Chromosomes. Indicate that chromosomes are composed of DNA, which is surrounded by two types of proteins: histone (basic) and non-histone (acidic). Note that chromosomes can be in two structural and functional states: spiralized and despiralized. Know which of these two chromosome states is working and what this means. Indicate at what period of life the cells of the chromosome are spiralized and clearly visible under a microscope. Know the structure of the chromosome, the types of chromosomes that differ in the location of the primary constriction.

The organisms of most living things have a cellular structure. In the process of evolution of the organic world, the cell was selected as an elementary system in which all the laws of the living could be manifested. Organisms with a cellular structure are divided into prenuclear organisms that do not have a typical nucleus (or prokaryotes), into those with a typical nucleus (or eukaryotes). Indicate which organisms belong to prokaryotes, which to eukaryotes.

To understand the organization of a biological system, it is necessary to know the molecular composition of a cell. According to their content, the elements that make up the cell are divided into three groups: macroelements, trace elements and ultramicroelements. Give examples of the elements that make up each group, characterize the role of the main inorganic components in the life of the cell. The chemical components of living things are divided into inorganic (water, mineral salts) and organic (proteins, carbohydrates, lipids, nucleic acids). With a few exceptions (bone and tooth enamel), water is the predominant component of cells. To know the properties of water, in what forms the water is in the cell, to characterize the biological significance of water. In terms of the content of organic matter in the cell, proteins occupy the first place. To characterize the composition of proteins, the spatial organization of proteins (primary, secondary, tertiary, quaternary structures), the role of proteins in the body. Carbohydrates are divided into 3 classes: monosaccharides, disaccharides and polysaccharides. Know the chemical composition and classification criteria for carbohydrates. Give examples of the most important representatives of the class and characterize their role in the life of the cell. Lipids are characterized by the greatest chemical diversity. The term "lipids" includes fats and fat-like substances - lipoids. Fats are esters of fatty acids and some alcohol. Know the chemical composition of lipids and lipoids. Emphasize the main functions: trophic, energy, and other functions that need to be characterized. The energy released during the decay of organic substances is not used immediately for work in cells, but is first stored in the form of a high-energy intermediate - adenosine triphosphate (ATP). Know the chemical composition of ATP. Disclose what AMP and ADP compounds are. Expand the concept of "macroergic connection". Indicate in which processes ADP and AMP are formed, and how ATP is formed, what is the energy value of these processes. Give examples of physiological processes that require a lot of energy.

Self-reproduction of genetic material. Replication.

Principles of recording genetic information. Genetic code and its properties.

Genetic code- inherent in all living organisms, a method of encoding the amino acid sequence of proteins using a sequence of nucleotides. In nature, 20 different amino acids are used to build proteins. Each protein is a chain or several chains in a strictly defined sequence. This sequence determines the structure of the protein, and hence its properties. The set of amino acids is universal for almost all living organisms.

Gene properties. code:

Triplet - a combination of 3 nucleotides

Continuity - there are no punctuation marks between triplets, i.e. information is read continuously

Non-overlapping - the same nucleotide cannot be part of several triplets at the same time

Specificity - a certain codon corresponds to only 1 amino acid

Degeneracy - several codons can correspond to the same amino acid

Versatility - the genetic code works the same in organisms of different levels of complexity

Immunity

In the process of replication of genetic material, hydrogen bonds between nitrogenous bases are broken, and two DNA strands are formed from the double helix. Each of them becomes a template for the synthesis of another complementary DNA strand. The latter, through a hydrogen bond, combines with the template DNA. So, any daughter DNA molecule consists of one old and one new polynucleotide chain. As a result, the daughter cells receive the same genetic information as the parent cells. The maintenance of such a situation is provided by a self-correction mechanism carried out by DNA polymerase. The ability of the genetic material, DNA, to reproduce itself (replicate) underlies the reproduction of living organisms, the transfer of hereditary properties from generation to generation and the development of a multicellular organism from the zygote.

Uncorrected changes in the chemical structure of genes, reproduced in successive replication cycles and manifested in offspring in the form of new variants of traits, are called gene mutations.

Changes in the structure of DNA can be divided into 3 groups: 1. Replacement of some bases with others.

2. shift of the reading frame with a change in the number of nucleotide pairs in the gene.

3. reordering of nucleotide sequences within a gene.

1. Replacing some bases with others. May occur accidentally or under the influence of specific chemical agents. If the altered form of the base remains unnoticed during repair, then during the next cycle of replication it can attach another nucleotide to itself.

Another reason may be the erroneous inclusion in the synthesized DNA strand of a nucleotide carrying a modified form of the base or its analogue. If this error goes unnoticed during repair, then the changed base is included in the replication process, which leads to the replacement of one pair for another.

As a result, a new triplet is formed in DNA. If this triplet encodes the same amino acid, then the changes will not affect the structure of the peptide (degeneracy of the genetic code). If the newly formed triplet encodes a different amino acid, the structure of the peptide chain and the properties of the protein change.

2. shift of the reading frame. These mutations occur due to the loss (deletion) or insertion of one or more pairs of complementary nucleotides into the DNA nucleotide sequence. This may be due to exposure of the genetic material to certain chemicals (acridine compounds). A large number of mutations occur due to the inclusion of movable genetic elements - transposons - into DNA. Errors during recombination in case of unequal intragenic crossing over may also be the reason.

With such mutations, the meaning of the biological information recorded in this DNA changes.

3... changing the order of nucleotide sequences. This type of mutation occurs due to a 180ᵒ rotation of the DNA region (inversion). This is due to the fact that the DNA molecule forms a loop within which replication goes in the wrong direction. Within the inverted region, the reading of information is disrupted and the amino acid sequence of the protein is disrupted.

Causes:- unequal crossing over between homologous chromosomes

Intrachromosomal crossing over

Chromosome breaks

Breaks followed by joining of chromosome elements

Copying a gene and transferring it to another part of the chromosome

The DNA molecule consists of two strands that form a double helix. Its structure was first deciphered by Francis Crick and James Watson in 1953.

At first, a DNA molecule, consisting of a pair of nucleotide chains twisted around each other, raised questions about why it has exactly this shape. Scientists have called this phenomenon complementarity, which means that only certain nucleotides can be located in its strands opposite each other. For example, adenine is always opposite thymine, and guanine is opposite cytosine. These nucleotides of the DNA molecule are called complementary.

This is schematically depicted as follows:

T - A

C - G

These pairs form a chemical nucleotide bond that determines the order in which the amino acids are arranged. In the first case, it is slightly weaker. The connection between C and G is stronger. Non-complementary nucleotides do not form pairs with each other.

About the structure

So, the structure of the DNA molecule is special. It has such a shape for a reason: the fact is that the number of nucleotides is very large, and a lot of space is needed to accommodate long chains. It is for this reason that spiral twisting is inherent in chains. This phenomenon is called spiralization, it allows the filaments to be shortened by about five to six times.

The body uses some molecules of this kind very actively, others rarely. The latter, in addition to spiralization, also undergo such "compact packaging" as supercoiling. And then the length of the DNA molecule decreases 25-30 times.

What is the "packing" of a molecule?

In the process of supercoiling, histone proteins are involved. They have the structure and appearance of a thread spool or rod. Spiralized threads are wound on them, which immediately become "compactly packed" and take up little space. When it becomes necessary to use one or another thread, it is unwound from a coil, for example, of a histone protein, and the spiral is unwound into two parallel chains. When the DNA molecule is in this state, the necessary genetic data can be read from it. However, there is one condition. Obtaining information is possible only if the structure of the DNA molecule is untwisted. The chromosomes available for reading are called euchromatins, and if they are supersypiralized, then these are already heterochromatins.

Nucleic acids

Nucleic acids, like proteins, are biopolymers. The main function is the storage, implementation and transmission of hereditary (genetic information). They are of two types: DNA and RNA (deoxyribonucleic and ribonucleic). Monomers in them are nucleotides, each of which contains a phosphoric acid residue, a five-carbon sugar (deoxyribose / ribose) and a nitrogenous base. The DNA code includes 4 types of nucleotides - adenine (A) / guanine (G) / cytosine (C) / thymine (T). They differ in the nitrogenous base they contain.

In a DNA molecule, the number of nucleotides can be enormous - from several thousand to tens and hundreds of millions. Such giant molecules can be viewed through an electron microscope. In this case, it will be possible to see a double strand of polynucleotide strands, which are interconnected by hydrogen bonds of the nitrogenous bases of nucleotides.

Research

In the course of research, scientists have found that the types of DNA molecules in different living organisms differ. It was also found that guanine of one chain can bind only with cytosine, and thymine - with adenine. The arrangement of the nucleotides of one strand strictly corresponds to the parallel one. Due to this complementarity of polynucleotides, the DNA molecule is capable of duplication and self-reproduction. But first, the complementary chains diverge under the influence of special enzymes that destroy paired nucleotides, and then synthesis of the missing chain begins in each of them. This is due to the free nucleotides present in large quantities in each cell. As a result, instead of the "parent molecule", two "daughter" molecules are formed, identical in composition and structure, and the DNA code becomes the original one. This process is a precursor to cell division. It ensures the transfer of all hereditary data from mother cells to daughter cells, as well as to all subsequent generations.

How is the gene code read?

Today, not only the mass of the DNA molecule is calculated, it is also possible to find out more complex data that were previously not available to scientists. For example, you can read information about how the body uses its own cell. Of course, at first, these information are encoded and have the form of some kind of matrix, and therefore it must be transported to a special carrier, which is RNA. Ribonucleic acid is able to infiltrate into the cell through the nuclear membrane and read the encoded information inside. Thus, RNA is a carrier of hidden data from the nucleus to the cell, and it differs from DNA in that it contains ribose instead of deoxyribose, and uracil instead of thymine. In addition, RNA is single stranded.

RNA synthesis

A deep analysis of DNA showed that after RNA leaves the nucleus, it enters the cytoplasm, where it can be incorporated as a matrix into ribosomes (special enzyme systems). Based on the information received, they can synthesize the appropriate sequence of protein amino acids. The ribosome learns from the triplet code what kind of organic compound must be attached to the forming protein chain. Each amino acid has its own specific triplet, which encodes it.

After the formation of the chain is completed, it acquires a specific spatial form and turns into a protein capable of performing its hormonal, building, enzymatic and other functions. For any organism, it is a gene product. It is from it that all kinds of qualities, properties and manifestations of genes are determined.

Genes

First of all, sequencing processes were developed in order to obtain information about how many genes the structure of a DNA molecule has. And, although the research has allowed scientists to make great headway in this matter, it is not yet possible to know the exact number of them.

A few years ago, it was assumed that DNA molecules contain approximately 100 thousand genes. A little later, the figure decreased to 80 thousand, and in 1998, geneticists announced that only 50 thousand genes are present in one DNA, which is only 3% of the entire length of DNA. But the latest conclusions of geneticists were amazed. Now they claim that the genome includes 25-40 thousand of these units. It turns out that only 1.5% of chromosomal DNA is responsible for coding proteins.

The research did not stop there. A parallel team of genetic engineering specialists found that the number of genes in one molecule is exactly 32 thousand. As you can see, it is still impossible to get a definitive answer. There are too many contradictions. All researchers rely only on their own results.

Has there been an evolution?

Despite the fact that there is no evidence of the evolution of the molecule (since the structure of the DNA molecule is fragile and small in size), still scientists have made one suggestion. Based on laboratory data, they voiced a version of the following content: at the initial stage of its appearance, the molecule looked like a simple self-replicating peptide, which included up to 32 amino acids found in ancient oceans.

After self-replication, thanks to the forces of natural selection, the molecules acquired the ability to protect themselves from the influence of external elements. They began to live longer and reproduce in large numbers. The molecules that found themselves in the lipid bladder got every chance for self-reproduction. As a result of a series of successive cycles, lipid bubbles acquired the form of cell membranes, and then - all known particles. It should be noted that today any part of the DNA molecule is a complex and clearly functioning structure, all the features of which have not yet been fully studied by scientists.

Modern world

Recently, Israeli scientists have developed a computer that can perform trillions of operations per second. Today it is the fastest car on Earth. The whole secret is that the innovative device is powered by DNA. The professors say that in the short term, such computers will even be able to generate power.

A year ago, specialists from the Weizmann Institute in Rehovot (Israel) announced the creation of a programmable molecular computer consisting of molecules and enzymes. They replaced silicon microchips with them. By now, the team is still moving forward. Now only one DNA molecule can provide the computer with the necessary data and provide the necessary fuel.

Biochemical "nanocomputers" are not fiction, they already exist in nature and are manifested in every living being. But often they are not controlled by humans. A person cannot yet operate on the genome of any plant in order to calculate, say, the number "pi".

The idea of ​​using DNA to store / process data first hit the bright minds of scientists in 1994. It was then that a molecule was used to solve a simple math problem. Since then, a number of research groups have proposed various projects related to DNA computers. But here all attempts were based only on the energy molecule. You cannot see such a computer with the naked eye; it looks like a transparent solution of water in a test tube. There are no mechanical parts in it, but only trillions of biomolecular devices - and that's just one drop of liquid!

Human DNA

What kind of human DNA, people became aware in 1953, when scientists were for the first time able to demonstrate to the world a double-stranded model of DNA. For this, Kirk and Watson received the Nobel Prize, since this discovery became fundamental in the 20th century.

Over time, of course, they proved that a structured human molecule can look not only like in the proposed version. After a more detailed analysis of DNA, the A-, B- and left-handed forms of Z- were discovered. Form A- is often an exception, since it is formed only if there is a lack of moisture. But this is possible only in laboratory studies, for the natural environment it is abnormal, in a living cell such a process cannot occur.

The B- shape is classic and is known as a double right-handed chain, but the Z- shape is not only twisted in the opposite direction, to the left, but also has a more zigzag appearance. Scientists have also identified the G-quadruplex form. There are not 2, but 4 threads in its structure. According to geneticists, this form occurs in those areas where there is an excess amount of guanine.

Artificial DNA

Artificial DNA already exists today, which is an identical copy of the real one; it perfectly repeats the structure of the natural double helix. But, unlike the primordial polynucleotide, in the artificial one there are only two additional nucleotides.

Since dubbing was created on the basis of information obtained in the course of various studies of real DNA, it can also be copied, self-replicated and evolved. Experts have been working on the creation of such an artificial molecule for about 20 years. The result is an amazing invention that can use the genetic code in the same way as natural DNA.

To the four available nitrogenous bases, genetics added an additional two, which they created by the method of chemical modification of natural bases. Unlike natural DNA, artificial DNA is quite short. It contains only 81 base pairs. However, it also reproduces and evolves.

Replication of an artificially obtained molecule takes place thanks to the polymerase chain reaction, but so far this does not happen on its own, but through the intervention of scientists. They independently add the necessary enzymes to the mentioned DNA, placing it in a specially prepared liquid medium.

Final result

The process and the final result of DNA development can be influenced by various factors, for example, mutations. This necessitates the study of samples of matter in order for the result of analyzes to be reliable and reliable. An example is the paternity test. But it is good news that such incidents as mutation are rare. Nevertheless, samples of matter are always rechecked in order to obtain more accurate information based on the analysis.

Plant DNA

Thanks to high technology sequencing (HTS), a revolution has been made in the field of genomics - the extraction of DNA from plants is also possible. Of course, obtaining high-quality DNA molecular weight from plant material causes some difficulties due to the large number of copies of DNA mitochondria and chloroplasts, as well as the high level of polysaccharides and phenolic compounds. To isolate the structure we are considering, in this case, a variety of methods are used.

Hydrogen bond in DNA

The hydrogen bond in the DNA molecule is responsible for the electromagnetic attraction created between a positively charged hydrogen atom, which is attached to an electronegative atom. This dipole interaction does not meet the chemical bond criterion. But it can be realized intermolecularly or in different parts of the molecule, that is, intramolecularly.

The hydrogen atom is attached to the electronegative atom, which is the donor of this bond. The electronegative atom can be nitrogen, fluorine, oxygen. It - through decentralization - attracts an electron cloud from the hydrogen core and makes the hydrogen atom charged (partially) positively. Since the size of H is small compared to other molecules and atoms, the charge is also small.

DNA decoding

Before decoding a DNA molecule, scientists first take a huge number of cells. For the most accurate and successful work, they need about a million. The results obtained in the course of the study are constantly compared and recorded. Today, genome decoding is no longer a rarity, but an affordable procedure.

Of course, deciphering the genome of one cell is an inappropriate exercise. The data obtained in the course of such studies are not of any interest to scientists. But it is important to understand that all currently existing decoding methods, despite their complexity, are not effective enough. They will only allow 40-70% of the DNA to be read.

However, Harvard professors recently announced a way through which 90% of the genome can be deciphered. The technique is based on the addition of primer molecules to the isolated cells, with the help of which DNA replication begins. But even this method cannot be considered successful; it still needs to be improved before being openly used in science.

MOSCOW, April 25 - RIA Novosti, Tatyana Pichugina. Exactly 65 years ago, British scientists James Watson and Francis Crick published an article on deciphering the structure of DNA, laying the foundations for a new science - molecular biology. This discovery has changed a lot in the life of mankind. RIA Novosti talks about the properties of the DNA molecule and why it is so important.

In the second half of the 19th century, biology was a very young science. Scientists were just starting to study the cell, and the concept of heredity, although they were already formulated by Gregor Mendel, did not gain wide acceptance.

In the spring of 1868, a young Swiss physician, Friedrich Miescher, came to the University of Tübingen (Germany) to do scientific work. He intended to find out what substances the cell is made of. For experiments, I chose leukocytes, which are easy to obtain from pus.

By separating the nucleus from protoplasm, proteins and fats, Misher discovered a compound with a high phosphorus content. He called this molecule nuclein ("nucleus" in Latin is the nucleus).

This compound exhibited acidic properties, hence the term "nucleic acid" was coined. Its prefix "deoxyribo" means that the molecule contains H groups and sugars. Then it turned out that it was actually salt, but the name was not changed.

At the beginning of the 20th century, scientists already knew that a nuclein is a polymer (that is, a very long flexible molecule of repeating units), the units are composed of four nitrogenous bases (adenine, thymine, guanine and cytosine), and the nuclein is contained in chromosomes - compact structures that arise in dividing cells. Their ability to transmit hereditary traits was demonstrated by the American geneticist Thomas Morgan in experiments on fruit flies.

The Model Explaining Genes

But what deoxyribonucleic acid, or DNA for short, does in the cell nucleus, was not understood for a long time. It was thought to play some kind of structural role in chromosomes. The units of heredity - genes - were attributed to the protein nature. The breakthrough was made by the American researcher Oswald Avery, who experimentally proved that genetic material is transmitted from bacteria to bacteria through DNA.

It became clear that DNA needed to be studied. But how? At that time, only X-rays were available to scientists. To shine biological molecules through them, they had to crystallize, and this is difficult. Deciphering the structure of protein molecules by X-ray diffraction patterns was carried out at the Cavendish Laboratory (Cambridge, Great Britain). Young researchers James Watson and Francis Crick who worked there did not have their own experimental data on DNA, so they used the radiographs of colleagues from King's College Maurice Wilkins and Rosalind Franklin.

Watson and Crick proposed a model of the structure of DNA that exactly corresponds to X-ray diffraction patterns: two parallel strands are twisted into a right-hand helix. Each chain is folded by an arbitrary set of nitrogenous bases, strung on the backbone of their sugars and phosphates, and held together by hydrogen bonds stretched between the bases. Moreover, adenine combines only with thymine, and guanine - with cytosine. This rule is called the principle of complementarity.

The Watson and Crick model explained the four main functions of DNA: replication of genetic material, its specificity, storage of information in a molecule, and its ability to mutate.

The scientists published their discovery in the journal Nature on April 25, 1953. Ten years later, he and Maurice Wilkins were awarded the Nobel Prize in Biology (Rosalind Franklin died in 1958 from cancer at the age of 37).

"Now, more than half a century later, we can state that the discovery of the structure of DNA played the same role in the development of biology as the discovery of the atomic nucleus in physics. The elucidation of the structure of the atom led to the birth of a new, quantum physics, and the discovery of the structure of DNA led to the birth of a new one, molecular biology ", - writes Maxim Frank-Kamenetsky, an outstanding geneticist, DNA researcher, author of the book" The Most Important Molecule ".

Genetic code

Now all that remained was to find out how this molecule worked. It was known that DNA contains instructions for the synthesis of cellular proteins that do all the work in the cell. Proteins are polymers made up of repeating sets (sequences) of amino acids. Moreover, there are only twenty amino acids. Animal species differ from each other by the set of proteins in the cells, that is, by different sequences of amino acids. Genetics argued that these sequences are given by genes, which were then believed to serve as the first building blocks of life. But what genes were, no one knew exactly.

The author of the Big Bang theory, physicist Georgy Gamov, an employee of the George Washington University (USA), made it clear. Based on the model of the double-stranded DNA helix by Watson and Crick, he suggested that a gene is a piece of DNA, that is, a certain sequence of links - nucleotides. Since each nucleotide is one of four nitrogenous bases, you just need to figure out how the four elements encode twenty. This was the idea behind the genetic code.

By the early 1960s, it was established that proteins are synthesized from amino acids in ribosomes - a kind of "factories" inside the cell. To start protein synthesis, an enzyme approaches the DNA, recognizes a specific site at the beginning of the gene, synthesizes a copy of the gene in the form of a small RNA (it is called a template), then a protein is grown from amino acids in the ribosome.

They also found out that the genetic code is three-letter. This means that three nucleotides correspond to one amino acid. The unit of the code was called a codon. In the ribosome, information from mRNA is read codon by codon, sequentially. And each of them corresponds to several amino acids. What does the cipher look like?

This question was answered by Marshall Nirenberg and Heinrich Mattei from the USA. In 1961, they presented their results for the first time at a biochemical congress in Moscow. By 1967, the genetic code had been completely deciphered. It turned out to be universal for all cells of all organisms, which had far-reaching consequences for science.

The discovery of the structure of DNA and the genetic code has completely reoriented biological research. The fact that each individual has a unique DNA sequence has fundamentally changed forensic science. Deciphering the human genome has given anthropologists a completely new method of studying the evolution of our species. The recently invented DNA editor CRISPR-Cas has taken genetic engineering forward a lot. Apparently, this molecule stores the solution to the most pressing problems of mankind: cancer, genetic diseases, aging.

The monomer units of which are nucliatides.

What is DNA?

All information about the structure and functioning of any living organism is contained in a coded form in its genetic material. The basis of the body's genetic material is deoxyribonucleic acid (DNA).

DNA in most organisms it is a long, double-stranded polymer molecule. Subsequence monomer units (deoxyribonucleotides) in one of its chains corresponds to ( complementary) the sequence of deoxyribonucleotides in another. The principle of complementarity ensures the synthesis of new DNA molecules, identical to the original ones, when they are doubled ( replication).

A section of a DNA molecule that encodes a certain trait - gene.

Genes- these are individual genetic elements that have a strictly specific nucleotide sequence and coding for certain characteristics of the organism. Some of them encode proteins, others only RNA molecules.

The information contained in the genes encoding proteins (structural genes) is decoded in two sequential processes:

• RNA synthesis (transcription): in a certain region of DNA, as on a matrix, is synthesized messenger RNA (mRNA).
• protein synthesis (translation): During the coordinated work of a multicomponent system with the participation of transport RNA (tRNA), mRNA, enzymes and various protein factors carried out protein synthesis.

All these processes ensure the correct translation of the genetic information encoded in DNA from the language of nucleotides into the language of amino acids. Amino acid sequence of a protein molecule defines its structure and functions.

DNA structure

DNA- this is linear organic polymer... His - nucleotides, which, in turn, consist of:

In this case, the phosphate group is attached to 5'-carbon atom monosaccharide residue, and the organic base - to 1'-atom.

There are two types of bases in DNA:

The structure of nucleotides in the DNA molecule

V DNA monosaccharide presented 2'-deoxyribose containing only 1 hydroxyl group (OH) and in RNA - ribose having 2 hydroxyl groups (OH).

Nucleotides are linked to each other phosphodiester bonds, while the phosphate group 5'-carbon atom one nucleotide linked to 3'-OH-group of deoxyribose adjacent nucleotide (Figure 1). At one end of the polynucleotide chain is Z'-OH-group (Z'-end), and on the other - 5'-phosphate group (5'-end).

DNA structure levels

It is customary to distinguish 3 levels of DNA structure:

• primary;
• secondary;
• tertiary.

Primary DNA structure Is the sequence of the arrangement of nucleotides in the polynucleotide chain of DNA.

Secondary structure of DNA stabilizes between complementary base pairs and is a double helix of two antiparallel chains twisted to the right around one axis.

The general turn of the spiral 3.4nm, distance between chains 2nm.

The tertiary structure of DNA is DNA supersperalization. The double helix of DNA in some regions can undergo further spiralization with the formation of a supercoil or an open circular shape, which is often caused by the covalent connection of their open ends. The supercoiled DNA structure allows economical packing of a very long DNA molecule in the chromosome. So, in an elongated form, the length of a DNA molecule is 8 cm, and in the form of a super spiral fits into 5 nm.

Chargaff's rule

E. Chargaff's rule Is the regularity of the quantitative content of nitrogenous bases in the DNA molecule:

1. DNA molar fractions purine and pyrimidine bases are equal: A +G = C+ T or (A +G)/(C + T) = 1.
2. In DNA the number of bases with amino groups (A +C) equals the number of bases with keto groups (G+ T):A +C= G+ T or (A +C)/(G+ T) = 1
3. Equivalence rule, that is: A = T, G = C; A / T = 1; G / C = 1.
4. DNA nucleotide composition in organisms of various groups is specific and characterized specificity coefficient: (G + C) / (A + T). In higher plants and animals specificity coefficient less than 1, and varies slightly: from 0,54 before 0,98 , in microorganisms it is more than 1.

Watson-Crick DNA Model

1953 James Watson and Francis Scream, based on the data of X-ray structural analysis of DNA crystals, came to the conclusion that native DNA consists of two polymer chains forming a double helix (Figure 3).

Coiled polynucleotide chains are held together hydrogen bonds formed between complementary bases of opposite chains (Figure 3). Wherein adenine pairs only with thymine, a guanine- with cytosine... A pair of bases AT stabilizes two hydrogen bonds and a couple G-C - three.

The length of double-stranded DNA is usually measured by the number of pairs of complementary nucleotides ( NS.n.). For DNA molecules consisting of thousands or millions of nucleotide pairs, units are accepted so on and m.p.n. respectively. For example, the DNA of human chromosome 1 is one double helix length 263 m.p.n.

Sugar-phosphate backbone of the molecule, which consists of phosphate groups and deoxyribose residues connected 5'-3'-phosphodiester linkages, forms the "sidewalls of a spiral staircase", and the base pairs AT and G-C- its steps (Figure 3).

Figure 3: Watson-Crick DNA Model

DNA molecule chains antiparallel: one of them has a direction 3 ’→ 5 ′, another 5 ’→ 3 ′... In accordance with the principle of complementarity if one of the strands contains a nucleotide sequence 5-TAGGCAT-3 ′, then in the complementary strand in this place there must be a sequence 3′-ATCCGTA-5 ′... In this case, the double-stranded form will look like this:

• 5′-TAGGCAT-3 ′
• 3-ATCCGTA-5 '.

In such a record 5'-end of the upper chain are always placed on the left, and 3'-end- on right.

The carrier of genetic information must meet two basic requirements: reproduce (replicate) with high fidelity and determine (encode) the synthesis of protein molecules.

Watson-Crick DNA Model fully meets these requirements, since:

• according to the principle of complementarity, each DNA strand can serve as a template for the formation of a new complementary strand. Therefore, after one round, two daughter molecules are formed, each of which has the same nucleotide sequence as the original DNA molecule.
• the nucleotide sequence of the structural gene uniquely defines the amino acid sequence of the protein it encodes.

1. One human DNA molecule contains about 1.5 gigabytes of information... At the same time, the DNA of all cells of the human body occupies 60 billion terabytes, which is stored in 150-160 grams of DNA.
2. International DNA Day celebrated on April 25th. It was on this day in 1953 James watson and Francis Creek published in the magazine Nature my article titled "Molecular structure of nucleic acids" , which described the double helix of the DNA molecule.

Bibliography: Molecular Biotechnology: Principles and Applications, B. Glick, J. Pasternak, 2002