4.2.1. Primary structure of nucleic acids called sequence of mononucleotides in a DNA or RNA chain . The primary structure of nucleic acids is stabilized by 3",5"-phosphodiester bonds. These bonds are formed by the interaction of the hydroxyl group in the 3 "-position of the pentose residue of each nucleotide with the phosphate group of the adjacent nucleotide (Figure 3.2),

Thus, at one end of the polynucleotide chain there is a free 5'-phosphate group (5'-end), and at the other end there is a free hydroxyl group in the 3'-position (3'-end). Nucleotide sequences are usually written in the direction from the 5" end to the 3" end.

Figure 4.2. The structure of a dinucleotide, which includes adenosine-5"-monophosphate and cytidine-5"-monophosphate.

4.2.2. DNA (deoxyribonucleic acid) is contained in the cell nucleus and has a molecular weight of about 1011 Da. Its nucleotides contain nitrogenous bases. adenine, guanine, cytosine, thymine , carbohydrate deoxyribose and phosphoric acid residues. The content of nitrogenous bases in a DNA molecule is determined by the Chargaff rules:

1) the number of purine bases is equal to the number of pyrimidine ones (A + G = C + T);

2) the amount of adenine and cytosine is equal to the amount of thymine and guanine, respectively (A = T; C = G);

3) DNA isolated from cells of different biological species differ from each other in the value of the specificity coefficient:

(G + C) / (A + T)

These patterns in the structure of DNA are explained by the following features of its secondary structure:

1) a DNA molecule is built from two polynucleotide chains interconnected by hydrogen bonds and oriented antiparallel (that is, the 3 "end of one chain is located opposite the 5" end of the other chain and vice versa);

2) hydrogen bonds are formed between complementary pairs of nitrogenous bases. Adenine is complementary to thymine; this pair is stabilized by two hydrogen bonds. Guanine is complementary to cytosine; this pair is stabilized by three hydrogen bonds (see figure b). The more G-C pairs in a DNA molecule, the greater its resistance to high temperatures and ionizing radiation;

Figure 3.3. Hydrogen bonds between complementary nitrogenous bases.

3) both DNA strands are twisted into a helix having a common axis. Nitrogenous bases face the inside of the helix; in addition to hydrogen interactions, hydrophobic interactions also arise between them. The ribose phosphate parts are located along the periphery, forming the backbone of the helix (see Figure 3.4).

Figure 3.4. Diagram of the structure of DNA.

4.2.3. RNA (ribonucleic acid) is contained mainly in the cytoplasm of the cell and has a molecular weight in the range of 104 - 106 Da. Its nucleotides contain nitrogenous bases. adenine, guanine, cytosine, uracil , carbohydrate ribose and phosphoric acid residues. Unlike DNA, RNA molecules are built from a single polynucleotide chain, which may contain sections complementary to each other (Figure 3.5). These sections can interact with each other, forming double helixes, alternating with non-spiralized sections.

Figure 3.5. Scheme of the structure of transfer RNA.

According to the features of the structure and function, three main types of RNA are distinguished:

1) messenger (messenger) RNA (mRNA) transmit information about the structure of the protein from the cell nucleus to the ribosomes;

2) transfer RNA (tRNA) carry out the transport of amino acids to the site of protein synthesis;

3) ribosomal RNA (rRNA) are part of ribosomes, participate in protein synthesis.

Deoxyribonucleic acids (DNA) are linear (or cyclic), unbranched polydeoxyribonucleotides. The structural unit of DNA is deoxyribonucleotides, namely deoxyribonucleoside monophosphates (DNMP).

DNMF are compounds consisting of a purine or pyrimidine nitrogenous base, deoxyribose, and one phosphoric acid residue.

As purine bases, DNMF includes adenine and guanine, pyrimidine bases are represented by thymine and cytosine. An important feature of the hydroxy derivatives of purine and pyrimidine is the possibility of their tautomeric (lactim-lactam) transformations. In the composition of DNA, all hydroxy derivatives of nitrogenous bases are present in the form of lactams (keto form).

Desocribonucleoside monophosphates.

Deoxyadenosine monophosphate Deoxyguanosine monophosphate

dAMP dGMP

Deoxycytidine Monophosphate Deoxythymidine Monophosphate

dCMP dTMP

In the composition of DNA, along with the indicated DNMPs, DNMPs with minor (exotic) bases are found in small amounts. Minor nitrogenous bases are methylated, hydroxymethylated or glucosylated bases resulting from the modification of the main bases in the polydeoxyribonucleotide during DNA processing (maturation). Examples of minor nitrogenous bases are:

Purine bases Pyrimidine bases

N 6 -methyladenine 5-methylcytosine

1(or 3, or 7)-methylguanine 5-hydroxymethylcytosine uranyl

N 2 -methyl (or dimethyl) -guanine hydroxymethyluracil

To study the nucleotide composition of DNA, DNA hydrolysis is used, followed by chromatography and the qualitative and quantitative determination of nitrogenous bases. Along with the classical methods of analysis, the nucleotide composition of DNA can also be determined from the melting temperature of DNA (the content of GC pairs is directly proportional to the melting temperature) and from the buoyant density of DNA during its ultracentrifugation in a cesium chloride density gradient (the content of GC pairs is directly proportional to the buoyant density).

When analyzing the nucleotide composition of the DNA of different types of organisms, a number of patterns were established that characterize the quantitative ratio of nitrogenous bases (Chargaff's rules).

1. The molar content of adenine is equal to the molar content of thymine, and the molar content of guanine is equal to the molar content of cytosine.

A = T, or A: T = 1.

G \u003d C, or G: C \u003d 1.

2. The sum of purine bases is equal to the sum of pyrimidine bases.

A + G \u003d T + C, or (A + G) : (T + C) \u003d 1.

purines = pyrimidines.

3. The nucleotide composition of the DNA of different cells of a multicellular organism is the same.

4. Each biological species is characterized by a constant specific nucleotide composition of DNA, which is reflected in the coefficient of specificity.

K = -----------;

Depending on the predominance of AT or GC, AT and GC DNA types are distinguished, respectively. The AT-type is typical, in particular, for chordates and invertebrates, higher plants, and yeasts. In different species of bacteria, there is a scatter in the nucleotide composition from a strongly pronounced GC type to the AT type. Based on the coefficient of specificity, the principles of gene systematics of objects of flora and fauna have been developed.

3.3 PRIMARY STRUCTURE OF DNA.

Deoxyribonucleic acids (DNA) are linear

(or cyclic) polydeoxyribonucleotides.

The primary structure of DNA is the sequence of alternating deoxyribonucleoside monophosphate (DNMP) residues in the polydeoxyribonucleotide chain.

The primary structure of DNA is a covalent structure, since the DNMP residues in the polydeoxyribonucleotide chain are connected to each other by 3", 5" phosphodiester bonds.

The skeleton (backbone, backbone) of a polydeoxyribonucleotide consists of monotonically alternating deoxyribose and phosphate groups attached to the backbone at equal distances from each other. The sugar-phosphate backbone of DNA, having a large negative charge, is a highly polar part of the molecule, while nitrogenous bases are non-polar, hydrophobic components.

The polydeoxyribonucleotide chain has vectority, it has a direction from the 5'-end (the beginning of the chain) to the 3'-end (the end of the chain), i.e. 5"---->3". The 5' end (phosphate end) and the 3' end (hydroxyl end) are the ends at which the 5' and 3' deoxyribose atoms, respectively, are free from the internucleotide bond. Vectority is determined by the direction of assembly of the polydeoxyribonucleotide chain.

The DNA polycondensation coefficient varies from 0.5 . 10 4 for viruses to 10 8 for nuclear DNA of higher eukaryotes. In accordance with this, the molecular weight of DNA also varies over a wide range, reaching several tens of billions of daltons in higher eukaryotes. At the same time, the number of encoded proteins in prokaryotes and eukaryotes differs by no more than an order of magnitude. This is due to both the complex organization of genes and the presence of repetitive DNA in eukaryotes.

In prokaryotes, DNA is represented by a single molecule. As species become more complex, the size and number of different DNAs increases. In eukaryotes, the number of DNA is equal to the number of chromosomes. Thus, there are 46 different DNAs in human cells.

Each DNA has a unique primary structure, and their primary structure in all cells of a multicellular organism seems to be exactly the same.

The nucleotide sequence of DNA is designated starting at the 5" end using the single letter symbols A, G, C, and T for nucleosides

(nucleotides) and f - for the phosphate group, for example: fAphTfGfGfC or fATHGC.

The complexity of studying the primary structure of DNA is due to the very long length of the polydeoxyribonucleotide chain and the presence of only four types of nucleotides. To decipher the primary structure of DNA, indirect methods were previously used:

by cohesion of purine and pyrimidine nucleotide units, elucidation of the number and structure of individual fractions of nucleotides (the so-called isoplates);

on the kinetics of DNA reassociation (presence of repeating sequences);

by distribution of minor bases;

for detection in DNA and determination of the sequence of palindromes.

Currently, direct methods are widely used, which are used in the following sequence:

cleavage with various restriction enzymes with the formation of overlapping sequences;

electrophoretic separation of DNA fragments in a polyacrylamide gel according to the number of nucleotides they contain;

deciphering the nucleotide sequence in the fragments;

establishing the order of arrangement of nucleotide fragments in overlapping areas.

FORMATION OF POLYDEOXYRIBONUCLEOTIDES.

Rice. Fragment of a polydeoxyribonucleoid chain

Nucleic acids are phosphorus-containing irregular heteropolymers. Opened in 1868 by G.F. Misher.

Nucleic acids are found in the cells of all living organisms. Moreover, each type of organism contains its own set of nucleic acids, characteristic only for it. In nature, there are more than 1,200,000 species of living organisms - from bacteria and humans. This means that there are about 10 10 different nucleic acids that are built from only four nitrogenous bases. How can four nitrogenous bases encode 10 10 nucleic acids? Approximately the same as we encode our thoughts on paper. We establish a sequence of letters of the alphabet, grouping them into words, and nature encodes hereditary information, establishing a sequence of many nucleotides.

Nucleotide - a relatively simple monomer, from the molecules of which nucleic acids are built. Each nucleotide consists of: a nitrogenous base, a five-carbon sugar (ribose or deoxyribose) and a phosphoric acid residue. The main part of a nucleotide is the nitrogenous base.

Nitrogenous bases have a cyclic structure, which, along with other atoms (C, O, H), includes nitrogen atoms. Because of this, these compounds are called nitrogenous. The most important properties of nitrogenous bases are also associated with nitrogen atoms, for example, their weakly basic (alkaline) properties. Hence, these compounds are called "bases".

In nature, nucleic acids contain only five of the known nitrogenous bases. They are found in all cell types, from mycoplasmas to human cells.

This is purine nitrogenous bases Adenine (A) and Guanine (G) and pyrimidine Uracil (U), Thymine (T) and Cytosine (C). Purine bases are derivatives of the purine heterocycle, and pyrimidine bases are derivatives of pyrimidine. Uracil is found only in RNA, while thymine is found in DNA. A, G, and C are found in both DNA and DNA.

There are two types of nucleotides in nucleic acids: deoxyribonucleotides - in DNA, ribonucleotides - in RNA. The structure of deoxyribose differs from that of ribose in that there is no hydroxyl group at the second carbon atom of deoxyribose.

As a result of the combination of a nitrogenous base and pentose, nucleoside. Nucleoside linked to a phosphoric acid residue nucleotide:

nitrogenous base + pentose = nucleoside + phosphoric acid residue = nucleotide

The ratio of nitrogenous bases in a DNA molecule is described Chargaff rules:

1. The amount of adenine is equal to the amount of thymine (A = T).

2. The amount of guanine is equal to the amount of cytosine (G = C).

3. The number of purines is equal to the number of pyrimidines (A + G = T + C), i.e. A + G / T + C \u003d 1.

4. The number of bases with six amino groups is equal to the number of bases with six keto groups (A + C = G + T).

5. The ratio of bases A + C / G + T is a constant value, strictly species-specific: man - 0.66; octopus - 0.54; mouse - 0.81; wheat - 0.94; algae - 0.64-1.76; bacteria - 0.45-2.57.

Based on E. Chargaff's data on the ratio of purine and pyrimidine bases and the results of X-ray diffraction analysis obtained by M. Wilkins and R. Franklin in 1953, J. Watson and F. Crick proposed a model of the DNA molecule. For the development of a double-stranded DNA molecule, Watson, Crick and Wilkins in 1962 were awarded the Nobel Prize.

The DNA molecule has two strands parallel to each other but in reverse order. DNA monomers are deoxyribonucleotides: adenyl (A), thymidyl (T), guanyl (G), and cytosyl (C). The chains are held together by hydrogen bonds: between A and T two, between G and C three hydrogen bonds. The double helix of the DNA molecule is twisted in the form of a spiral, and one turn includes 10 pairs of nucleotides. The coils of the helix are held together by hydrogen bonds and hydrophobic interactions. In the deoxyribose molecule, the free hydroxyl groups are in the 3' and 5' positions. At these positions, a diester bond can form between deoxyribose and phosphoric acid, which connects nucleotides to each other. In this case, one end of the DNA carries a 5'-OH group, and the other end carries a 3'-OH group. DNA is the largest organic molecules. Their length ranges from 0.25 nm to 40 mm in humans in bacteria (the length of the largest protein molecule is not more than 200 nm). The mass of a DNA molecule is 6 x 10 -12 g.

DNA postulates

1. Each DNA molecule consists of two antiparallel polynucleotide chains forming a double helix twisted (to the right or left) around the central axis. Antiparallelism is provided by connecting the 5' end of one strand to the 3' end of the other strand and vice versa.

2. Each nucleoside (pentose + base) is located in a plane perpendicular to the axis of the helix.

3. Two chains of the helix are held together by hydrogen bonds between the bases A–T (two) and G–C (three).

4. Base pairing is highly specific and occurs according to the principle of complementarity; as a result, only pairs A: T, G: C are possible.

5. The sequence of bases in one chain can vary significantly, but their sequence in another chain is strictly complementary.

DNA has unique properties of replication (the ability to self-doubling) and repair (the ability to self-repair).

DNA replication- the reaction of matrix synthesis, the process of doubling the DNA molecule by reduplication. In 1957, M. Delbrück and G. Stent, based on the results of experiments, proposed three models for doubling the DNA molecule:

To conservative: provides for the preservation of the original double-stranded DNA molecule and the synthesis of a new, also double-stranded molecule;

- semi-conservative: involves the separation of a DNA molecule into monochains as a result of breaking the hydrogen bonds between the nitrogenous bases of the two chains, after which a complementary base is attached to each base that has lost a partner; daughter molecules are obtained as exact copies of the parent molecule;

- dispersed: consists in the breakdown of the original molecule into nucleotide fragments that are replicated. After replication, new and parent fragments are randomly assembled.

In the same year, 1957, M. Meselson and F. Stahl experimentally proved the existence of a semi-conservative model based on Escherichia coli. And 10 years later, in 1967, the Japanese biochemist R. Okazaki deciphered the mechanism of DNA replication in a semi-conservative way.

Replication is carried out under the control of a number of enzymes and proceeds in several stages. The unit of replication is replicon - a section of DNA that in each cell cycle only 1 time comes into an active state. Replicon has starting points and end replication. In eukaryotes, many replicons appear simultaneously in each DNA. The origin of replication moves sequentially along the DNA strand in the same direction or in opposite directions. The moving front of replication is a fork - replicative or replication fork.

As in any matrix synthesis reaction, there are three stages in replication.

Initiation: enzyme attachment helicases (helicases) to the origin of replication. Helicase unwinds short stretches of DNA. After that, a DNA-binding protein (DBP) is attached to each of the separated chains, which prevents the reunion of the chains. Prokaryotes have an additional enzyme DNA gyrase, which helps the helicase unwind DNA.

Elongation: consecutive complementary addition of nucleotides, as a result of which the DNA chain is lengthened.

Synthesis of DNA occurs immediately on both of its chains. Since the DNA polymerase enzyme can only assemble a chain of nucleotides in the direction from 5' to 3', one of the chains replicates continuously (in the direction of the replication fork), and the other replicates discontinuously (with the formation of Okazaki fragments), in the opposite direction to the movement of the replication fork. The first chain is called leading, and the second is lagging behind. DNA synthesis is carried out with the participation of the enzyme DNA polymerase. Similarly, DNA fragments are synthesized on the lagging strand, which are then crosslinked by enzymes - ligases.

Termination: termination of DNA synthesis upon reaching the desired length of the molecule.

DNA repair- the ability of a DNA molecule to “correct” damage that has arisen in its chains. More than 20 enzymes (endonucleases, exonucleases, restriction enzymes, DNA polymerases, ligases) take part in this process. They are:

1) find changed areas;

2) cut and remove them from the chain;

3) restore the correct sequence of nucleotides;

4) the restored DNA fragment is fused with neighboring regions.

DNA performs special functions in the cell, which are determined by its chemical composition, structure and properties: storage, reproduction and implementation of hereditary information between new generations of cells and organisms.

RNAs are common in all living organisms and are represented by molecules of various sizes, structures, and functions. They consist of one polynucleotide chain formed by four types of monomers - ribonucleotides: adenyl (A), uracil (U), guanyl (G) and cytosyl (C). Each ribonucleotide consists of a nitrogenous base, a ribose, and a phosphoric acid residue. All RNA molecules are exact copies of certain sections of DNA (genes).

The structure of RNA is determined by the sequence of ribonucleotides:

- primary– the sequence of ribonucleotides in the RNA chain; it is a kind of record of genetic information; defines the secondary structure;

-secondary- a strand of RNA twisted into a spiral;

- tertiary– spatial arrangement of the entire RNA molecule; the tertiary structure includes the secondary structure and fragments of the primary, which connect one section of the secondary structure to another (transport, ribosomal RNA).

Secondary and tertiary structures are formed by hydrogen bonds and hydrophobic interactions between nitrogenous bases.

Messenger RNA (i-RNA)- programs the synthesis of cell proteins, since each protein is encoded by the corresponding mRNA (i-RNA contains information about the sequence of amino acids in the protein to be synthesized); weight 10 4 -2x10 6; short lived molecule.

Transfer RNA (t-RNA)- 70-90 ribonucleotides, weight 23,000-30,000; when implementing genetic information, it delivers activated amino acids to the site of polypeptide synthesis, “recognizes” the corresponding section of i-RNA; in the cytoplasm it is represented by two forms: t-RNA in free form and t-RNA associated with an amino acid; more than 40 types; ten%.

The human body contains a large number of organic compounds, without which it is impossible to imagine a stable course of metabolic processes that support the vital activity of all. One of these substances are nucleotides - these are phosphoric esters of nucleosides, which play a crucial role in the transmission of information data, as well as chemical reactions with the release of intracellular energy.

As independent organic units form the filling composition of all nucleic acids and most coenzymes. Let us consider in more detail what nucleoside phosphates are and what role they play in the human body.

What is a nucleotide made of. It is considered an extremely complex ester belonging to the group of phosphorus acids and nucleosides, which, according to their biochemical properties, are among the N-glycosides and contain heterocyclic fragments associated with glucose molecules and a nitrogen atom.

In nature, DNA nucleotides are the most common.

In addition, organic substances with similar structural characteristics are also distinguished: ribonucleotides, as well as deoxyribonucleotides. All of them, without exception, are monomeric molecules belonging to complex biological substances of the polymer type.

They form the RNA and DNA of all living beings, from the simplest microorganisms and viral infections to the human body.

The rest of the molecular structure of phosphorus among nucleoside phosphates forms an ester bond with two, three, and in some cases immediately with five hydroxyl groups. Almost without exception, nucleotides are among the essential substances that were formed from the residues of phosphoric acid, so their bonds are stable and do not break down under the influence of adverse factors of the internal and external environment.

Note! The structure of nucleotides is always complex and is based on monoesters. The sequence of nucleotides can change under the influence of stress factors.

Biological role

The influence of nucleotides on the course of all processes in the body of living beings is studied by scientists who study the molecular structure of the intracellular space.

Based on laboratory findings obtained as a result of many years of work by scientists from around the world, the following role of nucleoside phosphates is distinguished:

• a universal source of vital energy, due to which cells are nourished and, accordingly, the normal functioning of tissues that form internal organs, biological fluids, epithelial cover, and the vascular system is maintained;
• are transporters of glucose monomers in cells of any type (this is one of the forms of carbohydrate metabolism, when consumed sugar is transformed into glucose under the influence of digestive enzymes, which is carried to every corner of the body along with nucleoside phosphates);
• perform the function of a coenzyme (vitamin and mineral compounds that help provide cells with nutrients);
• complex and cyclic mononucleotides are biological conductors of hormones that spread along with the blood flow, and also enhance the effect of neuronal impulses;
• allosterically regulate the activity of digestive enzymes produced by pancreatic tissues.

Nucleotides are part of nucleic acids. They are connected by three and five bonds of the phosphodiester type. Geneticists and scientists who have devoted their lives to molecular biology continue laboratory research on nucleoside phosphates, so every year the world learns even more interesting things about the properties of nucleotides.

The sequence of nucleotides is a kind of genetic balance and the balance of the arrangement of amino acids in the DNA structure, a peculiar order of placement of ester residues in the composition of nucleic acids.

It is determined using the traditional method of sequencing the biological material selected for analysis.

T, thymine;

A - adenine;

G, guanine;

C, cytosine;

R – GA adenine in complex with guanine and purine bases;

Y, TC pyrimidine compounds;

K, GT nucleotides containing a keto group;

M - AC included in the amino group;

S - GC powerful, characterized by three hydrogen compounds;

W - AT are unstable, which form only two hydrogen bonds.

The sequence of nucleotides may change, and the designations in Latin letters are necessary in cases where the order of the ether compounds is unknown, is insignificant, or the results of primary studies are already available.

The greatest number of variants and combinations of nucleoside phosphates is characteristic of DNA. The symbols A, C, G, U are sufficient to write the essential compounds of RNA. The last letter designation is the substance uridine, which is found only in RNA. The symbolic sequence is always written without spaces.

Useful video: nucleic acids (DNA and RNA)

How many nucleotides are in DNA

In order to understand in as much detail as possible what is at stake, one should have a clear understanding of the DNA itself. This is a separate type of molecules that have an elongated shape and consist of structural elements, namely nucleoside phosphates. How many nucleotides are in DNA? There are 4 types of essential compounds of this type that are part of DNA. These are adenine, thymine, cytosine and guanine. All of them form a single chain, from which the molecular structure of DNA is formed.

The structure of DNA was first deciphered back in 1953 by American scientists Francis Crick and James Watson. One molecule of deoxyribonucleic acid contains two chains of nucleoside phosphates. They are placed in such a way that they look like a spiral twisting around its axis.

Note! The number of nucleotides in DNA is unchanged and limited to only four species - this discovery brought humanity closer to deciphering the complete human genetic code.

In this case, the structure of the molecule has one important feature. All nucleotide chains have the property of complementarity. This means that only essential compounds of a certain type are placed opposite each other. It is known that adenine is always located opposite thymine. No other substance other than guanine can be found opposite cytosine. Such nucleotide pairs form the principle of complementarity and are inseparable.

Weight and length

With the help of complex mathematical calculations and laboratory studies, scientists were able to establish the exact physical and biological properties of the essential compounds that form the molecular structure of deoxyribonucleic acid.

It is known that the length of one intracellular residue, consisting of amino acids in a single polypeptide chain, is 3.5 angstroms. The average mass of one molecular residue is 110 amu.

In addition, nucleotide-type monomers are also isolated, which are formed not only from amino acids, but also have ether components. These are DNA and RNA monomers. Their linear length is measured directly inside the nucleic acid and is at least 3.4 angstroms. The molecular weight of one nucleoside phosphate is in the range of 345 amu. These are the initial data that are used in practical laboratory work devoted to experiments, genetic studies and other scientific activities.

Medical designations

Genetics, as a science, developed back in the period when there were no studies of the DNA structure of humans and other living beings at the molecular level. Therefore, in the period of premolecular genetics, nucleotide bonds were designated as the smallest element in the structure of the DNA molecule. Both previously and at the present time, essential substances of this type were subject to. It could be spontaneous or induced, therefore, the term “recon” is also used to refer to nucleoside phosphates with a damaged structure.

To define the concept of the onset of a possible mutation in nitrogenous compounds of nucleotide bonds, the term "muton" is used. These designations are more in demand in laboratory work with biological material. They are also used by geneticists who study the structure of DNA molecules, the ways in which hereditary information is transmitted, how it is encrypted, and possible combinations of genes resulting from the fusion of the genetic potential of two sexual partners.

In contact with

Nucleic acids are natural high-molecular organic compounds, polynucleotides that provide storage and transmission of hereditary (genetic) information in living organisms.

These organic compounds were discovered in 1869 by a Swiss doctor in cells rich in nuclear material (leukocytes, salmon spermatozoa). Nucleic acids are an integral part of cell nuclei, which is why they got their name (from lat. nucleus- core). In addition to the nucleus, nucleic acids are also found in the cytoplasm, centrioles, mitochondria, and chloroplasts.

There are two types of nucleic acids in nature: deoxyribonucleic (DNA) and ribonucleic (RNA). They differ in composition, structure and functions. DNA is double-stranded and RNA is single-stranded.

Nucleic acids are biopolymers that reach enormous sizes. The length of their molecules is hundreds of thousands of nanometers (1 nm = 10–9 m), which is thousands of times longer than the length of protein molecules. The DNA molecule is especially large. The molecular weight of nucleic acids reaches tens of millions and billions (105–109). For example, the mass of E. coli DNA is 2.5x109, and in the nucleus of a human germ cell (haploid set of chromosomes), the length of DNA molecules is 102 cm.

2. NC - non-periodic polymers. Types of nucleotides and their structure

Nucleic acids are non-periodic biopolymers whose polymer chains are formed by monomers called nucleotides. DNA and RNA molecules contain four types of nucleotides.

Composition of DNA and RNA nucleotides

Consider the structure of a nucleotide. Nucleotides are complex organic compounds that include three components.

Deoxyribonucleotides contain pyrimidine bases thymine and cytosine , and in the composition of ribonucleotides - cytosine and uracil . adenine and guanine are part of the nucleotides of both DNA and RNA.

Task. The DNA molecule consists of two chains - the main one, on which mRNA is synthesized, and the complementary one. Write down the order of nucleotides in the synthesized mRNA, if the order of nucleotides in the main (working) DNA strand is as follows: C-G-C-T-G-A-T-A-G.

Decision

Using the principle of complementarity, we determine the order of nucleotides in the mRNA synthesized along the working DNA chain: G-C-G-A-C-U-A-U-C.

Answer: G-Ts-G-A-Ts-U-A-U-Ts

Task. Chemical analysis showed that 28% of the total number of nucleotides of this mRNA is adenine, 6% is guanine, and 40% is uracil. What should be the nucleotide composition of the corresponding section of double-stranded DNA, the information from which is “rewritten” by this mRNA?

Decision

1. Knowing that the chain of the RNA molecule and the working chain of the DNA molecule are complementary to each other, we determine the content of nucleotides (in%) in the working chain of DNA:

· in the mRNA chain G = 6%, which means that in the working DNA chain C = 6%;

In the mRNA chain A = 28%, then in the working DNA chain T = 28%;

In the mRNA chain Y = 40%, which means that in the working DNA chain A = 40%;

2. Determine the content of the mRNA chain (in%) of cytosine.

Let's summarize the content of three other types of nucleotides in the mRNA chain: 6% + 28% + +40% = 74% (G+A+U);

Determine the proportion of cytosine in the mRNA chain: 100% - 74% = 26% (C);

If in the mRNA chain C=26%, then in the working DNA chain G=26%.

Answer: C=6%; T=28%; A=40%; G=26%

Task . On a fragment of one DNA chain, the nucleotides are arranged in the sequence: A-A-G-T-C-T-A-A-C-G-T-A-T. Draw a diagram of the structure of a double-stranded DNA molecule. What is the length of this DNA fragment? How many (in%) nucleotides are in this DNA strand?

Decision

1. By the principle of complementarity, it builds the second strand of a given DNA molecule: T-T-C-A-G-A-T-T-G-C-A-T-A.

2. Knowing the length of one nucleotide (0.34 nm), we determine the length of this DNA fragment (in DNA, the length of one chain is equal to the length of the entire molecule): 13x0.34 = 4.42 nm.

3. Calculate the percentage of nucleotides in this DNA chain:

13 nucleotides - 100%
5 A - x%, x \u003d 38% (A).
2 G - x%, x \u003d 15.5% (G).
4 T – x%, x=31% (T).
2 C - x%, x \u003d 15.5% (C).

Answer: T-T-C-A-G-A-T-T-G-C-A-T-A; 4.42 nm; A=38; T=31%; G=15.5%; C=15.5%.

Task. A section of one of the DNA molecule chains was examined in the laboratory. It turned out that it consists of 20 monomers, which are arranged in the following sequence: G-T-G-T-A-A-C-G-A-C-C-G-A-T-A-C-T-G -T-A.
What can be said about the structure of the corresponding section of the second strand of the same DNA molecule?

Decision

Knowing that the chains of a DNA molecule are complementary to each other, we determine the sequence of nucleotides of the second chain of the same DNA molecule: C-A-C-A-T-T-G-C-T-G-G-C-T-A-T- G-A-C-A-T.

Task. On a fragment of one DNA chain, the nucleotides are arranged in the sequence: A-A-G-T-C-T-A-C-G-T-A-T ...

1. Draw a diagram of the structure of the second strand of this DNA molecule.
2. What is the length in nm of this DNA fragment if one nucleotide is about 0.34 nm?
3. How many (in%) nucleotides are contained in this fragment of the DNA molecule?

Decision

1. We complete the second strand of this fragment of the DNA molecule, using the rule of complementarity: T-T-C-A-G-A-T-G-C-A-T-A.
2. Determine the length of this DNA fragment: 12x0.34=4.08 nm.
3. Calculate the percentage of nucleotides in this DNA fragment.

24 nucleotides - 100%
8A - x%, hence x = 33.3% (A);
because according to the Chargaff rule A=T, then the content of T=33.3%;
24 nucleotides - 100%
4D - x%, hence x \u003d 16.7% (G);
since according to the Chargaff rule G=C, it means that the content of C=16.6%.

Answer: T-T-C-A-G-A-T-G-C-A-T-A; 4.08 nm; A=T=33.3%; G=C=16.7%

Task. What will be the composition of the second DNA strand if the first contains 18% guanine, 30% adenine and 20% thymine?

Decision

1. Knowing that the chains of the DNA molecule are complementary to each other, we determine the content of nucleotides (in%) in the second chain:

since in the first chain G = 18%, then in the second chain C = 18%;
since in the first chain A = 30%, then in the second chain T = 30%;
since in the first chain T = 20%, then in the second chain A = 20%;

2. Determine the content in the first chain of cytosine (in%).

Let's summarize the content of three other types of nucleotides in the first DNA strand: 18% + 30% + 20% = 68% (G+A+T);

Determine the proportion of cytosine in the first DNA strand: 100% - 68% = 32% (C);

If in the first chain C=32%, then in the second chain G=32%.

Answer: C=18%; T=30%; A=20%; G=32%

Task. In a DNA molecule, there are 23% of adenyl nucleotides of the total number of nucleotides. Determine the amount of thymidyl and cytosyl nucleotides.

Decision

1. According to the Chargaff rule, we find the content of thymidyl nucleotides in a given DNA molecule: A=T=23%.
2. Find the sum (in%) of the content of adenyl and thymidyl nucleotides in a given DNA molecule: 23% + 23% = 46%.
3. Find the sum (in%) of the content of guanyl and cytosyl nucleotides in this DNA molecule: 100% - 46% = 54%.
4. According to the Chargaff rule, in the DNA molecule G=C, in total they account for 54%, and individually: 54% : 2 = 27%.

Answer: T=23%; C=27%

Task. Given a DNA molecule with a relative molecular weight of 69 thousand, of which 8625 are adenyl nucleotides. The relative molecular weight of one nucleotide is on average 345. How many nucleotides are there individually in this DNA? What is the length of its molecule?

Decision

1. Determine how many adenyl nucleotides are in a given DNA molecule: 8625: 345 = 25.
2. According to Chargaff's rule, A=G, i.e., in this DNA molecule A=T=25.
3. Determine how much of the total molecular weight of this DNA is the share of guanyl nucleotides: 69,000 - (8625x2) = 51,750.
4. Determine the total number of guanyl and cytosyl nucleotides in this DNA: 51 750:345=150.
5. Determine the content of guanyl and cytosyl nucleotides separately: 150:2 = 75;
6. Determine the length of this DNA molecule: (25 + 75) x 0.34 = 34 nm.

Answer: A=T=25; G=C=75; 34 nm.

Task. According to some scientists, the total length of all DNA molecules in the nucleus of one human germ cell is about 102 cm. How many base pairs are there in the DNA of one cell (1 nm = 10–6 mm)?

Decision

1. Convert centimeters to millimeters and nanometers: 102 cm = 1020 mm = 1,020,000,000 nm.
2. Knowing the length of one nucleotide (0.34 nm), we determine the number of base pairs contained in the DNA molecules of the human gamete: (102 x 107): 0.34 = 3 x 109 pairs.

Answer: 3x109 pairs.

Homework

1. Learn abstract

2. solve problems

Option 1

1. Fragments of one chain of the DNA molecule are given: C-A-A-A-T-T-G-G-A-C-G-G-G. Determine the content (in%) of each type of nucleotide and the length of this fragment of the DNA molecule.

2. 880 guanyl nucleotides were found in the DNA molecule, which make up 22% of the total number of nucleotides of this DNA? Determine how many other nucleotides are contained (individually) in this DNA molecule. What is the length of this DNA?

Option 2

1. Fragments of one chain of the DNA molecule are given: A-G-C-C-G-G-G-A-A-T-T-A. Determine the content (in%) of each type of nucleotide and the length of this fragment of the DNA molecule.

2. In the DNA molecule, 250 thymidyl nucleotides were found, which make up 22.5% of the total number of nucleotides of this DNA. Determine how many other nucleotides are contained (individually) in this DNA molecule. What is the length of this DNA?

3. Distribute the abstracts by options. Option 1 - DNA; option 2 - RNA.

1. Single-stranded molecule.
2. Double-stranded molecule.
3. Contains adenine, uracil, guanine, cytosine.
4. Contains adenine, thymine, guanine, cytosine.
5. Ribose is a part of nucleotides.
6. Nucleotides contain deoxyribose.
7. Contained in the nucleus, chloroplasts, mitochondria, centrioles, ribosomes, cytoplasm.
8. Contained in the nucleus, chloroplasts, mitochondria.
9. Participates in the storage, reproduction and transmission of hereditary information.
10. Participates in the transfer of hereditary information.