomal RNAs form the scaffold to which specific proteins bind to form ribosomes. Ribosomes- These are nucleoprotein organelles that provide protein synthesis on mRNA. The number of ribosomes in a cell is very large: from 104 in prokaryotes to 106 in eukaryotes. Ribosomes are localized mainly in the cytoplasm, in eukaryotes, in addition, in the nucleolus, in the mitochondrial matrix and the stroma of chloroplasts. Ribosomes consist of two subunits: large and small. Based on size and molecular weight, all studied ribosomes are divided into 3 groups - 70S ribosomes of prokaryotes (S-sedimentation coefficient), consisting of small 30S and large 50S subparticles; 80S ribosomes of eukaryotes, consisting of 40S small and 60S large subunits.

Small subparticle The 80S ribosome is formed by one rRNA molecule (18S) and 33 molecules of various proteins. Large subparticle formed by three rRNA molecules (5S, 5.8S and 28S) and approximately 50 proteins.

Secondary structure of rRNA is formed due to short double-stranded sections of the molecule - hairpins (about 2/3 of rRNA), 1/3 is represented single-strand sections, rich in purine nucleotides.

TO nucleic acids include high-polymer compounds that decompose during hydrolysis into purine and pyrimidine bases, pentose and phosphoric acid. Nucleic acids contain carbon, hydrogen, phosphorus, oxygen and nitrogen. There are two classes of nucleic acids: ribonucleic acids (RNA) And deoxyribonucleic acids (DNA).

Structure and functions of DNA

DNA- a polymer whose monomers are deoxyribonucleotides. A model of the spatial structure of the DNA molecule in the form of a double helix was proposed in 1953 by J. Watson and F. Crick (to build this model they used the work of M. Wilkins, R. Franklin, E. Chargaff).

DNA molecule formed by two polynucleotide chains, helically twisted around each other and together around an imaginary axis, i.e. is a double helix (with the exception that some DNA-containing viruses have single-stranded DNA). The diameter of the DNA double helix is ​​2 nm, the distance between adjacent nucleotides is 0.34 nm, and there are 10 nucleotide pairs per turn of the helix. The length of the molecule can reach several centimeters. Molecular weight - tens and hundreds of millions. The total length of DNA in the nucleus of a human cell is about 2 m. In eukaryotic cells, DNA forms complexes with proteins and has a specific spatial conformation.

DNA monomer - nucleotide (deoxyribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of nucleic acids belong to the classes of pyrimidines and purines. DNA pyrimidine bases(have one ring in their molecule) - thymine, cytosine. Purine bases(have two rings) - adenine and guanine.

The DNA nucleotide monosaccharide is deoxyribose.

The name of a nucleotide is derived from the name of the corresponding base. Nucleotides and nitrogenous bases are indicated by capital letters.

The polynucleotide chain is formed as a result of nucleotide condensation reactions. In this case, between the 3"-carbon of the deoxyribose residue of one nucleotide and the phosphoric acid residue of another, phosphoester bond(belongs to the category of strong covalent bonds). One end of the polynucleotide chain ends with a 5" carbon (called the 5" end), the other ends with a 3" carbon (3" end).

Opposite one strand of nucleotides is a second strand. The arrangement of nucleotides in these two chains is not random, but strictly defined: thymine is always located opposite the adenine of one chain in the other chain, and cytosine is always located opposite guanine, two hydrogen bonds arise between adenine and thymine, and three hydrogen bonds arise between guanine and cytosine. The pattern according to which the nucleotides of different DNA chains are strictly ordered (adenine - thymine, guanine - cytosine) and selectively connect with each other is called the principle of complementarity. It should be noted that J. Watson and F. Crick came to understand the principle of complementarity after familiarizing themselves with the works of E. Chargaff. E. Chargaff, having studied a huge number of samples of tissues and organs of various organisms, found that in any DNA fragment the content of guanine residues always exactly corresponds to the content of cytosine, and adenine to thymine ( "Chargaff's rule"), but he could not explain this fact.

From the principle of complementarity it follows that the nucleotide sequence of one chain determines the nucleotide sequence of the other.

The DNA strands are antiparallel (multidirectional), i.e. nucleotides of different chains are located in opposite directions, and, therefore, opposite the 3" end of one chain is the 5" end of the other. The DNA molecule is sometimes compared to a spiral staircase. The “railing” of this staircase is a sugar-phosphate backbone (alternating residues of deoxyribose and phosphoric acid); “steps” are complementary nitrogenous bases.

Function of DNA- storage and transmission of hereditary information.

DNA replication (reduplication)

- the process of self-duplication, the main property of the DNA molecule. Replication belongs to the category of matrix synthesis reactions and occurs with the participation of enzymes. Under the action of enzymes, the DNA molecule unwinds, and a new chain is built around each chain, acting as a template, according to the principles of complementarity and antiparallelism. Thus, in each daughter DNA, one strand is the mother strand, and the second is newly synthesized. This synthesis method is called semi-conservative.

The “building material” and source of energy for replication are deoxyribonucleoside triphosphates(ATP, TTP, GTP, CTP) containing three phosphoric acid residues. When deoxyribonucleoside triphosphates are incorporated into a polynucleotide chain, two terminal phosphoric acid residues are cleaved off, and the released energy is used to form a phosphodiester bond between nucleotides.

The following enzymes are involved in replication:

1. helicases (“unwind” DNA);
2. destabilizing proteins;
3. DNA topoisomerases (cut DNA);
4. DNA polymerases (select deoxyribonucleoside triphosphates and complementarily attach them to the DNA template strand);
5. RNA primases (form RNA primers);
6. DNA ligases (link DNA fragments together).

With the help of helicases, DNA is unraveled in certain sections, single-stranded sections of DNA are bound by destabilizing proteins, and a replication fork. With a divergence of 10 nucleotide pairs (one turn of the helix), the DNA molecule must make a full revolution around its axis. To prevent this rotation, DNA topoisomerase cuts one strand of DNA, allowing it to rotate around the second strand.

DNA polymerase can attach a nucleotide only to the 3" carbon of the deoxyribose of the previous nucleotide, therefore this enzyme is able to move along the template DNA in only one direction: from the 3" end to the 5" end of this template DNA. Since in the mother DNA the chains are antiparallel , then on its different chains the assembly of daughter polynucleotide chains occurs differently and in opposite directions. On chain 3"-5", the synthesis of the daughter polynucleotide chain proceeds without interruption; this daughter chain will be called leading. On a 5"-3" chain - intermittently, in fragments ( fragments of Okazaki), which, after completion of replication, are stitched into one strand by DNA ligases; this child chain will be called lagging (lagging behind).

A special feature of DNA polymerase is that it can begin its work only with "seeds" (primer). The role of “primers” is performed by short RNA sequences formed by the enzyme RNA primase and paired with template DNA. RNA primers are removed after completion of the assembly of polynucleotide chains.

Replication proceeds similarly in prokaryotes and eukaryotes. The rate of DNA synthesis in prokaryotes is an order of magnitude higher (1000 nucleotides per second) than in eukaryotes (100 nucleotides per second). Replication begins simultaneously in several parts of the DNA molecule. A fragment of DNA from one origin of replication to another forms a replication unit - replicon.

Replication occurs before cell division. Thanks to this ability of DNA, hereditary information is transferred from the mother cell to the daughter cells.

Reparation (“repair”)

Reparations is the process of eliminating damage to the DNA nucleotide sequence. Carried out by special enzyme systems of the cell ( repair enzymes). In the process of restoring the DNA structure, the following stages can be distinguished: 1) DNA repair nucleases recognize and remove the damaged area, as a result of which a gap is formed in the DNA chain; 2) DNA polymerase fills this gap, copying information from the second (“good”) strand; 3) DNA ligase “crosslinks” nucleotides, completing repair.

Three repair mechanisms have been most studied: 1) photorepair, 2) excisional, or pre-replicative, repair, 3) post-replicative repair.

Changes in the DNA structure occur in the cell constantly under the influence of reactive metabolites, ultraviolet radiation, heavy metals and their salts, etc. Therefore, defects in repair systems increase the rate of mutation processes and cause hereditary diseases (xeroderma pigmentosum, progeria, etc.).

Structure and functions of RNA

- a polymer whose monomers are ribonucleotides. Unlike DNA, RNA is formed not by two, but by one polynucleotide chain (with the exception that some RNA-containing viruses have double-stranded RNA). RNA nucleotides are capable of forming hydrogen bonds with each other. RNA chains are much shorter than DNA chains.

RNA monomer - nucleotide (ribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of RNA also belong to the classes of pyrimidines and purines.

The pyrimidine bases of RNA are uracil and cytosine, and the purine bases are adenine and guanine. The RNA nucleotide monosaccharide is ribose.

Highlight three types of RNA: 1) informational(messenger) RNA - mRNA (mRNA), 2) transport RNA - tRNA, 3) ribosomal RNA - rRNA.

All types of RNA are unbranched polynucleotides, have a specific spatial conformation and take part in the processes of protein synthesis. Information about the structure of all types of RNA is stored in DNA. The process of synthesizing RNA on a DNA template is called transcription.

Transfer RNAs usually contain 76 (from 75 to 95) nucleotides; molecular weight - 25,000-30,000. tRNA accounts for about 10% of the total RNA content in the cell. Functions of tRNA: 1) transport of amino acids to the site of protein synthesis, to ribosomes, 2) translational intermediary. There are about 40 types of tRNA found in a cell, each of them has a unique nucleotide sequence. However, all tRNAs have several intramolecular complementary regions, due to which the tRNAs acquire a clover-leaf-like conformation. Any tRNA has a loop for contact with the ribosome (1), an anticodon loop (2), a loop for contact with the enzyme (3), an acceptor stem (4), and an anticodon (5). The amino acid is added to the 3" end of the acceptor stem. Anticodon- three nucleotides that “identify” the mRNA codon. It should be emphasized that a specific tRNA can transport a strictly defined amino acid corresponding to its anticodon. The specificity of the connection between amino acid and tRNA is achieved due to the properties of the enzyme aminoacyl-tRNA synthetase.

Ribosomal RNA contain 3000-5000 nucleotides; molecular weight - 1,000,000-1,500,000. rRNA accounts for 80-85% of the total RNA content in the cell. In complex with ribosomal proteins, rRNA forms ribosomes - organelles that carry out protein synthesis. In eukaryotic cells, rRNA synthesis occurs in the nucleoli. Functions of rRNA: 1) a necessary structural component of ribosomes and, thus, ensuring the functioning of ribosomes; 2) ensuring the interaction of the ribosome and tRNA; 3) initial binding of the ribosome and the initiator codon of the mRNA and determination of the reading frame, 4) formation of the active center of the ribosome.

Messenger RNAs varied in nucleotide content and molecular weight (from 50,000 to 4,000,000). mRNA accounts for up to 5% of the total RNA content in the cell. Functions of mRNA: 1) transfer of genetic information from DNA to ribosomes, 2) matrix for the synthesis of a protein molecule, 3) determination of the amino acid sequence of the primary structure of a protein molecule.

Structure and functions of ATP

Adenosine triphosphoric acid (ATP)- a universal source and main energy accumulator in living cells. ATP is found in all plant and animal cells. The amount of ATP is on average 0.04% (of the wet weight of the cell), the largest amount of ATP (0.2-0.5%) is found in skeletal muscles.

ATP consists of residues: 1) a nitrogenous base (adenine), 2) a monosaccharide (ribose), 3) three phosphoric acids. Since ATP contains not one, but three phosphoric acid residues, it belongs to ribonucleoside triphosphates.

Most of the work that happens in cells uses the energy of ATP hydrolysis. In this case, when the terminal residue of phosphoric acid is eliminated, ATP transforms into ADP (adenosine diphosphoric acid), and when the second phosphoric acid residue is eliminated, it turns into AMP (adenosine monophosphoric acid). The free energy yield upon elimination of both the terminal and second residues of phosphoric acid is 30.6 kJ. The elimination of the third phosphate group is accompanied by the release of only 13.8 kJ. The bonds between the terminal and second, second and first residues of phosphoric acid are called high-energy (high-energy).

ATP reserves are constantly replenished. In the cells of all organisms, ATP synthesis occurs in the process of phosphorylation, i.e. addition of phosphoric acid to ADP. Phosphorylation occurs with varying intensity during respiration (mitochondria), glycolysis (cytoplasm), and photosynthesis (chloroplasts).

ATP is the main link between processes accompanied by the release and accumulation of energy, and processes occurring with energy expenditure. In addition, ATP, along with other ribonucleoside triphosphates (GTP, CTP, UTP), is a substrate for RNA synthesis.

Go to lectures No. 3“Structure and functions of proteins. Enzymes"

Go to lectures No. 5"Cell theory. Types of cellular organization"