DNA changes due to transition. What is DNA - deoxyribonucleic acid? The structure of nucleotides in a DNA molecule

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 of a new science - molecular biology. This discovery 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 beginning to study the cell, and ideas about heredity, although already formulated by Gregor Mendel, were not widely accepted.

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

Separating the nucleus from protoplasm, proteins and fats, Miescher discovered a compound with a high phosphorus content. He called this molecule nuclein ("nucleus" in Latin - nucleus).

This compound exhibited acidic properties, which is why the term “nucleic acid” arose. Its prefix "deoxyribo" means that the molecule contains H-groups and sugars. Then it turned out that it was actually salt, but they did not change the name.

At the beginning of the 20th century, scientists already knew that 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 nuclein is contained in chromosomes - compact structures that occur in dividing cells. Their ability to transmit hereditary characteristics was demonstrated by the American geneticist Thomas Morgan in experiments on fruit flies.

The model that explained the genes

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

It became clear that DNA needed to be studied. But how? At that time, only X-rays were available to scientists. In order to illuminate biological molecules with it, they had to be crystallized, and this is difficult. The structure of protein molecules was deciphered from X-ray diffraction patterns at the Cavendish Laboratory (Cambridge, UK). The young researchers who worked there, James Watson and Francis Crick, did not have their own experimental data on DNA, so they used the X-ray photographs of colleagues from King's College Maurice Wilkins and Rosalind Franklin.

Watson and Crick proposed a model of DNA structure that exactly matched the X-ray patterns: two parallel strands twisted into a right-handed helix. Each chain is composed of a random set of nitrogenous bases strung on the backbone of their sugars and phosphates, and is held together by hydrogen bonds 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: the replication of genetic material, its specificity, the storage of information in the molecule, and its ability to mutate.

The scientists published their discovery in the journal Nature on April 25, 1953. Ten years later, together with Maurice Wilkins, they 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, and 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, which 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 in the set of proteins in their cells, that is, in different amino acid sequences. Genetics claimed that these sequences were determined by genes, which were then believed to serve as the building blocks of life. But no one knew exactly what genes were.

Clarity was brought by the author of the Big Bang theory, physicist Georgiy Gamow, an employee of George Washington University (USA). Based on Watson and Crick's model of a double-stranded DNA helix, he suggested that a gene is a section of DNA, that is, a certain sequence of links - nucleotides. Since each nucleotide is one of four nitrogenous bases, we simply need to figure out how four elements code for twenty. This was the idea of ​​the genetic code.

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

They also found out that the genetic code is three-letter. This means that one amino acid corresponds to three nucleotides. The unit of code is 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 first reported their results at the 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 completely redirected biological research. The fact that each individual has a unique DNA sequence has revolutionized forensic science. Deciphering the human genome has given anthropologists an entirely new method for studying the evolution of our species. The recently invented DNA editor CRISPR-Cas has greatly advanced genetic engineering. Apparently, this molecule contains the solution to the most pressing problems of humanity: cancer, genetic diseases, aging.

Watson And Scream showed that DNA consists of two polynucleotide chains. Each chain is twisted into a spiral to the right, and both of them are twisted together, that is, twisted to the right around the same axis, forming a double helix.

The chains are antiparallel, that is, directed in opposite directions. Each strand of DNA consists of a sugar-phosphate backbone along which the bases are located perpendicular to the long axis of the double helix; The opposing bases of two opposite strands of a double helix are connected by hydrogen bonds.

Sugar phosphate backbones two double helix strands are clearly visible on the spatial DNA model. The distance between the sugar-phosphate backbones of the two chains is constant and equal to the distance occupied by a pair of bases, i.e., one purine and one pyrimidine. Two purines would take up too much space and two pyrimidines would take up too little space to fill the gaps between the two chains.

Along the axis of the molecule, neighboring base pairs are located at a distance of 0.34 nm from one another, which explains the periodicity detected in the X-ray diffraction patterns. Full revolution of the spiral accounts for 3.4 nm, i.e., 10 base pairs. There are no restrictions regarding the sequence of nucleotides in one chain, but due to the rule of base pairing, this sequence in one chain determines the sequence of nucleotides in the other chain. Therefore we say that the two strands of the double helix are complementary to each other.

Watson And Scream published a message about your DNA model in the magazine "" in 1953, and in 1962 they, along with Maurice Wilkins, were awarded the Nobel Prize for this work. In the same year, Kendrew and Perutz received the Nobel Prize for their work on determining the three-dimensional structure of proteins, also performed by X-ray diffraction analysis. Rosalind Franklin, who died of cancer before the prizes were awarded, was not included as a recipient because the Nobel Prize is not awarded posthumously.

In order to recognize the proposed structure as genetic material, it was necessary to show that it is capable of: 1) carrying encoded information and 2) accurately reproducing (replicating). Watson and Crick were aware that their model satisfied these requirements. At the end of their first paper, they cautiously noted: “It has not escaped our attention that the specific base pairing we postulated immediately allows us to postulate a possible copying mechanism for genetic material.”

In a second paper, published in 1953, they discussed the genetic implications of their model. This discovery showed how explicit structure may be associated with function already at the molecular level, giving a powerful impetus to the development of molecular biology.

According to its chemical structure, DNA ( Deoxyribonucleic acid) is biopolymer, whose monomers are nucleotides. That is, DNA is polynucleotide. Moreover, a DNA molecule usually consists of two chains twisted relative to each other along a helical line (often called “helically twisted”) and connected to each other by hydrogen bonds.

The chains can be twisted both to the left and to the right (most often) side.

Some viruses have single strand DNA.

Each DNA nucleotide consists of 1) a nitrogenous base, 2) deoxyribose, 3) a phosphoric acid residue.

Double right-handed DNA helix

The composition of DNA includes the following: adenine, guanine, thymine And cytosine. Adenine and guanine are purins, and thymine and cytosine - to pyrimidines. Sometimes DNA contains uracil, which is usually characteristic of RNA, where it replaces thymine.

The nitrogenous bases of one chain of a DNA molecule are connected to the nitrogenous bases of another strictly according to the principle of complementarity: adenine only with thymine (form two hydrogen bonds with each other), and guanine only with cytosine (three bonds).

The nitrogenous base in the nucleotide itself is connected to the first carbon atom of the cyclic form deoxyribose, which is a pentose (a carbohydrate with five carbon atoms). The bond is covalent, glycosidic (C-N). Unlike ribose, deoxyribose lacks one of its hydroxyl groups. The deoxyribose ring is formed by four carbon atoms and one oxygen atom. The fifth carbon atom is outside the ring and is connected through an oxygen atom to a phosphoric acid residue. Also, through the oxygen atom at the third carbon atom, the phosphoric acid residue of the neighboring nucleotide is attached.

Thus, in one strand of DNA, adjacent nucleotides are linked to each other by covalent bonds between deoxyribose and phosphoric acid (phosphodiester bond). A phosphate-deoxyribose backbone is formed. Directed perpendicular to it, towards the other DNA chain, are nitrogenous bases, which are connected to the bases of the second chain by hydrogen bonds.

The structure of DNA is such that the backbones of the chains connected by hydrogen bonds are directed in different directions (they say “multidirectional”, “antiparallel”). On the side where one ends with phosphoric acid connected to the fifth carbon atom of deoxyribose, the other ends with a “free” third carbon atom. That is, the skeleton of one chain is turned upside down relative to the other. Thus, in the structure of DNA chains, 5" ends and 3" ends are distinguished.

During DNA replication (doubling), the synthesis of new chains always proceeds from their 5th end to the third, since new nucleotides can only be added to the free third end.

Ultimately (indirectly through RNA), every three consecutive nucleotides in the DNA chain code for one protein amino acid.

The discovery of the structure of the DNA molecule occurred in 1953 thanks to the work of F. Crick and D. Watson (which was also facilitated by the early work of other scientists). Although DNA was known as a chemical substance back in the 19th century. In the 40s of the 20th century, it became clear that DNA is the carrier of genetic information.

The double helix is ​​considered the secondary structure of the DNA molecule. In eukaryotic cells, the overwhelming amount of DNA is located in chromosomes, where it is associated with proteins and other substances, and is also more densely packaged.

A person's birth plan is ready when the reproductive cells of the mother and father merge into one. This formation is called a zygote or fertilized egg. The very plan for the development of the organism is contained in the DNA molecule located in the nucleus of this single cell. It is in it that hair color, height, nose shape and everything else that makes a person individual are encoded.

Of course, the fate of a person depends not only on the molecule, but also on many other factors. But the genes laid down at birth also largely influence the fateful path. And they represent a sequence of nucleotides.

Each time a cell divides, DNA doubles. Therefore, each cell carries information about the structure of the entire organism. It is as if, when constructing a brick building, each brick had an architectural plan for the entire structure. You look at just one brick and you already know which building structure it is part of.

The true structure of the DNA molecule was first demonstrated by British biologist John Gurdon in 1962. He took a cell nucleus from a frog's intestine and, using microsurgical techniques, transplanted it into a frog egg. Moreover, in this egg, its own nucleus was previously killed by ultraviolet irradiation.

A normal frog grew from the hybrid egg. Moreover, it was absolutely identical to the one whose cell nucleus was taken. This marked the beginning of the era of cloning. And the first successful result of cloning among mammals was Dolly the sheep. She lived for 6 years and then died.

However, nature itself also creates doubles. This happens when, after the first division of the zygote, two new cells do not remain together, but move apart, and each produces its own organism. This is how identical twins are born. Their DNA molecules are exactly the same, which is why twins are so similar.

In appearance, DNA resembles a rope ladder twisted into a right-handed spiral. And it consists of polymer chains, each of which is formed from 4 types of units: adenine (A), guanine (G), thymine (T) and cytosine (C).

It is in their sequence that the genetic program of any living organism is contained. The figure below, for example, shows nucleotide T. Its top ring is called a nitrogenous base, the five-membered ring at the bottom is a sugar, and on the left is a phosphate group.

The figure shows a thymine nucleotide, which is part of DNA. The remaining 3 nucleotides have a similar structure, but differ in their nitrogenous base. The upper right ring is a nitrogenous base. The lower five-membered ring is sugar. Left group PO - phosphate

Dimensions of a DNA molecule

The diameter of the double helix is ​​2 nm (nm is a nanometer, equal to 10 -9 meters). The distance between adjacent base pairs along the helix is ​​0.34 nm. The double helix makes a full revolution every 10 pairs. But the length depends on the organism to which the molecule belongs. The simplest viruses have only a few thousand links. Bacteria have several million of them. And higher organisms have billions of them.

If you stretch all the DNA contained in one human cell into one line, you will get a thread approximately 2 m long. This shows that the length of the thread is billions of times greater than its thickness. To better imagine the size of a DNA molecule, you can imagine that its thickness is 4 cm. Such a thread, taken from one human cell, can encircle the globe along the equator. At this scale, a person will correspond to the size of the Earth, and the cell nucleus will grow to the size of a stadium.

Is the Watson and Crick model correct?

Considering the structure of the DNA molecule, the question arises of how it, having such a huge length, is located in the nucleus. It must lie in such a way that it is accessible along its entire length for RNA polymerase, which reads the desired genes.

How is replication carried out? After all, after doubling, the two complementary chains must separate. This is quite difficult, since the chains are initially twisted into a spiral.

Such questions initially raised doubts about the validity of the Watson and Crick model. But this model was too specific and simply teased specialists with its inviolability. Therefore, everyone rushed to look for flaws and contradictions.

Some experts assumed that if the ill-fated molecule consists of 2 polymer chains connected by weak non-covalent bonds, then they should diverge when the solution is heated, which can be easily verified experimentally.

The second specialists became interested in nitrogenous bases that form hydrogen bonds with each other. This can be verified by measuring the spectra of the molecule in the infrared region.

Still others thought that if nitrogenous bases were indeed hidden inside the double helix, then it would be possible to find out whether the molecule was affected by those substances that could react only with these hidden groups.

Many experiments were carried out and by the end of the 50s of the 20th century it became clear that the model proposed by Watson and Crick passed all tests. Attempts to refute it failed.

The monomer units of which are nucliatides.

What is DNA?

All information about the structure and functioning of any living organism is contained in encoded form in its genetic material. The basis of the genetic material of an organism is deoxyribonucleic acid (DNA).

DNA in most organisms it is a long, double-chain polymer molecule. Subsequence monomer units (deoxyribonucleotides) in one of its chains corresponds to ( complementary) deoxyribonucleotide sequences into another. 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 specific trait - gene.

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

The information contained in genes encoding proteins (structural genes) is deciphered through two sequential processes:

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

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

DNA structure

DNA- This 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 a DNA molecule

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

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

Levels of DNA structure

It is customary to distinguish 3 levels of DNA structure:

• primary;
• secondary;
• tertiary

Primary structure of DNA is the sequence of arrangement of nucleotides in a 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 the same axis.

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

Tertiary structure of DNA - super-specialization of DNA. The DNA double helix may undergo further helicalization at some sites to form a supercoil or open circular shape, often caused by covalent joining of their open ends. The supercoiled structure of DNA ensures the economical packaging of a very long DNA molecule in a chromosome. Thus, in an elongated form, the length of a DNA molecule is 8 cm, and in the form of a superspiral fits into 5 nm.

Chargaff's rule

E. Chargaff's rule is a pattern of the quantitative content of nitrogenous bases in a DNA molecule:

1. In DNA mole fractions purine and pyrimidine bases are equal: A+G = C+ T or (A +G)/(C + T)=1 .
2. In DNA number of bases with amino groups (A +C) equals number of bases with keto groups (G+ T):A+C= G+ T or (A +C)/(G+ T)= 1
3. The equivalence rule, that is: A=T, G=C; A/T = 1; G/C=1.
4. Nucleotide composition of DNA 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 fluctuates slightly: from 0,54 before 0,98 , in microorganisms it is more than 1.

Watson-Crick DNA model

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

Polynucleotide chains wound on top of each other are held together hydrogen bonds, formed between the complementary bases of opposite chains (Figure 3). Wherein adenine forms a pair only with thymine, A guanine- With cytosine. Base pair A-T is stabilizing two hydrogen bonds, and a couple G-C - three.

The length of double-stranded DNA is usually measured by the number of complementary nucleotide pairs ( P.n.). For DNA molecules consisting of thousands or millions of nucleotide pairs, units are taken t.b.s. And m.p.n. respectively. For example, the DNA of human chromosome 1 is one double helix of length 263 m.b..

Sugar phosphate backbone of the molecule, which consists of phosphate groups and deoxyribose residues connected 5'-3'-phosphodiester bonds, forms the “sidewalls of a spiral staircase”, and the base pairs A-T 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′, other 5’→3′. In accordance with the principle of complementarity, if one of the chains contains a nucleotide sequence 5-TAGGCAT-3′, then in the complementary chain in this place there should be a sequence 3′-ATCCGTA-5′. In this case, the double-stranded form would look like this:

• 5′-TAGGCAT-3′
• 3-ATCCGTA-5′.

In such a recording 5′ end of the top chain always placed on the left, and 3′ end- on right.

The carrier of genetic information must satisfy two basic requirements: reproduce (replicate) with high accuracy And determine (encode) the synthesis of protein molecules.

Watson-Crick DNA model fully meets these requirements because:

• According to the principle of complementarity, each DNA strand can serve as a template for the formation of a new complementary chain. Consequently, 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 a structural gene uniquely determines 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 takes up 60 billion terabytes, which are stored on 150-160 grams of DNA.
2. International DNA Day celebrated on April 25. On this day in 1953 James Watson And Francis Creek published in a magazine Nature his article entitled "Molecular structure of nucleic acids" , where the double helix of the DNA molecule was described.

Bibliography: Molecular biotechnology: principles and applications, B. Glick, J. Pasternak, 2002