On April 25, 1953, an article by James Watson and Francis Crick "The Structure of Deoxyribonucleic Acid" was published in the American journal Nature. The publication occupied a little more than one page, it contained only one very simple drawing. So, 50 years ago, a model of the spatial structure of DNA was first proposed.
Undoubtedly, the solution to the structure of the DNA molecule caused a revolution in natural science and led to a number of new discoveries, without which it is impossible to imagine not only modern science, but also modern life generally. Watson and Crick's discovery was followed by an explosion of genetic research. Knowing the structure of DNA helped to understand the process of DNA replication (doubling) and, thus, to establish how genetic information is transmitted from generation to generation. Subsequently, a genetic code was discovered that carries information about the primary structure of proteins - the main components of all cells. The solution to the structure of the hereditary apparatus of the cell served as a starting point in the development new science — molecular biology. The emergence of its methods such as polymerase chain reaction, molecular cloning, sequencing would be unthinkable without knowing the structure of DNA.
Without a doubt, this discovery served as a significant impetus for the development of genetics, the apogee of which was scientific program"Human Genome". Watson became the first leader of this project, in which the hereditary information of Homo sapiens was completely deciphered. Knowledge of the human genome in the future will make it possible to reveal the cause of many diseases, to create drugs for the so-called gene therapy aimed at correcting "diseased genes" or replacing "spoiled" genes with "healthy ones".
Over the past 50 years, it has become clear that Watson and Crick's work on the structure of DNA has changed the whole of biology and has proved to be the most important for medicine. It is difficult to name the area of natural sciences, the development of which was not influenced by their discovery. In 1962, James Watson, Francis Crick, along with Maurice Wilkins, a specialist in X-ray diffraction analysis, received the Nobel Prize. This is perhaps the most outstanding event in the history of natural science in the 20th century.
By the way, another significant event of this year is the anniversary of one of the "fathers" of the double helix, James Watson, who turns 75 years old. It is hard to believe that at the time of the publication of that very article in the journal Nature, which turned the whole world, he was only 25 years old. Now Professor Watson runs the Cold Spring Harbor Laboratory in New York.
Discovery of DNA
More than fifty years ago, a remarkable scientific discovery was made. On April 25, 1953, an article was published on how the most mysterious molecule, the deoxyribonucleic acid molecule, works.
It is called DNA for short. This molecule is found in all living cells of all living organisms. Scientists discovered it more than a hundred years ago. But then no one knew how this molecule is arranged and what role it plays in the life of living beings.
Finally solved the mystery English physicist Francis Crick and American biologist James Watson.
Their discovery was very important.
And not only for biologists, who finally found out how the molecule works, which controls all the properties of a living organism.
One of the largest discoveries of mankind was made in such a way that it is absolutely impossible to say what science this discovery belongs to - chemistry, physics and biology are so closely merged in it.
This fusion of sciences is the most striking feature of the discovery of Crick and Watson.
History of the discovery of DNA
Scientists have long been interested in the secret of the main property of all living organisms - reproduction.
Why do children - whether we are talking about people, bears, viruses - look like their parents, grandparents? In order to discover the secret, biologists have examined a variety of organisms.
And scientists have found that special particles of a living cell - chromosomes - are responsible for the similarity of children and parents. They are like small sticks. Small sections of the rod-chromosome are called genes. There are a lot of genes, and each is responsible for some sign of the future organism.
If we talk about a person, then one gene determines the color of the eyes, the other determines the shape of the nose ... But what the gene consists of and how it works, scientists did not know this. True, it was already known: chromosomes contain DNA and DNA has something to do with genes.
Different scientists wanted to unravel the mystery of the gene: each looked at this mystery from the point of view of his science. But in order to find out how a gene, a small particle of DNA, is arranged, it was necessary to find out how the molecule itself is arranged and what it consists of.
Chemists who research chemical composition substances, studied the chemical composition of the DNA molecule.
Physicists began to shine through DNA x-rays, as usual, they shine through the crystals to find out how these crystals are arranged. And they found out that DNA is like a helix.
Biologists were interested in the mystery of the gene, of course, more than anyone else.
And Watson decided to tackle the problem of the gene. In order to learn from advanced biochemists and learn more about the nature of the gene, he traveled from America to Europe.
At that time, Watson and Crick did not yet know each other. Watson, after working for some time in Europe, made no significant progress in elucidating the nature of the gene.
But on one of scientific conferences he learned that physicists study the structure of the DNA molecule using their own physical methods.
Upon learning this, Watson realized that physicists would help him discover the secret of the gene, and went to England, where he got a job in a physical laboratory in which biological molecules were studied.
It was here that Watson and Crick met.
How Physicist Crick Became Interested in Biology
Crick had no interest in biology at all. Until a book caught his eye famous physicist Schrödinger "What is life from the point of view of physics?".
Schrödinger noticed that the "reproduction" of genes resembles the growth of a crystal, and suggested that scientists consider a gene a crystal. This proposal interested Crick and other physicists. That's why.
A crystal is a very simple physical body in structure: the same group of atoms repeats in it all the time. And the device of the gene was considered very complicated, since there are so many of them and they are all different. If genes are made up of the substance of DNA, and the DNA molecule is arranged in the same way as a crystal, then it turns out that it is both complex and simple.
How so?
Watson and Crick realized that physicists and biologists knew too little about the DNA molecule. True, something was known about DNA to chemists.
How Watson helped the chemists, and the chemists helped the cry
Chemists knew that the DNA molecule contained four chemical compounds: adenine, thymine, guanine, and cytosine.
They were designated by the first letters - A, T, G, C. Moreover, there was as much adenine as thymine, and guanine as much as cytosine. Why? Chemists could not understand this.
They guessed: it had something to do with the structure of the molecule.
But how, they did not know. The biologist Watson helped the chemists.
Watson was accustomed to the fact that in wildlife many things occur in pairs: a pair of eyes, a pair of arms, a pair of legs, there are, for example, two sexes: male and female ... It seemed to him that a DNA molecule could also consist of two chains. But if DNA is like a helix, as X-ray physicists have found out, then how do two strands in this helix hold each other?
Watson suggested that with the help of A, G, C and T, which, like hands, are stretched out to each other. Having cut out the contours of these chemical compounds from cardboard, Watson applied them for a long time this way and that, until he suddenly saw: adenine combines perfectly with thymine, and guanine with cytosine.
Watson told Crick about it.
He quickly figured out how the double helix should actually look - in space, and not in the picture.
Both scientists began to build a model of DNA.
How is it to "build"? That's how. From a molecular constructor that resembles a children's toy constructor. In the molecular designer, the parts are balls-atoms, which are fastened to each other with buttons in the order in which the atoms are located in the substance.
The molecular designer was invented by another scientist, the chemist Pauling. He built models of protein molecules and found out that they must have sections that look like spirals.
Very soon this was confirmed by the physicists of the laboratory where Crick worked. An important biological problem was solved theoretically.
Crick liked Pauling's method so much that he suggested that Watson build a model of DNA using a molecular constructor. This is how the model of the famous DNA Double Helix was created, which you can see in the picture.
And what is remarkable: due to the fact that A in one chain can "stick" only with T in another, and G - only with C, the "chemical" rule is automatically fulfilled, according to which the amount of A is equal to the amount of T, and the amount of G is the amount of C.
But the most important thing is that, looking at the Double Helix of DNA, it is immediately clear how to solve the puzzle of gene reproduction. It is enough to "unwind" the DNA pigtail, and each chain will be able to complete a new one on itself so that A sticks together with T, and G - with C: there was one gene - there are two. Due to the fact that the dimensions steam A-T and G-C are the same, the DNA molecule actually resembles a crystal in structure, as physicists assumed.
And at the same time, this "crystal" can contain a variety of combinations of A, T, C, G, and therefore all genes are different.
The solution of the gene problem by Watson and Crick led to the fact that literally in 2–3 years a whole new area natural science, which was called molecular biology.
Often it is called physico-chemical biology.
Importance of DNA discovery
The question of what and how is written in DNA has accelerated decoding genetic code.
The realization that genes are DNA, the universal carrier of genetic information, led to the emergence of genetic engineering. Today, university students are already deciphering the alternation of nucleotides in DNA, connecting the genes of different organisms, transferring them between species, genera, and much more distant taxa. On the basis of genetic engineering, biotechnology arose, which the famous science fiction writer S.
Lem defined it as the use of patterns of biogenesis in production.
Let us recall what V.L.
Johannsen, the man who in 1909 gave the very name of the gene: “The properties of organisms are determined by special, under certain circumstances separable from each other and therefore to a certain extent independent units or elements in germ cells, which we call genes.
Since then, the situation has changed significantly.
We have seen that, apart from atoms and molecules, there is nothing in the cell. And it obeys the same physical laws as inanimate objects, as physicists who rushed into biology in the 1940s were able to verify, precisely in search of some fundamentally new laws of nature unknown to physics.
All reactions of cellular metabolism are carried out under the control of biocatalysts - enzymes, the structure of which is recorded in the DNA of genes.
This record is transmitted in the chain of information transfer DNA RNA PROTEIN.
First, the information recorded in the form of an alternation of deoxyribonucleotides on one of the two complementary chains in the DNA of the gene is copied to a single-stranded molecule of informational ribonucleic acid - mRNA (aka mRNA from the English.
messenger - carrier). This is the process of transcription. At the next stage, the sequence of amino acid residues of the polypeptide is built using the mRNA template. This creates the primary structure of the future protein molecule. This is the translation process. The primary structure determines the way the protein molecule folds and thus determines its enzymatic or any other, for example structural or regulatory, function.
These ideas originated in the early 40s, when J.
Beadle and E. Tatum put forward their famous slogan "One gene - one enzyme"*. He, like political slogans, divided the scientific community into supporters and opponents of the stated hypothesis about the equality of the number of genes and the number of enzymes in a cell.
The arguments in the discussion that arose were the facts obtained during the development of the so-called gene-enzyme systems, in which gene mutations were studied, their location within genes was determined, and changes in the enzymes encoded by these genes were taken into account: substitutions of amino acid residues in their polypeptide chains, their effect on enzymatic activity etc. Now we know that one enzyme can be encoded in several genes if it consists of different subunits, that is, different polypeptide chains.
We know that there are genes that do not encode polypeptides at all. These are genes encoding transfer RNA (tRNA) or ribosomal RNA (rRNA) involved in protein synthesis.
In its original form, the One Gene, One Enzyme principle is of historical interest but deserves a monument because it stimulated the creation of an entire scientific field- comparative molecular biology of the gene, in which genes - units of hereditary information appear as independent subjects of study.
In addition, the development of numerous gene-enzyme systems helped to formulate the question: what and how is written in the genetic code?
To this question in general form answered F.
Crick with his colleagues in 1961. It turned out that the code is triplet - each coding unit-codon consists of three nucleotides. In each gene, triplets are read from a fixed point, in one direction and without commas, that is, codons are not separated from each other in any way.
The codon sequence determines the sequence of amino acid residues in polypeptides.
Thus, due to the specific organization of the genetic code, nonsense codons play a special role as translation terminators. Therefore, arising by mutation, they, like frameshift mutations, appear much more often and more clearly than missense mutations that change the meaning of codons. *
* Kapitsa S.P., Kurdyumov S.P., Malinetsky G.G.
Synergetics and forecasts of the future. - M., 2001. - S. 97.
Nonsense and readout shifts are often found in the so-called pseudogenes, which were discovered in the early 1980s as a result of studying nucleotide sequences in the genomes of higher eukaryotes.
Pseudogenes are very similar to ordinary genes, but their expression is reliably "locked" by clearly manifested mutations: readout shifts and nonsense. Pseudogenes represent the reserve of the evolutionary process. Their fragments are used in the emergence of new genes.
Evidence for the role of DNA in heredity
Counteraction of DNA and chromosomes to the influences of the external environment
Enzymatic functions of RNA, vaccines
What is DNA, content in cells
Participation in heredity, properties of molecules
Methods for obtaining DNA, biosynthesis
Stages of DNA cloning, chem.
compound
Biological role of DNA
DNA, RNA-containing viruses
Repair, recombination, replication, types, DNA synthesis
Deoxyribonucleic acid (DNA) is one of two types of nucleic acids that provide storage, transmission from generation to generation and implementation of the genetic program for the development and functioning of living organisms.
The main role of DNA in cells is the long-term storage of information about the structure of RNA and proteins.
Deciphering the structure of DNA (1953) was one of the turning points in the history of biology.
In the scientific literature devoted to the study of DNA, the names of J. Watson and Francis Crick are most often found as scientists who created a model of the structure of the DNA molecule in 1953.
However, the molecule itself was discovered much earlier and not by these scientists. The name of the discoverer is not mentioned in every textbook, reference book or encyclopedia.
The primacy of the discovery of deoxyribonucleic acid is attributed to the young Swiss physician Johann Friedrich Miescher. In 1869, while working in Germany, he studied the chemical composition of animal cells. He chose leukocyte cells as the object of his research. The scientist isolated leukocytes from purulent material, because
it is in the pus that there are a lot of these white blood cells that perform a protective function in the body and destroy microbes. From the local surgical hospital, he was supplied with bandages taken from fresh, festering wounds. Misher washed leukocytes from the tissue of the bandages, and then isolated protein molecules from the washed cells. In the process of research, he managed to establish that, in addition to protein, leukocytes contain some other unexplored substance.
It was isolated in the form of a precipitate of a filamentous or flocculent structure when an acidic environment was created. When the solution was alkalized, the precipitate dissolved. Examining the preparation of leukocytes under a microscope, Misher discovered that in the process of washing the leukocytes with dilute hydrochloric acid, only nuclei remain. Based on this, he concluded that the nuclei of cells contained an unknown substance, and called it nuclein, from Latin word nucleus, which means "core" in translation.
Upon closer examination, Miescher developed a whole system for the isolation and purification of nucleins.
He subjected the isolated compound to treatment with ether and other organic solvents, and made sure that it was not a fatty compound, i.e.
because it did not dissolve in these substances. They did not have nucleins and protein nature, tk. when treated with enzymes that decompose proteins, they did not undergo any changes.
Chemical analysis, in those days, was imperfect, inaccurate and time-consuming.
Slowly but surely, the scientist went through it and determined that nuclein is composed of carbon, hydrogen, oxygen and phosphorus. Phosphoric organic compounds then they were still practically unknown, so Misher concluded that he had discovered a class of compounds contained inside the cell, unknown to science.
He wanted to publish an article about his new discovery in the journal Medico-Chemical Research, which was published by his teacher, one of the founders of biochemistry, Felix Hoppe-Seyler.
But before publishing the material, he decided to check his data in his laboratory. This research took a whole year, and only in 1871, in one of the issues of the journal, Misher's work was published. It was accompanied by two articles by Hoppe-Seyler himself and his associate, with confirmation of data on the composition and properties of nucleins.
After returning to Switzerland, Miescher accepted an offer to take the position of head of the department of physiology at the University of Basel.
There he continued his research. At the new place, the scientist used for experiments a more pleasant, and no less nuclein-rich material - the milk of salmon fish (they are still used for the same purposes). On the banks of the salmon-rich Rhine, which flows through Basel, he had no shortage of research material.
It seemed to the scientist that the compound he discovered was so simple and uniform that he could not in any way imagine that it was in it that the whole variety of hereditary characteristics of living organisms could be stored. The methods of biochemical analysis that existed at that time did not yet make it possible to detect significant differences between human and salmon nucleins and, moreover, to recognize such a complex structure, which has not yet been fully recognized.
Nucleic acids were first discovered in the nucleus human cells Swiss explorer Friedrich Miescher in 1869. At the beginning of the 20th century, biologists and biochemists managed to find out the structure and basic properties of the cell. It was found that one of the nucleic acids, DNA, is an extremely large molecule consisting of structural units called nucleotides, each of which contains nitrogenous bases.
Maurice Wilkins and Rosalyn Franklin, scientists from the University of Cambridge, conducted X-ray diffraction analysis of DNA molecules and showed that they are a double helix resembling a spiral staircase. The data they obtained led the American biochemist James Watson to the idea of studying the chemical structure of nucleic acids. The National Society for the Study of Infantile Paralysis provided a grant. In October 1951, at the Cavendish Laboratory at the University of Cambridge, Watson took up the study of the spatial structure of DNA with John C. Kendrew and Francis Crick, a physicist who was interested in biology and was writing his doctoral dissertation at that time.
Watson and Crick knew that there are two types of nucleic acids - deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), each of which consists of a monosaccharide of the pentose group, phosphate and four nitrogenous bases: adenine, thymine (in RNA - uracil), guanine and cytosine. Over the next eight months, Watson and Crick summarized their results with those already available and in February 1953 made a report on the structure of DNA. A month later, they created a three-dimensional model of the DNA molecule, made from balloons, pieces of cardboard and wire.
According to the Crick-Watson model, DNA is a double helix, consisting of two chains of deoxyribose phosphate connected by base pairs, similar to the rungs of a ladder. Through hydrogen bonding, adenine combines with thymine, and guanine with cytosine. With this model, it was possible to trace the replication of the DNA molecule itself. According to Watson and Crick, the two parts of the DNA molecule are separated from each other at the sites of hydrogen bonds, which is very similar to unzipping a zipper. From each half of the old molecule, a new DNA molecule is synthesized. The base sequence functions as a template, or template, for the formation of new DNA molecules. The discovery of the chemical structure of DNA has been hailed worldwide as one of the most outstanding biological discoveries century.
DNA plays an extremely important role in both the maintenance and reproduction of life. Firstly, it is the storage of hereditary information, which is contained in the nucleotide sequence of one of its chains. The smallest unit of genetic information after a nucleotide is three consecutive nucleotides - a triplet. Triplets located one after another, which determine the structure of one chain, represent the so-called gene. The second function of DNA is the transmission of hereditary information from generation to generation. DNA is involved as a template in the process of transferring genetic information from the nucleus to the cytoplasm to the site of protein synthesis.
Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine "for their discoveries about the molecular structure of nucleic acids and for determining their role in the transmission of information in living matter." In a presentation speech, A. V. Engstrom of the Karolinska Institute described DNA as “a polymer composed of several types of building blocks—a monosaccharide, a phosphate, and nitrogenous bases... The monosaccharide and phosphate are the repeating elements of the giant DNA molecule, in addition, it contains four type of nitrogenous bases. The discovery is the order of the spatial connection of these building blocks.
What has this discovery changed in our lives over the past 50+ years??
In 1969, scientists first synthesized an artificial enzyme, in 1971 - an artificial gene. At the end of the 20th century, it became possible to create completely artificial microorganisms. So, artificial bacteria were created in laboratories that produce amino acids unusual for them, as well as viable "synthetic" viruses. Work is underway to create more complex artificial organisms - plants and animals.
The study of the structure and biochemistry of DNA led to the creation of a technique for modifying the genome and cloning. In 1980, the first patent for experiments with mammalian genes was issued, and a year later, a transgenic mouse with an artificially modified genome was created. In 1996, the first cloned mammal, Dolly the Sheep, was born, followed by cloned mice, rats, cows, and monkeys.
In 2002, the Human Genome Project was successfully completed, during which a complete genetic map of human cells was created. And in the same year, attempts to clone a person began, although so far none of them has been completed (at least, there are no scientific data on successful human cloning).
Back in 1978, insulin was created, almost completely identical to human, and then its gene was introduced into the genome of bacteria, which turned into an “insulin factory”. In 1990, a gene therapy method was first tested, which saved the life of a four-year-old girl who suffered from a severe immune disorder. Now the study of the genetic mechanisms of the development of a variety of diseases - from cancer to arthritis - and the search for methods to correct the genetic "mistakes" that cause them, are in full swing. In total, more than 350 drugs and vaccines are used in clinical practice, the creation of which uses genetic engineering.
DNA analysis has found wide application even in forensics. It is used during litigation to recognize paternity (by the way, this method has become a real gift for musicians, politicians and actors who were forced to prove in court their innocence in the birth of children attributed to them), as well as to identify the criminal. It is worth noting that James Watson himself spoke about such a possibility of using DNA, who proposed creating a database that would include personal DNA structures of all the inhabitants of the planet, which would speed up the process of identifying criminals and their victims.
Using DNA, you can "catch" not only criminals, but also, for example, drugs or biological weapons. American forensic scientists use the system of control of the DNA structure of drug plants to create a database of all varieties of marijuana. This database will allow you to trace the source of almost any drug sample. In the near future, DNA-based methods for detecting biological attacks will be used in the United States - it is planned to install special sensors in public places that will automatically "catch" dangerous microorganisms from the air and give a warning signal.
In 1982, the first successful modification of the plant genome was carried out. And five years later, the first agricultural plants with a modified genome appeared on the fields (these were tomatoes resistant to viral diseases).
Now almost all food is grown with the help of genetic engineering, especially crops such as soybeans and corn. Since 1996, when the commercial use of genetically modified products began, the total area under their crops has increased 50 times. The total sown area under transgenic crops in the world in 2005 amounted to 90 million hectares. True, the governments of many countries have banned the cultivation and import of such products, as a number of studies have shown that they can pose a danger to human health (allergies, damage to reproductive function, etc.).
The ability to study the structure of DNA has provided a new impetus historical research. For example, the remains of Nicholas II and his family were identified, and some historical gossip was confirmed and refuted (in particular, it was proved that one of the founders of the United States, Thomas Jefferson, had illegitimate children from a black slave).
With the help of DNA analysis, it was possible to trace the origin of both people and entire nations. For example, it was shown that the genes of the Japanese are almost identical to the genes of one of the tribes Central America. And for only $349, black Americans can find out what region of Africa and even what tribe their ancestors came from, brought on slave ships many years ago.
What will DNA give us in the near future??
Obviously, this will be the cloning of a person and his organs, which will solve the problem of the lack of donor hearts and lungs for transplantation. There will be new drugs that will make incurable genetic diseases a thing of the past...
More than fifty years ago, a remarkable scientific discovery was made. On April 25, 1953, an article was published on how the most mysterious molecule, the deoxyribonucleic acid molecule, works. It is called DNA for short. This molecule is found in all living cells of all living organisms. Scientists discovered it more than a hundred years ago. But then no one knew how this molecule is arranged and what role it plays in the life of living beings.
The English physicist Francis Crick and the American biologist James Watson managed to finally solve the mystery.
Their discovery was very important. And not only for biologists, who finally found out how the molecule works, which controls all the properties of a living organism.
One of the largest discoveries of mankind was made in such a way that it is absolutely impossible to say what science this discovery belongs to - chemistry, physics and biology are so closely merged in it. This fusion of sciences is the most striking feature of the discovery of Crick and Watson.
History of the discovery of DNA
Scientists have long been interested in the secret of the main property of all living organisms - reproduction. Why do children - whether we are talking about people, bears, viruses - look like their parents, grandparents? In order to discover the secret, biologists have examined a variety of organisms.
And scientists have found that special particles of a living cell - chromosomes - are responsible for the similarity of children and parents. They are like small sticks. Small sections of the rod-chromosome are called genes. There are a lot of genes, and each is responsible for some sign of the future organism. If we talk about a person, then one gene determines the color of the eyes, the other determines the shape of the nose ... But what the gene consists of and how it works, scientists did not know this. True, it was already known: chromosomes contain DNA and DNA has something to do with genes.
Different scientists wanted to unravel the mystery of the gene: each looked at this mystery from the point of view of his science. But in order to find out how a gene, a small particle of DNA, is arranged, it was necessary to find out how the molecule itself is arranged and what it consists of.
Chemists who study the chemical composition of substances have studied the chemical composition of the DNA molecule. Physicists began to scan DNA with X-rays, as they usually do with crystals, to find out how these crystals are arranged. And they found out that DNA is like a helix.
Biologists were interested in the mystery of the gene, of course, more than anyone else. And Watson decided to tackle the problem of the gene. In order to learn from advanced biochemists and learn more about the nature of the gene, he traveled from America to Europe.
At that time, Watson and Crick did not yet know each other. Watson, after working for some time in Europe, made no significant progress in elucidating the nature of the gene.
But at one of the scientific conferences, he learned that physicists study the structure of the DNA molecule using their own physical methods. Upon learning this, Watson realized that physicists would help him discover the secret of the gene, and went to England, where he got a job in a physical laboratory in which biological molecules were studied. It was here that Watson and Crick met.
How Physicist Crick Became Interested in Biology
Crick had no interest in biology at all. Until he came across a book by the famous physicist Schrödinger "What is life from the point of view of physics?".
In this book, the author suggested that the chromosome is like a crystal. Schrödinger noticed that the "reproduction" of genes resembles the growth of a crystal, and suggested that scientists consider a gene a crystal. This proposal interested Crick and other physicists. That's why.
A crystal is a very simple physical body in structure: the same group of atoms repeats in it all the time. And the device of the gene was considered very complicated, since there are so many of them and they are all different. If genes are made up of the substance of DNA, and the DNA molecule is arranged in the same way as a crystal, then it turns out that it is both complex and simple. How so?
Watson and Crick realized that physicists and biologists knew too little about the DNA molecule. True, something was known about DNA to chemists.
How Watson helped the chemists, and the chemists helped the cry
Chemists knew that the DNA molecule contained four chemical compounds: adenine, thymine, guanine, and cytosine. They were designated by the first letters - A, T, G, C. Moreover, there was as much adenine as thymine, and guanine as much as cytosine. Why? Chemists could not understand this.
They guessed: it had something to do with the structure of the molecule. But how, they did not know. The biologist Watson helped the chemists.
Watson was accustomed to the fact that in wildlife many things occur in pairs: a pair of eyes, a pair of arms, a pair of legs, there are, for example, two sexes: male and female ... It seemed to him that a DNA molecule could also consist of two chains. But if DNA is like a helix, as X-ray physicists have found out, then how do two strands in this helix hold each other? Watson suggested that with the help of A, G, C and T, which, like hands, are stretched out to each other. Having cut out the contours of these chemical compounds from cardboard, Watson applied them for a long time this way and that, until he suddenly saw: adenine combines perfectly with thymine, and guanine with cytosine.
Watson told Crick about it. He quickly figured out how the double helix should actually look - in space, and not in the picture. Both scientists began to build a model of DNA.
How is it to "build"? That's how. From a molecular constructor that resembles a children's toy constructor. In the molecular designer, the parts are balls-atoms, which are fastened to each other with buttons in the order in which the atoms are located in the substance.
The molecular designer was invented by another scientist, the chemist Pauling. He built models of protein molecules and found out that they must have sections that look like spirals. Very soon this was confirmed by the physicists of the laboratory where Crick worked. An important biological problem was solved theoretically.
Crick liked Pauling's method so much that he suggested that Watson build a model of DNA using a molecular constructor. This is how the model of the famous DNA Double Helix was created, which you can see in the picture.
And what is remarkable: due to the fact that A in one chain can "stick" only with T in another, and G - only with C, the "chemical" rule is automatically fulfilled, according to which the amount of A is equal to the amount of T, and the amount of G is the number of Cs. But the most important thing is that, looking at the Double Helix of DNA, it is immediately clear how to solve the riddle of gene reproduction. It is enough to "unwind" the DNA pigtail, and each chain will be able to complete a new one on itself so that A sticks together with T, and G - with C: there was one gene - there are two. Due to the fact that the sizes of the A-T and G-C pairs are the same, the DNA molecule actually resembles a crystal in structure, as physicists assumed.
And at the same time, this "crystal" can contain a variety of combinations of A, T, C, G, and therefore all genes are different.
The solution of the gene problem by Watson and Crick led to the fact that literally in 2–3 years a whole new field of natural science was formed, which was called molecular biology. Often it is called physico-chemical biology.
Importance of DNA discovery
The question of what is written in DNA and how has accelerated the deciphering of the genetic code. The realization that genes are DNA, the universal carrier of genetic information, led to the emergence of genetic engineering. Today, university students are already deciphering the alternation of nucleotides in DNA, connecting the genes of different organisms, transferring them between species, genera, and much more distant taxa. On the basis of genetic engineering, biotechnology arose, which the famous science fiction writer S. Lem defined as the use of the laws of biogenesis in production.
Let us recall what V.L. Johannsen, the man who in 1909 gave the very name of the gene: “The properties of organisms are determined by special, under certain circumstances separable from each other and therefore to a certain extent independent units or elements in germ cells, which we call genes.
Since then, the situation has changed significantly. We have seen that, apart from atoms and molecules, there is nothing in the cell. And it obeys the same physical laws as inanimate objects, as physicists who rushed into biology in the 1940s were able to verify, precisely in search of some fundamentally new laws of nature unknown to physics. All reactions of cellular metabolism are carried out under the control of biocatalysts - enzymes, the structure of which is recorded in the DNA of genes. This record is transmitted in the chain of information transfer DNA RNA PROTEIN.
First, the information recorded in the form of an alternation of deoxyribonucleotides on one of the two complementary chains in the DNA of the gene is copied to a single-stranded molecule of informational ribonucleic acid - mRNA (it is also mRNA from the English messenger - carrier). This is the process of transcription. At the next stage, the sequence of amino acid residues of the polypeptide is built using the mRNA template. This creates the primary structure of the future protein molecule. This is the translation process. The primary structure determines the way the protein molecule folds and thus determines its enzymatic or any other, for example structural or regulatory, function.
These ideas originated in the early 40s, when J. Beadle and E. Tatum put forward their famous slogan "One gene - one enzyme" *. He, like political slogans, divided the scientific community into supporters and opponents of the stated hypothesis about the equality of the number of genes and the number of enzymes in a cell. The arguments in the discussion that arose were the facts obtained during the development of the so-called gene-enzyme systems, in which gene mutations were studied, their location within genes was determined, and changes in the enzymes encoded by these genes were taken into account: substitutions of amino acid residues in their polypeptide chains, their effect on enzymatic activity etc. Now we know that one enzyme can be encoded in several genes if it consists of different subunits, that is, different polypeptide chains. We know that there are genes that do not encode polypeptides at all. These are genes encoding transfer RNA (tRNA) or ribosomal RNA (rRNA) involved in protein synthesis.
In its original form, the "One gene - one enzyme" principle is of rather historical interest, but deserves a monument, since it stimulated the creation of an entire scientific field - comparative molecular biology of the gene, in which genes - units of hereditary information appear as independent subjects of study.
DNA is a universal source and keeper of hereditary information, which is recorded using a special sequence of nucleotides; it determines the properties of all living organisms.
Medium molecular mass nucleotide is taken equal to 345, and the number of nucleotide residues can reach several hundreds, thousands and even millions. DNA is mostly found in the nuclei of cells. Little is found in chloroplasts and mitochondria. However, the DNA of the cell nucleus is not a single molecule. It consists of many molecules that are distributed over different chromosomes, their number varies depending on the organism. This is the structure of DNA.
History of the discovery of DNA
The structure and functions of DNA were discovered by James Watson and Francis Crick, they were even given Nobel Prize in 1962.
But for the first time, the Swiss scientist Friedrich Johann Miescher, who worked in Germany, discovered nucleic acids. In 1869 he studied animal cells - leukocytes. To obtain them, he used bandages with pus, which he got from hospitals. Misher washed out leukocytes from pus, and isolated protein from them. In the course of these studies, the scientist managed to establish that in addition to proteins, there is something else in leukocytes, some unknown substance at that time. It was a filamentous or flaky precipitate that stood out if an acidic environment was created. The precipitate immediately dissolved upon addition of alkali.
Using a microscope, the scientist discovered that when leukocytes are washed with hydrochloric acid, nuclei remain from the cells. Then he concluded that there was an unknown substance in the nucleus, which he called nuclein (the word nucleus in translation means nucleus).
After conducting a chemical analysis, Misher found out that the new substance in its composition has carbon, hydrogen, oxygen and phosphorus. At that time, few organophosphorus compounds were known, so Friedrich thought he had discovered a new class of compounds found in the cell nucleus.
Thus, in the 19th century, the existence of nucleic acids was discovered. However, at that time, no one could even think about what an important role they played.
The substance of heredity
The structure of DNA continued to be studied, and in 1944 a group of bacteriologists led by Oswald Avery received evidence that this molecule deserved serious attention. The scientist has been studying pneumococci, organisms that cause pneumonia or lung disease, for many years. Avery conducted experiments mixing pneumococci that cause disease with those that are safe for living organisms. First, disease-causing cells were killed, and then those that did not cause diseases were added to them.
The results of the research amazed everyone. There were such living cells that, after interacting with the dead, learned to cause disease. The scientist found out the nature of the substance that is involved in the process of transmitting information to living cells from dead ones. The DNA molecule turned out to be this substance.
Structure
So, it is necessary to understand what structure the DNA molecule has. The discovery of its structure was a significant event, it led to the formation of molecular biology - a new branch of biochemistry. DNA in large quantities ax is located in the nuclei of cells, but the size and number of molecules depend on the type of organism. It has been established that the nuclei of mammalian cells contain many of these cells, they are distributed over chromosomes, there are 46 of them.
Studying the structure of DNA, in 1924 Felgen first established its localization. The evidence obtained during the experiments showed that DNA is located in mitochondria (1-2%). In other places, these molecules can be found during a viral infection, in basal bodies, and also in the eggs of some animals. It is known that the more complex the organism, the greater the mass of DNA. The number of molecules in the cell depends on the function and is usually 1-10%. The least of them is in myocytes (0.2%), more - in germ cells (60%).
The structure of DNA showed that in the chromosomes of higher organisms they are associated with simple proteins - albumins, histones and others, which together form DNP (deoxyribonucleoprotein). Usually a large molecule is unstable, and in order for it to remain intact and unchanged during evolution, a so-called repair system has been created, which consists of enzymes - ligases and nucleases responsible for the "repair" of the molecule.
Chemical structure of DNA
DNA is a polymer, a polynucleotide, consisting of a huge number (up to tens of thousands of millions) of mononucleotides. The structure of DNA is as follows: mononucleotides contain nitrogenous bases - cytosine (C) and thymine (T) - from pyrimidine derivatives, adenine (A) and guanine (G) - from purine derivatives. In addition to nitrogenous bases, the human and animal molecule contains 5-methylcytosine, a minor pyrimidine base. Nitrogenous bases bind to phosphoric acid and deoxyribose. The structure of DNA is shown below.
Chargaff rules
The structure and biological role of DNA were studied by E. Chargaff in 1949. In the course of his research, he revealed patterns that are observed in the quantitative distribution of nitrogenous bases:
- ∑T + C \u003d ∑A + G (that is, the number of pyrimidine bases is equal to the number of purines).
- The number of adenine residues is always equal to the number of thymine residues, and the amount of guanine is equal to cytosine.
- The specificity coefficient has the formula: G+C/A+T. For example, in humans it is 1.5, in a bull it is 1.3.
- The sum of "A + C" is equal to the sum of "G + T", that is, there is as much adenine and cytosine as there is guanine and thymine.
DNA structure model
It was created by Watson and Crick. Phosphate residues and deoxyribose are located along the ridge of two polynucleotide chains twisted in a spiral manner. It has been determined that the planar structures of pyrimidine and purine bases are located perpendicular to the chain axis and form, as it were, steps of a staircase in the form of a spiral. It has also been established that A is always connected to T with the help of two hydrogen bonds, and G is attached to C by three of the same bonds. This phenomenon was given the name "principle of selectivity and complementarity".
Levels of structural organization
A polynucleotide chain bent like a helix is a primary structure that has a certain qualitative and quantitative set of mononucleotides linked by a 3',5'-phosphodiester bond. Thus, each of the chains has a 3' end (deoxyribose) and a 5' end (phosphate). The regions that contain genetic information are called structural genes.
The double helix molecule is a secondary structure. Moreover, its polynucleotide chains are antiparallel and are linked by hydrogen bonds between the complementary bases of the chains. It has been established that each turn of this helix contains 10 nucleotide residues, its length is 3.4 nm. This structure is also supported by the van der Waals interaction forces that are observed between the bases of the same chain, including repulsive and attractive components. These forces are explained by the interaction of electrons in neighboring atoms. The electrostatic interaction also stabilizes the secondary structure. It occurs between positively charged histone molecules and a negatively charged DNA strand.
Tertiary structure is the winding of DNA strands around histones or supercoiling. Five types of histones have been described: H1, H2A, H2B, H3, H4.
The folding of nucleosomes into chromatin is a quaternary structure, so a DNA molecule that is several centimeters long can fold up to 5 nm.
Functions of DNA
The main functions of DNA are:
- Storage of hereditary information. The sequence of amino acids in a protein molecule is determined by the order in which the nucleotide residues are located in the DNA molecule. It also encodes all the information about the properties and characteristics of the body.
- DNA is capable of passing hereditary information to the next generation. This is possible because of the ability to replicate - self-doubling. DNA is capable of splitting into two complementary chains, and on each of them (in accordance with the principle of complementarity) the original nucleotide sequence is restored.
- With the help of DNA, the biosynthesis of proteins, enzymes and hormones occurs.
Conclusion
The structure of DNA allows it to be the custodian of genetic information, as well as to pass it on to the next generations. What are the characteristics of this molecule?
- Stability. This is possible due to glycosidic, hydrogen and phosphodiester bonds, as well as the mechanism of repair of induced and spontaneous damage.
- Replication capability. This mechanism allows somatic cells to maintain the diploid number of chromosomes.
- The existence of the genetic code. With the help of the processes of translation and transcription, the sequence of bases found in DNA is converted into a sequence of amino acids found in the polypeptide chain.
- The ability to genetic recombination. In this case, new combinations of genes are formed that are linked to each other.
Thus, the structure and functions of DNA allow it to play an invaluable role in the organisms of living beings. It is known that the length of 46 DNA molecules in each human cell is almost 2 m, and the number of nucleotide pairs is 3.2 billion.
The rubric contains articles and notes from the following publications: The Economist and New Scientist (England), American Scientist, Discover, IEEE Spectrum, Science News, Scientific American Mind, and Wired "(USA), "Ça m'interesse", "Ciel et Espace", "Le Journal du CNRS", "La Recherche" and "Science et Vie" (France), as well as press reports and information from the Internet.
Friedrich Miescher in last years life.
The University of Tübingen has a test tube containing Miescher's DNA.
As a rule, the names of the English biologists J. Watson and F. Crick, who discovered the structure of this molecule in 1953, are associated with the DNA molecule. However, the connection itself was not opened by them. But the discoverer is not mentioned in every reference book or textbook.
Deoxyribonucleic acid was discovered in 1869 by a young Swiss physician, Friedrich Miescher, who was then working in Germany. He decided to study the chemical composition of animal cells, and chose leukocytes as the material. These protective germ-eating cells are plentiful in pus, and Misher enlisted the cooperation of colleagues at the local surgical hospital. They began to bring him baskets with purulent dressings taken from wounds. Misher tested different methods of washing leukocytes from gauze bandages and began to isolate proteins from the washed cells. In the process of work, he realized that in addition to proteins in leukocytes there is some kind of mysterious compound. It precipitated as white flakes or threads when the solution was acidified and dissolved again when it was alkalized. Examining his preparation of leukocytes under a microscope, the scientist found that after washing the leukocytes from the bandages with dilute hydrochloric acid, only nuclei remained from them. And he concluded: an unknown compound is contained in the nuclei of cells. Misher called it nuclein, from the Latin nucleus - nucleus.
At that time, almost nothing was known about the cell nucleus, although three years before Miescher's discovery, in 1866, the famous German biologist Ernst Haeckel suggested that the nucleus is responsible for the transmission of hereditary traits.
Wanting to study nuclein in more detail, Miescher developed a procedure for isolating and purifying it. Having processed the sediment with protein-digesting enzymes, he was convinced that this was not a protein compound - the enzymes were unable to decompose the nuclein. It did not dissolve in ether and other organic solvents, that is, it was not a fatty substance. Chemical analysis was then extremely laborious, slow and not very accurate, but Misher carried it out and made sure that nuclein consists of carbon, oxygen, hydrogen, nitrogen and large amounts of phosphorus. At the time they were practically unknown. organic molecules with phosphorus in them. All this convinced Misher that he had discovered some new class of intracellular compounds.
Having written an article about the new discovery, he sent it to his teacher, one of the founders of biochemistry, Felix Hoppe-Seyler, who published the journal Medico-Chemical Research. He decided to check such an unusual message in his laboratory. The verification took a whole year, and Misher was already afraid that someone would independently discover the same nuclein and publish the results first. But in the next issue of the journal for 1871, Miescher's article was accompanied by two articles by Hoppe-Seyler himself and his collaborator, confirming the properties of nuclein.
Returning to Switzerland, Miescher took up the post of head of the Department of Physiology at the University of Basel and continued his research on nuclein. Here he found another richer and more pleasant source of the new compound - the milk of salmon fish (they are still used for the mass production of DNA). The Rhine that flows through Basel was then full of salmon, and Miescher himself caught hundreds of them for his research.
In an article published in 1874 on the discovery of nuclein in milk, Miescher wrote that this substance was clearly associated with the process of fertilization. But he rejected the idea that hereditary information could be encoded in the nuclein: the connection seemed to him too simple and uniform to store the whole variety of hereditary traits. The then methods of analysis did not allow finding significant differences between human and salmon nuclein.
Later, Miescher studied the physiology of salmon, commissioned by the Swiss government to develop a cheap and healthy diet for prisons, wrote a cookbook for workers, founded the Institute of Anatomy and Physiology in Basel, studied the role of blood in the breathing process ... Even during his lifetime, nuclein was renamed " nucleic acid”, which irritated the discoverer very much. Misher died of tuberculosis in 1895. Almost half a century after his death, it was believed that the DNA molecule, consisting of only four types of blocks, was too simple to store hereditary information, and much more diverse proteins were put forward for this role.
