What is the relationship in DNA. DNA molecule. The structure of the DNA molecule. Structure and functions of DNA

After discovering the principle molecular organization such a substance as DNA in 1953, molecular biology began to develop. Further, in the process of research, scientists found out how DNA is recombined, its composition, and how our human genome is arranged.

Every day on molecular level complex processes take place. How is the DNA molecule arranged, what does it consist of? What role do DNA molecules play in a cell? Let's talk in detail about all the processes occurring inside the double chain.

What is hereditary information?

So how did it all start? Back in 1868 found in the nuclei of bacteria. And in 1928, N. Koltsov put forward the theory that it is in DNA that all genetic information about a living organism is encrypted. Then J. Watson and F. Crick found a model for the now well-known DNA helix in 1953, for which they deserved recognition and an award - the Nobel Prize.

What is DNA anyway? This substance consists of 2 combined threads, more precisely spirals. A section of such a chain with certain information is called a gene.

DNA stores all the information about what kind of proteins will be formed and in what order. A DNA macromolecule is a material carrier of incredibly voluminous information, which is recorded in a strict sequence of individual building blocks - nucleotides. There are 4 nucleotides in total, they complement each other chemically and geometrically. This principle of complementation, or complementarity, in science will be described later. This rule plays a key role in encoding and decoding genetic information.

Since the DNA strand is incredibly long, there are no repetitions in this sequence. Every living being has its own unique DNA strand.

Functions of DNA

The functions include the storage of hereditary information and its transmission to offspring. Without this function, the genome of a species could not be preserved and developed over millennia. Organisms that have undergone major gene mutations are more likely to not survive or lose their ability to produce offspring. So there is a natural protection against the degeneration of the species.

Another essential function is the implementation of stored information. The cell cannot make any vital protein without the instructions that are stored in the double strand.

Composition of nucleic acids

Now it is already reliably known what the nucleotides themselves, the building blocks of DNA, consist of. They include 3 substances:

• Orthophosphoric acid.
• nitrogenous base. Pyrimidine bases - which have only one ring. These include thymine and cytosine. Purine bases containing 2 rings. These are guanine and adenine.
• Sucrose. DNA contains deoxyribose, RNA contains ribose.

The number of nucleotides is always equal to the number of nitrogenous bases. In special laboratories, a nucleotide is cleaved and a nitrogenous base is isolated from it. So they study the individual properties of these nucleotides and possible mutations in them.

Levels of organization of hereditary information

There are 3 levels of organization: gene, chromosomal and genomic. All the information needed for the synthesis of a new protein is contained in a small section of the chain - the gene. That is, the gene is considered the lowest and simplest level of encoding information.

Genes, in turn, are assembled into chromosomes. Thanks to such an organization of the carrier of hereditary material, groups of traits alternate according to certain laws and are transmitted from one generation to another. It should be noted that there are incredibly many genes in the body, but information is not lost, even when it is recombined many times.

There are several types of genes:

• according to their functional purpose, 2 types are distinguished: structural and regulatory sequences;
• according to the influence on the processes occurring in the cell, there are: supervital, lethal, conditionally lethal genes, as well as mutator and antimutator genes.

Genes are arranged along the chromosome in a linear order. In chromosomes, information is not randomly focused, there is a certain order. There is even a map showing positions, or gene loci. For example, it is known that data on the color of the eyes of a child is encrypted in chromosome number 18.

What is a genome? This is the name of the entire set of nucleotide sequences in the cell of the body. The genome characterizes the whole species, not a single individual.

What is the human genetic code?

The fact is that the whole huge potential of human development is laid down already in the period of conception. All hereditary information that is necessary for the development of the zygote and the growth of the child after birth is encrypted in the genes. Sections of DNA are the most basic carriers of hereditary information.

Humans have 46 chromosomes, or 22 somatic pairs plus one sex-determining chromosome from each parent. This diploid set of chromosomes encodes the entire physical appearance of a person, his mental and physical abilities and predisposition to diseases. Somatic chromosomes are outwardly indistinguishable, but they carry different information, since one of them is from the father, the other is from the mother.

The male code differs from the female code in the last pair of chromosomes - XY. The female diploid set is the last pair, XX. Males get one X chromosome from their biological mother, and then it is passed on to their daughters. The sex Y chromosome is passed on to sons.

Human chromosomes vary greatly in size. For example, the smallest pair of chromosomes is #17. And the biggest pair is 1 and 3.

The diameter of the double helix in humans is only 2 nm. The DNA is so tightly coiled that it fits in the small nucleus of the cell, although it will be up to 2 meters long if unwound. The length of the helix is ​​hundreds of millions of nucleotides.

How is the genetic code transmitted?

So, what role do DNA molecules play in a cell during division? Genes - carriers of hereditary information - are inside every cell of the body. In order to pass on their code to a daughter organism, many creatures divide their DNA into 2 identical helices. This is called replication. In the process of replication, DNA unwinds and special "machines" complete each chain. After the genetic helix bifurcates, the nucleus and all organelles begin to divide, and then the whole cell.

But a person has a different process of gene transfer - sexual. The signs of the father and mother are mixed, the new genetic code contains information from both parents.

The storage and transmission of hereditary information is possible due to the complex organization of the DNA helix. After all, as we said, the structure of proteins is encrypted in genes. Once created at the time of conception, this code will copy itself throughout life. The karyotype (personal set of chromosomes) does not change during the renewal of organ cells. The transmission of information is carried out with the help of sex gametes - male and female.

Only viruses containing a single strand of RNA are unable to transmit their information to their offspring. Therefore, in order to reproduce, they need human or animal cells.

Implementation of hereditary information

Important processes are constantly taking place in the cell nucleus. All information recorded in chromosomes is used to build proteins from amino acids. But the DNA strand never leaves the nucleus, so another important compound, RNA, is needed here. Just RNA is able to penetrate the nuclear membrane and interact with the DNA chain.

Through the interaction of DNA and 3 types of RNA, all encoded information is realized. At what level is the implementation of hereditary information? All interactions occur at the nucleotide level. Messenger RNA copies a segment of the DNA chain and brings this copy to the ribosome. Here begins the synthesis of the nucleotides of a new molecule.

In order for the mRNA to copy the necessary part of the chain, the helix unfolds and then, upon completion of the recoding process, is restored again. Moreover, this process can occur simultaneously on 2 sides of 1 chromosome.

The principle of complementarity

They consist of 4 nucleotides - these are adenine (A), guanine (G), cytosine (C), thymine (T). They are connected by hydrogen bonds according to the rule of complementarity. The works of E. Chargaff helped to establish this rule, since the scientist noticed some patterns in the behavior of these substances. E. Chargaff discovered that the molar ratio of adenine to thymine is equal to one. And in the same way, the ratio of guanine to cytosine is always equal to one.

Based on his work, geneticists have formed a rule for the interaction of nucleotides. The rule of complementarity states that adenine combines only with thymine, and guanine with cytosine. During the decoding of the helix and the synthesis of a new protein in the ribosome, this alternation rule helps to quickly find the necessary amino acid that is attached to the transfer RNA.

RNA and its types

What is hereditary information? nucleotides in the DNA double strand. What is RNA? What is her job? RNA, or ribonucleic acid, helps to extract information from DNA, decode it, and, based on the principle of complementarity, create proteins necessary for cells.

In total, 3 types of RNA are isolated. Each of them performs strictly its function.

1. Informational (mRNA), or it is also called matrix. It goes right into the center of the cell, into the nucleus. It finds in one of the chromosomes the necessary genetic material for building a protein and copies one of the sides of the double chain. Copying occurs again according to the principle of complementarity.
2. Transport is a small molecule that has nucleotide decoders on one side, and amino acids corresponding to the main code on the other side. The task of tRNA is to deliver it to the "workshop", that is, to the ribosome, where it synthesizes the necessary amino acid.
3. rRNA is ribosomal. It controls the amount of protein that is produced. Consists of 2 parts - amino acid and peptide site.

The only difference when decoding is that RNA does not have thymine. Instead of thymine, uracil is present here. But then, in the process of protein synthesis, with tRNA, it still correctly establishes all the amino acids. If there are any failures in the decoding of information, then a mutation occurs.

Repair of a damaged DNA molecule

The process of repairing a damaged double strand is called reparation. During the repair process, damaged genes are removed.

Then the required sequence of elements is exactly reproduced and crashes back into the same place on the chain from where it was extracted. All this happens thanks to special chemicals - enzymes.

Why do mutations occur?

Why do some genes begin to mutate and cease to fulfill their function - the storage of vital hereditary information? This is due to a decoding error. For example, if adenine is accidentally replaced with thymine.

There are also chromosomal and genomic mutations. Chromosomal mutations occur when pieces of hereditary information are missing, duplicated, or even transferred and integrated into another chromosome.

Genomic mutations are the most serious. Their cause is a change in the number of chromosomes. That is, when instead of a pair - a diploid set, a triploid set is present in the karyotype.

The most famous example of a triploid mutation is Down syndrome, in which the personal set of chromosomes is 47. In such children, 3 chromosomes are formed in place of the 21st pair.

There is also such a mutation as polyploidy. But polyploidy is found only in plants.

In my own way 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 “spiral twisted”) and interconnected by hydrogen bonds.

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 DNA contains the following: adenine, guanine, thymine And cytosine. Adenine and guanine are purines, 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 (they form two hydrogen bonds between themselves), 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 (carbohydrate with five carbon atoms). The bond is covalent, glycosidic (C-N). Unlike ribose, deoxyribose lacks one of its hydroxyl groups. The ring of deoxyribose 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 by covalent bonds between deoxyribose and phosphoric acid (phosphodiester bond). A phosphate-deoxyribose backbone is formed. Perpendicular to it, towards another strand of DNA, nitrogenous bases are directed, which are connected to the bases of the second strand by hydrogen bonds.

The structure of DNA is such that the backbones of 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, as it were, relative to the other. Thus, in the structure of DNA chains, 5 "ends and 3" ends are distinguished.

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

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

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 early work other scientists). Although DNA was known as a chemical substance in the 19th century. In the 1940s, 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 vast majority of DNA is located in the chromosomes, where it is associated with proteins and other substances, and also undergoes denser packaging.

All life on the planet consists of many cells that maintain the orderliness of their organization due to the genetic information contained in the nucleus. It is stored, implemented and transmitted by complex high-molecular compounds - nucleic acids, consisting of monomer units - nucleotides. The role of nucleic acids cannot be overestimated. The stability of their structure determines the normal vital activity of the organism, and any deviations in the structure inevitably lead to a change in the cellular organization, the activity of physiological processes and the viability of cells as a whole.

The concept of a nucleotide and its properties

Each or RNA is assembled from smaller monomeric compounds - nucleotides. In other words, a nucleotide is construction material for nucleic acids, coenzymes and many other biological compounds that are essential for the cell in the course of its life.

The main properties of these irreplaceable substances include:

Storage of information about and inherited traits;
. exercising control over growth and reproduction;
. participation in metabolism and many others physiological processes flowing in the cell.

Speaking of nucleotides, one cannot but dwell on such an important issue as their structure and composition.

Each nucleotide is made up of:

sugar residue;
. nitrogenous base;
. a phosphate group or a phosphoric acid residue.

We can say that a nucleotide is a complex organic compound. Depending on the species composition of nitrogenous bases and the type of pentose in the nucleotide structure, nucleic acids are divided into:

Deoxyribonucleic acid, or DNA;
. ribonucleic acid, or RNA.

Composition of nucleic acids

In nucleic acids, sugar is represented by pentose. This is a five-carbon sugar, in DNA it is called deoxyribose, in RNA it is called ribose. Each pentose molecule has five carbon atoms, four of which, together with an oxygen atom, form a five-membered ring, and the fifth is included in the HO-CH2 group.

The position of each carbon atom in a pentose molecule is indicated by an Arabic numeral with a prime (1C´, 2C´, 3C´, 4C´, 5C´). Since all reading processes from a nucleic acid molecule have a strict direction, the numbering of carbon atoms and their arrangement in the ring serve as a kind of indicator of the correct direction.

On the hydroxyl group, a phosphoric acid residue is attached to the third and fifth carbon atoms (3С´ and 5С´). It determines the chemical affiliation of DNA and RNA to a group of acids.

A nitrogenous base is attached to the first carbon atom (1C´) in the sugar molecule.

Species composition of nitrogenous bases

DNA nucleotides according to the nitrogenous base are represented by four types:

Adenine (A);
. guanine (G);
. cytosine (C);
. thymine (T).

The first two belong to the class of purines, the last two are pyrimidines. In terms of molecular weight, purines are always heavier than pyrimidines.

RNA nucleotides by nitrogenous base are represented by:

Adenine (A);
. guanine (G);
. cytosine (C);
. uracil (U).

Uracil, like thymine, is a pyrimidine base.

In the scientific literature, one can often find another designation of nitrogenous bases - in Latin letters (A, T, C, G, U).

Let us dwell in more detail on the chemical structure of purines and pyrimidines.

Pyrimidines, namely cytosine, thymine and uracil, in their composition are represented by two nitrogen atoms and four carbon atoms, forming a six-membered ring. Each atom has its own number from 1 to 6.

Purines (adenine and guanine) are composed of pyrimidine and imidazole or two heterocycles. The purine base molecule is represented by four nitrogen atoms and five carbon atoms. Each atom is numbered from 1 to 9.

As a result of the combination of a nitrogenous base and a pentose residue, a nucleoside is formed. A nucleotide is a compound of a nucleoside and a phosphate group.

Formation of phosphodiester bonds

It is important to understand the question of how nucleotides are connected into a polypeptide chain and form a nucleic acid molecule. This happens due to the so-called phosphodiester bonds.

The interaction of two nucleotides gives a dinucleotide. The formation of a new compound occurs by condensation, when a phosphodiester bond occurs between the phosphate residue of one monomer and the hydroxy group of the pentose of another.

The synthesis of a polynucleotide is the repeated repetition of this reaction (several million times). The polynucleotide chain is built through the formation of phosphodiester bonds between the third and fifth carbons of sugars (3C' and 5C').

Polynucleotide assembly is a complex process that occurs with the participation of the enzyme DNA polymerase, which ensures the growth of the chain from only one end (3´) with a free hydroxyl group.

DNA molecule structure

A DNA molecule, like a protein, can have a primary, secondary, and tertiary structure.

The sequence of nucleotides in the DNA chain determines its primary formation due to hydrogen bonds, which are based on the principle of complementarity. In other words, during the synthesis of a double, a certain pattern operates: adenine of one chain corresponds to the thymine of the other, guanine to cytosine, and vice versa. Pairs of adenine and thymine or guanine and cytosine are formed due to two in the first and three in the last case hydrogen bonds. Such a connection of nucleotides provides a strong bond between the chains and an equal distance between them.

Knowing the nucleotide sequence of one strand of DNA, by the principle of complementarity or addition, you can complete the second one.

The tertiary structure of DNA is formed by complex three-dimensional bonds, which makes its molecule more compact and able to fit in a small cell volume. So, for example, the length of E. coli DNA is more than 1 mm, while the length of the cell is less than 5 microns.

The number of nucleotides in DNA, namely their quantitative ratio, obeys the Chergaff rule (the number of purine bases is always equal to the number of pyrimidine bases). The distance between nucleotides is a constant value equal to 0.34 nm, as is their molecular weight.

Structure of the RNA molecule

RNA is represented by a single polynucleotide chain formed through between the pentose (in this case, ribose) and the phosphate residue. It is much shorter than DNA in length. There are also differences in the species composition of nitrogenous bases in the nucleotide. In RNA, uracil is used instead of the pyrimidine base of thymine. Depending on the functions performed in the body, RNA can be of three types.

Ribosomal (rRNA) - usually contains from 3000 to 5000 nucleotides. As needed structural component takes part in the formation of the active center of ribosomes, the site of one of the most important processes in the cell - protein biosynthesis.
. Transport (tRNA) - consists of an average of 75 - 95 nucleotides, carries out the transfer of the desired amino acid to the site of polypeptide synthesis in the ribosome. Each type of tRNA (at least 40) has its own unique sequence of monomers or nucleotides.
. Information (mRNA) - the nucleotide composition is very diverse. Transfers genetic information from DNA to ribosomes, acts as a matrix for the synthesis of a protein molecule.

The role of nucleotides in the body

Nucleotides in the cell perform a number of important functions:

They are used as structural blocks for nucleic acids (nucleotides of the purine and pyrimidine series);
. participate in many metabolic processes in the cell;
. are part of ATP - the main source of energy in cells;
. act as carriers of reducing equivalents in cells (NAD+, NADP+, FAD, FMN);
. perform the function of bioregulators;
. can be considered as the second messengers of extracellular regular synthesis (for example, cAMP or cGMP).

A nucleotide is a monomeric unit that forms more complex compounds - nucleic acids, without which the transfer of genetic information, its storage and reproduction is impossible. Free nucleotides are the main components involved in signaling and energy processes that support the normal functioning of cells and the body as a whole.

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

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

Schematically, this is shown as follows:

T - A

C - G

These pairs form a chemical nucleotide bond, which determines the order in which the amino acids are arranged. In the first case, she is a little 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 chains are inherent in spiral twisting. This phenomenon is called spiralization, it allows the threads to be shortened by a factor of five or six.

Some molecules of such a plan are used by the body very actively, others rarely. The latter, in addition to spiralization, are also subjected to such a “compact packing” as supercoiling. And then the length of the DNA molecule decreases by 25-30 times.

What is the "packaging" of a molecule?

Histone proteins are involved in the process of supercoiling. They have the structure and appearance of a spool for thread or a 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 helix unwinds 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. Chromosomes available for reading are called euchromatins, and if they are superspiralized, 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). The monomers in them are nucleotides, each of which has 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 huge - 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 chain 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 are different. It was also found that guanine of one chain can only bind to cytosine, and thymine to adenine. The arrangement of nucleotides of one chain strictly corresponds to the parallel one. Due to this complementarity of polynucleotides, the DNA molecule is capable of duplication and self-replication. But first, complementary chains, under the influence of special enzymes that destroy paired nucleotides, diverge, and then the synthesis of the missing chain begins in each of them. This is due to the free nucleotides available in large quantities in each cell. As a result, instead of the “parent molecule”, two “daughter” ones are formed, identical in composition and structure, and the DNA code becomes the original one. This process is the precursor of 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 a DNA molecule is calculated - it is also possible to find out more complex data that were not previously available to scientists. For example, you can read information about how the body uses its own cell. Of course, at first this information is in an encoded form and has the form of a certain matrix, and therefore it must be transported to a special carrier, which is RNA. Ribonucleic acid is able to seep into the cell through the nuclear membrane and read the encoded information already 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 integrated as a template into ribosomes (special enzyme systems). Guided by the information received, they can synthesize the appropriate sequence of protein amino acids. About what kind organic compound needs to be attached to the nascent protein chain, the ribosome learns from the triplet code. 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 with the aim of obtaining information about how many genes the structure of a DNA molecule has. And, although research has allowed scientists to advance far in this matter, it is not yet possible to know their exact number.

A few years ago it was assumed that DNA molecules contain approximately 100,000 genes. A little later, the figure decreased to 80,000, and in 1998, geneticists stated that only 50,000 genes are present in one DNA, which are only 3% of the entire length of DNA. But they were struck by the latest conclusions of geneticists. Now they claim that the genome contains 25-40 thousand of the mentioned units. It turns out that only 1.5% of chromosomal DNA is responsible for encoding proteins.

The research didn't stop there. A parallel team of genetic engineering specialists found that the number of genes in one molecule is exactly 32,000. As you can see, it is still impossible to get a definitive answer. Too many contradictions. All researchers rely only on their findings.

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 has a small size), scientists nevertheless made one assumption. Based on laboratory data, they voiced a version of the following content: a molecule on initial stage of its appearance had the form of a simple self-replicating peptide, which included up to 32 amino acids contained in the ancient oceans.

After self-replication, due to the forces of natural selection, molecules have the ability to protect themselves from the effects of external elements. They began to live longer and reproduce in large quantities. Molecules that found themselves in the lipid bubble got every chance to reproduce themselves. As a result of a series of successive cycles, lipid bubbles took the form of cell membranes, and only further - well-known particles. It should be noted that today any part of the DNA molecule is a complex and well-functioning structure, all the features of which have not yet been fully studied by scientists.

Modern world

Recently, scientists from Israel have developed a computer that can perform trillions of operations per second. Today it is the fastest car on Earth. The whole secret lies in the fact that the innovative device functions from DNA. Professors say that in the near future such computers will even be able to generate energy.

Specialists from the Weizmann Institute in Rehovot (Israel) a year ago announced the creation of a programmable molecular computer, consisting of molecules and enzymes. They replaced silicon microchips with them. To date, the team has moved 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 people. 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 heads of scientists in 1994. It was then that a molecule was used to solve a simple mathematical 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 this is just in one drop of liquid!

Human DNA

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

Over time, of course, they proved that not only as in the proposed version, a structured human molecule can look like. After a more detailed DNA analysis, they discovered the A-, B- and left-handed form of Z-. 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 this is abnormal, in a living cell such a process cannot occur.

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

Artificial DNA

Today, artificial DNA already exists, which is an identical copy of the real one; it perfectly repeats the structure of the natural double helix. But, unlike the original 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 evolve. 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 existing nitrogenous bases, genetics added an additional two, which were created by the method of chemical modification of natural bases. Unlike natural, artificial DNA turned out to be quite short. It contains only 81 base pairs. However, it also reproduces and evolves.

Replication of a molecule obtained artificially takes place due to polymerase chain reaction, but so far this is not happening 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 final outcome of DNA development can be influenced by various factors, such as mutations. This causes the mandatory study of samples of matter so that the result of the analyzes is reliable and reliable. An example is a paternity test. But one cannot but rejoice 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 isolation of DNA from plants is also possible. Of course, obtaining from plant material the molecular weight of DNA High Quality causes some difficulties due to the large number of copies of mitochondria and chloroplasts of DNA, as well as high level polysaccharides and phenolic compounds. In this case, a variety of methods are used to isolate the structure we are considering.

Hydrogen bond in DNA

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

A hydrogen atom is attached to an electronegative atom that is the donor of this bond. An electronegative atom can be nitrogen, fluorine, oxygen. It - by decentralization - attracts an electron cloud from the hydrogen nucleus to itself 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.

Deciphering DNA

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

Of course, deciphering the genome of a single cell is an inappropriate exercise. The data obtained in the course of such studies are of no interest to scientists. But it is important to understand that all existing on this moment decoding methods, despite their complexity, are not efficient enough. They will allow you to read only 40-70% of DNA.

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

On the right is the largest human DNA helix built from people on the beach in Varna (Bulgaria), which was included in the Guinness Book of Records on April 23, 2016

Deoxyribonucleic acid. General information

DNA (deoxyribonucleic acid) is a kind of blueprint of life, a complex code that contains data on hereditary information. This complex macromolecule is capable of storing and transmitting hereditary genetic information from generation to generation. DNA determines such properties of any living organism as heredity and variability. The information encoded in it determines the entire development program of any living organism. Genetically embedded factors predetermine the entire course of life of both a person and any other organism. Artificial or natural influence of the external environment can only slightly affect the overall severity of individual genetic traits or affect the development of programmed processes.

Deoxyribonucleic acid(DNA) is a macromolecule (one of the three main ones, the other two are RNA and proteins), which provides storage, transmission from generation to generation and implementation of the genetic program for the development and functioning of living organisms. DNA contains information about the structure various kinds RNA and proteins.

In eukaryotic cells (animals, plants, and fungi), DNA is found in the cell nucleus as part of chromosomes, as well as in some cell organelles (mitochondria and plastids). In the cells of prokaryotic organisms (bacteria and archaea), a circular or linear DNA molecule, the so-called nucleoid, is attached from the inside to the cell membrane. They and lower eukaryotes (for example, yeast) also have small autonomous, mostly circular DNA molecules called plasmids.

WITH chemical point DNA is a long polymer molecule made up of repeating units called nucleotides. Each nucleotide is made up of a nitrogenous base, a sugar (deoxyribose), and a phosphate group. The bonds between nucleotides in a chain are formed by deoxyribose ( WITH) and phosphate ( F) groups (phosphodiester bonds).

Rice. 2. Nuclertide consists of a nitrogenous base, sugar (deoxyribose) and a phosphate group

In the overwhelming majority of cases (except for some viruses containing single-stranded DNA), the DNA macromolecule consists of two chains oriented by nitrogenous bases to each other. This double-stranded molecule is twisted in a helix.

There are four types of nitrogenous bases found in DNA (adenine, guanine, thymine, and cytosine). The nitrogenous bases of one of the chains are connected to the nitrogenous bases of the other chain by hydrogen bonds according to the principle of complementarity: adenine combines only with thymine ( A-T), guanine - only with cytosine ( G-C). It is these pairs that make up the "rungs" of the helical "ladder" of DNA (see: Fig. 2, 3 and 4).

Rice. 2. Nitrogenous bases

The sequence of nucleotides allows you to "encode" information about various types of RNA, the most important of which are informational or template (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized on the DNA template by copying the DNA sequence into the RNA sequence synthesized during transcription and take part in protein biosynthesis (translation process). In addition to coding sequences, cell DNA contains sequences that perform regulatory and structural functions.

Rice. 3. DNA replication

The location of the basic combinations of DNA chemical compounds and the quantitative ratios between these combinations provide encoding of hereditary information.

Education new DNA (replication)

1. The process of replication: the unwinding of the DNA double helix - the synthesis of complementary strands by DNA polymerase - the formation of two DNA molecules from one.
2. The double helix "unzips" into two branches when enzymes break the bond between the base pairs of chemical compounds.
3. Each branch is a new DNA element. New base pairs are connected in the same sequence as in the parent branch.

Upon completion of the duplication, two independent helices are formed, created from the chemical compounds of the parent DNA and having the same genetic code with it. In this way, DNA is able to rip through information from cell to cell.

More detailed information:

STRUCTURE OF NUCLEIC ACIDS

Rice. 4 . Nitrogenous bases: adenine, guanine, cytosine, thymine

Deoxyribonucleic acid(DNA) refers to nucleic acids. Nucleic acids is a class of irregular biopolymers whose monomers are nucleotides.

NUCLEOTIDES consist of nitrogenous base, connected to a five-carbon carbohydrate (pentose) - deoxyribose(in the case of DNA) or ribose(in the case of RNA), which combines with a phosphoric acid residue (H 2 PO 3 -).

Nitrogenous bases There are two types: pyrimidine bases - uracil (only in RNA), cytosine and thymine, purine bases - adenine and guanine.

Rice. Fig. 5. The structure of nucleotides (left), the location of the nucleotide in DNA (bottom) and the types of nitrogenous bases (right): pyrimidine and purine

The carbon atoms in a pentose molecule are numbered from 1 to 5. Phosphate combines with the third and fifth carbon atoms. This is how nucleic acids are linked together to form a chain of nucleic acids. Thus, we can isolate the 3' and 5' ends of the DNA strand:

Rice. 6. Isolation of the 3' and 5' ends of the DNA strand

Two strands of DNA form double helix. These chains in a spiral are oriented in opposite directions. In different strands of DNA, nitrogenous bases are connected to each other by means of hydrogen bonds. Adenine always combines with thymine, and cytosine always combines with guanine. It is called complementarity rule(cm. principle of complementarity).

Complementarity rule:

For example, if we are given a DNA strand that has the sequence

3'-ATGTCCTAGCTGCTCG - 5',

then the second chain will be complementary to it and directed in the opposite direction - from the 5'-end to the 3'-end:

5'- TACAGGATCGACGAGC- 3'.

Rice. 7. The direction of the chains of the DNA molecule and the connection of nitrogenous bases using hydrogen bonds

DNA REPLICATION

DNA replication is the process of doubling a DNA molecule by template synthesis. In most cases of natural DNA replicationprimerfor DNA synthesis is short snippet (created again). Such a ribonucleotide primer is created by the enzyme primase (DNA primase in prokaryotes, DNA polymerase in eukaryotes), and is subsequently replaced by deoxyribonucleotide polymerase, which normally performs repair functions (correcting chemical damage and breaks in the DNA molecule).

Replication occurs in a semi-conservative manner. This means that the double helix of DNA unwinds and a new chain is completed on each of its chains according to the principle of complementarity. The daughter DNA molecule thus contains one strand from the parent molecule and one newly synthesized. Replication occurs in the 3' to 5' direction of the parent strand.

Rice. 8. Replication (doubling) of the DNA molecule

DNA synthesis- this is not such a complicated process as it might seem at first glance. If you think about it, then first you need to figure out what synthesis is. It is the process of bringing something together. The formation of a new DNA molecule takes place in several stages:

1) DNA topoisomerase, located in front of the replication fork, cuts the DNA in order to facilitate its unwinding and unwinding.
2) DNA helicase, following topoisomerase, affects the process of "unwinding" the DNA helix.
3) DNA-binding proteins carry out the binding of DNA strands, and also carry out their stabilization, preventing them from sticking to each other.
4) DNA polymerase δ(delta) , coordinated with the speed of movement of the replication fork, performs the synthesisleadingchains subsidiary DNA in the direction 5" → 3" on the matrix maternal strands of DNA in the direction from its 3" end to the 5" end (speed up to 100 base pairs per second). These events on this maternal strands of DNA are limited.

Rice. 9. Schematic representation of the DNA replication process: (1) Lagging strand (lag strand), (2) Leading strand (leading strand), (3) DNA polymerase α (Polα), (4) DNA ligase, (5) RNA -primer, (6) Primase, (7) Okazaki fragment, (8) DNA polymerase δ (Polδ ), (9) Helicase, (10) Single-stranded DNA-binding proteins, (11) Topoisomerase.

The synthesis of the lagging daughter DNA strand is described below (see below). scheme replication fork and function of replication enzymes)

For more information on DNA replication, see

5) Immediately after the unwinding and stabilization of another strand of the parent molecule, it joinsDNA polymerase α(alpha)and in the direction 5 "→3" synthesizes a primer (RNA primer) - an RNA sequence on a DNA template with a length of 10 to 200 nucleotides. After that, the enzymeremoved from the DNA strand.

Instead of DNA polymeraseα attached to the 3" end of the primer DNA polymeraseε .

6) DNA polymeraseε (epsilon) as if continues to lengthen the primer, but as a substrate embedsdeoxyribonucleotides(in the amount of 150-200 nucleotides). As a result, a solid thread is formed from two parts -RNA(i.e. primer) and DNA. DNA polymerase εworks until it encounters the primer of the previousfragment Okazaki(synthesized a little earlier). This enzyme is then removed from the chain.

7) DNA polymerase β(beta) stands in place ofDNA polymerases ε,moves in the same direction (5" → 3") and removes primer ribonucleotides while inserting deoxyribonucleotides in their place. The enzyme works until the complete removal of the primer, i.e. until a deoxyribonucleotide (even more previously synthesizedDNA polymerase ε). The enzyme is not able to link the result of its work and the DNA in front, so it leaves the chain.

As a result, a fragment of the daughter DNA "lies" on the matrix of the mother thread. It is calledfragment of Okazaki.

8) DNA ligase ligates two adjacent fragments Okazaki , i.e. 5 "-end of the segment, synthesizedDNA polymerase ε,and 3" chain end built-inDNA polymeraseβ .

STRUCTURE OF RNA

Ribonucleic acid(RNA) is one of the three main macromolecules (the other two are DNA and proteins) that are found in the cells of all living organisms.

Just like DNA, RNA is made up of a long chain in which each link is called nucleotide. Each nucleotide is made up of a nitrogenous base, a ribose sugar, and a phosphate group. However, unlike DNA, RNA usually has one rather than two strands. Pentose in RNA is represented by ribose, not deoxyribose (ribose has an additional hydroxyl group on the second carbohydrate atom). Finally, DNA differs from RNA in the composition of nitrogenous bases: instead of thymine ( T) uracil is present in RNA ( U) , which is also complementary to adenine.

The sequence of nucleotides allows RNA to encode genetic information. All cellular organisms use RNA (mRNA) to program protein synthesis.

Cellular RNAs are formed in a process called transcription , that is, the synthesis of RNA on a DNA template, carried out by special enzymes - RNA polymerases.

Messenger RNAs (mRNAs) then take part in a process called broadcast, those. protein synthesis on the mRNA template with the participation of ribosomes. Other RNAs undergo chemical modifications after transcription, and after the formation of secondary and tertiary structures, they perform functions that depend on the type of RNA.

Rice. 10. The difference between DNA and RNA in terms of the nitrogenous base: instead of thymine (T), RNA contains uracil (U), which is also complementary to adenine.

TRANSCRIPTION

This is the process of RNA synthesis on a DNA template. DNA unwinds at one of the sites. One of the chains contains information that needs to be copied onto the RNA molecule - this chain is called coding. The second strand of DNA, which is complementary to the coding strand, is called the template strand. In the process of transcription on the template chain in the 3'-5' direction (along the DNA chain), an RNA chain complementary to it is synthesized. Thus, an RNA copy of the coding strand is created.

Rice. 11. Schematic representation of transcription

For example, if we are given the sequence of the coding strand

3'-ATGTCCTAGCTGCTCG - 5',

then, according to the rule of complementarity, the matrix chain will carry the sequence

5'- TACAGGATCGACGAGC- 3',

and the RNA synthesized from it is the sequence

BROADCAST

Consider the mechanism protein synthesis on the RNA matrix, as well as the genetic code and its properties. Also, for clarity, at the link below, we recommend watching a short video about the processes of transcription and translation occurring in a living cell:

Rice. 12. Process of protein synthesis: DNA codes for RNA, RNA codes for protein

GENETIC CODE

Genetic code- a method of encoding the amino acid sequence of proteins using a sequence of nucleotides. Each amino acid is encoded by a sequence of three nucleotides - a codon or a triplet.

Genetic code common to most pro- and eukaryotes. The table lists all 64 codons and lists the corresponding amino acids. The base order is from the 5" to the 3" end of the mRNA.

Table 1. Standard genetic code

Among the triplets, there are 4 special sequences that act as "punctuation marks":

• *Triplet AUG, also encoding methionine, is called start codon. This codon begins the synthesis of a protein molecule. Thus, during protein synthesis, the first amino acid in the sequence will always be methionine.

• **Triplets UAA, UAG And UGA called stop codons and do not code for any amino acids. At these sequences, protein synthesis stops.

Properties of the genetic code

1. Tripletity. Each amino acid is encoded by a sequence of three nucleotides - a triplet or codon.

2. Continuity. There are no additional nucleotides between the triplets, information is read continuously.

3. Non-overlapping. One nucleotide cannot be part of two triplets at the same time.

4. Uniqueness. One codon can code for only one amino acid.

5. Degeneracy. One amino acid can be encoded by several different codons.

6. Versatility. The genetic code is the same for all living organisms.

Example. We are given the sequence of the coding strand:

3’- CCGATTGCACGTCGATCGTATA- 5’.

The matrix chain will have the sequence:

5’- GGCTAACGTGCAGCTAGCATAT- 3’.

Now we “synthesize” informational RNA from this chain:

3’- CCGAUUGCACGUCGAUCGUAUA- 5’.

Protein synthesis goes in the direction 5' → 3', therefore, we need to flip the sequence in order to "read" the genetic code:

5’- AUAUGCUAGCUGCACGUUAGCC- 3’.

Now find the start codon AUG:

5’- AU AUG CUAGCUGCACGUUAGCC- 3’.

Divide the sequence into triplets:

sounds like this: information from DNA is transferred to RNA (transcription), from RNA to protein (translation). DNA can also be duplicated by replication, and the process of reverse transcription is also possible, when DNA is synthesized from an RNA template, but such a process is mainly characteristic of viruses.

Rice. 13. Central dogma of molecular biology

GENOM: GENES AND CHROMOSOMES

(general concepts)

Genome - the totality of all the genes of an organism; its complete chromosome set.

The term "genome" was proposed by G. Winkler in 1920 to describe the totality of genes contained in the haploid set of chromosomes of organisms of one species. The original meaning of this term indicated that the concept of the genome, in contrast to the genotype, is a genetic characteristic of the species as a whole, and not of an individual. With the development of molecular genetics, the meaning of this term has changed. It is known that DNA, which is the carrier of genetic information in most organisms and, therefore, forms the basis of the genome, includes not only genes in the modern sense of the word. Most of the DNA of eukaryotic cells is represented by non-coding (“redundant”) nucleotide sequences that do not contain information about proteins and nucleic acids. Thus, the main part of the genome of any organism is the entire DNA of its haploid set of chromosomes.

Genes are segments of DNA molecules that code for polypeptides and RNA molecules.

Over the past century, our understanding of genes has changed significantly. Previously, a genome was a region of a chromosome that encodes or determines one trait or phenotypic(visible) property, such as eye color.

In 1940, George Beadle and Edward Tatham proposed a molecular definition of a gene. Scientists processed fungus spores Neurospora crassa x-rays and other agents that cause changes in the DNA sequence ( mutations), and found mutant strains of the fungus that lost some specific enzymes, which in some cases led to disruption of the entire metabolic pathway. Beadle and Tatham came to the conclusion that a gene is a section of genetic material that defines or codes for a single enzyme. This is how the hypothesis "one gene, one enzyme". This concept was later extended to the definition "one gene - one polypeptide", since many genes encode proteins that are not enzymes, and a polypeptide can be a subunit of a complex protein complex.

On fig. 14 shows a diagram of how DNA triplets determine a polypeptide, the amino acid sequence of a protein, mediated by mRNA. One of the DNA strands plays the role of a template for the synthesis of mRNA, the nucleotide triplets (codons) of which are complementary to the DNA triplets. In some bacteria and many eukaryotes, coding sequences are interrupted by non-coding regions (called introns).

Modern biochemical definition of a gene even more specifically. Genes are all sections of DNA that encode the primary sequence of end products, which include polypeptides or RNA that have a structural or catalytic function.

Along with genes, DNA also contains other sequences that perform an exclusively regulatory function. Regulatory sequences may mark the beginning or end of genes, affect transcription, or indicate the site of initiation of replication or recombination. Some genes can be expressed in different ways, with the same piece of DNA serving as a template for the formation of different products.

We can roughly calculate minimum gene size coding for the intermediate protein. Each amino acid in a polypeptide chain is encoded by a sequence of three nucleotides; the sequences of these triplets (codons) correspond to the chain of amino acids in the polypeptide encoded by the given gene. A polypeptide chain of 350 amino acid residues (medium length chain) corresponds to a sequence of 1050 bp. ( bp). However, many eukaryotic genes and some prokaryotic genes are interrupted by DNA segments that do not carry information about the protein, and therefore turn out to be much longer than a simple calculation shows.

How many genes are on one chromosome?

Rice. 15. View of chromosomes in prokaryotic (left) and eukaryotic cells. Histones are a broad class of nuclear proteins that perform two main functions: they are involved in the packaging of DNA strands in the nucleus and in the epigenetic regulation of nuclear processes such as transcription, replication, and repair.

The DNA of prokaryotes is more simple: their cells do not have a nucleus, so the DNA is located directly in the cytoplasm in the form of a nucleoid.

As you know, bacterial cells have a chromosome in the form of a DNA strand, packed into a compact structure - a nucleoid. prokaryotic chromosome Escherichia coli, whose genome is completely decoded, is a circular DNA molecule (in fact, this is not a regular circle, but rather a loop without beginning and end), consisting of 4,639,675 bp. This sequence contains approximately 4300 protein genes and another 157 genes for stable RNA molecules. IN human genome approximately 3.1 billion base pairs corresponding to almost 29,000 genes located on 24 different chromosomes.

Prokaryotes (Bacteria).

Bacterium E. coli has one double-stranded circular DNA molecule. It consists of 4,639,675 b.p. and reaches a length of approximately 1.7 mm, which exceeds the length of the cell itself E. coli about 850 times. In addition to the large circular chromosome as part of the nucleoid, many bacteria contain one or more small circular DNA molecules that are freely located in the cytosol. These extrachromosomal elements are called plasmids(Fig. 16).

Most plasmids consist of only a few thousand base pairs, some contain more than 10,000 bp. They carry genetic information and replicate to form daughter plasmids, which enter the daughter cells during the division of the parent cell. Plasmids are found not only in bacteria, but also in yeast and other fungi. In many cases, plasmids offer no advantage to the host cells and their only job is to reproduce independently. However, some plasmids carry genes useful to the host. For example, genes contained in plasmids can confer resistance to antibacterial agents in bacterial cells. Plasmids carrying the β-lactamase gene confer resistance to β-lactam antibiotics such as penicillin and amoxicillin. Plasmids can pass from antibiotic-resistant cells to other cells of the same or different bacterial species, causing those cells to also become resistant. Intensive use of antibiotics is a powerful selective factor that promotes the spread of plasmids encoding antibiotic resistance (as well as transposons that encode similar genes) among pathogenic bacteria, and leads to the emergence of bacterial strains with resistance to several antibiotics. Doctors are beginning to understand the dangers of widespread use of antibiotics and prescribe them only when absolutely necessary. For similar reasons, the widespread use of antibiotics for the treatment of farm animals is limited.

See also: Ravin N.V., Shestakov S.V. Genome of prokaryotes // Vavilov Journal of Genetics and Breeding, 2013. V. 17. No. 4/2. pp. 972-984.

Eukaryotes.

Table 2. DNA, genes and chromosomes of some organisms

Note. Information is constantly updated; For more up-to-date information, refer to individual genomic project websites.

* For all eukaryotes, except yeast, the diploid set of chromosomes is given. diploid kit chromosomes (from Greek diploos - double and eidos - view) - double set of chromosomes(2n), each of which has a homology to itself.
**Haploid set. Wild strains of yeast typically have eight (octaploid) or more sets of these chromosomes.
***For females with two X chromosomes. Males have an X chromosome, but no Y, i.e. only 11 chromosomes.

A yeast cell, one of the smallest eukaryotes, has 2.6 times more DNA than a cell E. coli(Table 2). fruit fly cells Drosophila, a classic object of genetic research, contains 35 times more DNA, and human cells contain about 700 times more DNA than cells E. coli. Many plants and amphibians contain even more DNA. The genetic material of eukaryotic cells is organized in the form of chromosomes. Diploid set of chromosomes (2 n) depends on the type of organism (Table 2).

For example, in a human somatic cell there are 46 chromosomes ( rice. 17). Each chromosome in a eukaryotic cell, as shown in Fig. 17, A, contains one very large double-stranded DNA molecule. Twenty-four human chromosomes (22 paired chromosomes and two sex chromosomes X and Y) differ in length by more than 25 times. Each eukaryotic chromosome contains a specific set of genes.

Rice. 17. eukaryotic chromosomes.A- a pair of connected and condensed sister chromatids from the human chromosome. In this form, eukaryotic chromosomes remain after replication and in metaphase during mitosis. b- a complete set of chromosomes from a leukocyte of one of the authors of the book. Each normal human somatic cell contains 46 chromosomes.

The size and function of DNA as a matrix for storing and transmitting hereditary material explains the presence of special structural elements in the organization of this molecule. In higher organisms, DNA is distributed between chromosomes.

The set of DNA (chromosomes) of an organism is called the genome. Chromosomes are located in the cell nucleus and form a structure called chromatin. Chromatin is a complex of DNA and basic proteins (histones) in a 1:1 ratio. The length of DNA is usually measured by the number of pairs of complementary nucleotides (bp). For example, the 3rd human chromosomecentury is a DNA molecule with a size of 160 million bp. has a length of approximately 1 mm, therefore, a linearized molecule of the 3rd human chromosome would be 5 mm in length, and the DNA of all 23 chromosomes (~ 3 * 10 9 bp, MR = 1.8 * 10 12) of a haploid cell - egg or sperm cell - in a linearized form would be 1 m. With the exception of germ cells, all cells of the human body (there are about 1013 of them) contain a double set of chromosomes. During cell division, all 46 DNA molecules replicate and reorganize into 46 chromosomes.

If you connect the DNA molecules of the human genome (22 chromosomes and chromosomes X and Y or X and X) to each other, you get a sequence about one meter long. Note: In all mammals and other heterogametic male organisms, females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY).

Most human cells, so the total DNA length of such cells is about 2m. An adult human has about 10 14 cells, so the total length of all DNA molecules is 2・10 11 km. For comparison, the circumference of the Earth is 4・10 4 km, and the distance from the Earth to the Sun is 1.5・10 8 km. That's how amazingly compactly packaged DNA is in our cells!

In eukaryotic cells, there are other organelles containing DNA - these are mitochondria and chloroplasts. Many hypotheses have been put forward regarding the origin of mitochondrial and chloroplast DNA. The generally accepted point of view today is that they are the rudiments of the chromosomes of ancient bacteria that penetrated into the cytoplasm of the host cells and became the precursors of these organelles. Mitochondrial DNA codes for mitochondrial tRNA and rRNA, as well as several mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nuclear DNA.

STRUCTURE OF GENES

Consider the structure of the gene in prokaryotes and eukaryotes, their similarities and differences. Despite the fact that a gene is a section of DNA encoding only one protein or RNA, in addition to the direct coding part, it also includes regulatory and other structural elements that have a different structure in prokaryotes and eukaryotes.

coding sequence- the main structural and functional unit of the gene, it is in it that the triplets of nucleotides encodingamino acid sequence. It starts with a start codon and ends with a stop codon.

Before and after the coding sequence are untranslated 5' and 3' sequences. They perform regulatory and auxiliary functions, for example, ensure the landing of the ribosome on mRNA.

Untranslated and coding sequences make up the unit of transcription - the transcribed DNA region, that is, the DNA region from which mRNA is synthesized.

Terminator A non-transcribed region of DNA at the end of a gene where RNA synthesis stops.

At the beginning of the gene is regulatory area, which includes promoter And operator.

promoter- the sequence with which the polymerase binds during transcription initiation. Operator- this is the area to which special proteins can bind - repressors, which can reduce the activity of RNA synthesis from this gene - in other words, reduce it expression.

The structure of genes in prokaryotes

The general plan for the structure of genes in prokaryotes and eukaryotes does not differ - both contain a regulatory region with a promoter and operator, a transcription unit with coding and non-translated sequences, and a terminator. However, the organization of genes in prokaryotes and eukaryotes is different.

Rice. 18. Scheme of the structure of the gene in prokaryotes (bacteria) -the image is enlarged

At the beginning and at the end of the operon, there are common regulatory regions for several structural genes. From the transcribed region of the operon, one mRNA molecule is read, which contains several coding sequences, each of which has its own start and stop codon. From each of these areasone protein is synthesized. Thus, Several protein molecules are synthesized from one i-RNA molecule.

Prokaryotes are characterized by the combination of several genes into a single functional unit - operon. The work of the operon can be regulated by other genes, which can be noticeably removed from the operon itself - regulators. The protein translated from this gene is called repressor. It binds to the operator of the operon, regulating the expression of all the genes contained in it at once.

Prokaryotes are also characterized by the phenomenon transcription and translation conjugations.

Rice. 19 The phenomenon of conjugation of transcription and translation in prokaryotes - the image is enlarged

This pairing does not occur in eukaryotes due to the presence of a nuclear membrane that separates the cytoplasm, where translation occurs, from the genetic material, on which transcription occurs. In prokaryotes, during the synthesis of RNA on a DNA template, a ribosome can immediately bind to the synthesized RNA molecule. Thus, translation begins even before transcription is complete. Moreover, several ribosomes can simultaneously bind to one RNA molecule, synthesizing several molecules of one protein at once.

The structure of genes in eukaryotes

The genes and chromosomes of eukaryotes are very complexly organized.

Bacteria of many species have only one chromosome, and in almost all cases there is one copy of each gene on each chromosome. Only a few genes, such as rRNA genes, are contained in multiple copies. Genes and regulatory sequences make up almost the entire genome of prokaryotes. Moreover, almost every gene strictly corresponds to the amino acid sequence (or RNA sequence) that it encodes (Fig. 14).

The structural and functional organization of eukaryotic genes is much more complex. The study of eukaryotic chromosomes, and later the sequencing of complete eukaryotic genome sequences, has brought many surprises. Many, if not most, eukaryotic genes have interesting feature: their nucleotide sequences contain one or more DNA regions that do not encode the amino acid sequence of the polypeptide product. Such non-translated inserts disrupt the direct correspondence between the nucleotide sequence of the gene and the amino acid sequence of the encoded polypeptide. These untranslated segments in the genes are called introns, or built-in sequences, and the coding segments are exons. In prokaryotes, only a few genes contain introns.

So, in eukaryotes, there is practically no combination of genes into operons, and the coding sequence of a eukaryotic gene is most often divided into translated regions. - exons, and untranslated sections - introns.

In most cases, the function of introns has not been established. In general, only about 1.5% of human DNA is "coding", that is, it carries information about proteins or RNA. However, taking into account large introns, it turns out that 30% of human DNA consists of genes. Since genes make up a relatively small proportion of the human genome, a significant amount of DNA remains unaccounted for.

Rice. 16. Scheme of the structure of the gene in eukaryotes - the image is enlarged

From each gene, an immature, or pre-RNA, is first synthesized, which contains both introns and exons.

After that, the splicing process takes place, as a result of which the intron regions are excised, and a mature mRNA is formed, from which a protein can be synthesized.

Rice. 20. Alternative splicing process - the image is enlarged

Such an organization of genes allows, for example, when different forms of a protein can be synthesized from one gene, due to the fact that exons can be fused in different sequences during splicing.

Rice. 21. Differences in the structure of genes of prokaryotes and eukaryotes - the image is enlarged

MUTATIONS AND MUTAGENESIS

mutation called a persistent change in the genotype, that is, a change in the nucleotide sequence.

The process that leads to mutation is called mutagenesis, and the organism All whose cells carry the same mutation mutant.

mutation theory was first formulated by Hugh de Vries in 1903. Its modern version includes the following provisions:

1. Mutations occur suddenly, abruptly.

2. Mutations are passed down from generation to generation.

3. Mutations can be beneficial, deleterious or neutral, dominant or recessive.

4. The probability of detecting mutations depends on the number of individuals studied.

5. Similar mutations can occur repeatedly.

6. Mutations are not directed.

Mutations can occur under the influence of various factors. Distinguish between mutations caused by mutagenic impacts: physical (eg ultraviolet or radiation), chemical (eg colchicine or reactive oxygen species) and biological (eg viruses). Mutations can also be caused replication errors.

Depending on the conditions for the appearance of mutations are divided into spontaneous- that is, mutations that have arisen under normal conditions, and induced- that is, mutations that arose under special conditions.

Mutations can occur not only in nuclear DNA, but also, for example, in the DNA of mitochondria or plastids. Accordingly, we can distinguish nuclear And cytoplasmic mutations.

As a result of the occurrence of mutations, new alleles can often appear. If the mutant allele overrides the normal allele, the mutation is called dominant. If the normal allele suppresses the mutated one, the mutation is called recessive. Most mutations that give rise to new alleles are recessive.

Mutations are distinguished by effect adaptive, leading to an increase in the adaptability of the organism to the environment, neutral that do not affect survival harmful that reduce the adaptability of organisms to environmental conditions and lethal leading to the death of the organism in the early stages of development.

According to the consequences, mutations are distinguished, leading to loss of protein function, mutations leading to emergence the protein has a new function, as well as mutations that change the dose of a gene, and, accordingly, the dose of protein synthesized from it.

A mutation can occur in any cell of the body. If a mutation occurs in a germ cell, it is called germinal(germinal, or generative). Such mutations do not appear in the organism in which they appeared, but lead to the appearance of mutants in the offspring and are inherited, so they are important for genetics and evolution. If the mutation occurs in any other cell, it is called somatic. Such a mutation can manifest itself to some extent in the organism in which it arose, for example, lead to the formation of cancerous tumors. However, such a mutation is not inherited and does not affect offspring.

Mutations can affect parts of the genome of different sizes. Allocate genetic, chromosomal And genomic mutations.

Gene mutations

Mutations that occur on a scale smaller than one gene are called genetic, or dotted (dotted). Such mutations lead to a change in one or more nucleotides in the sequence. Gene mutations includesubstitutions, leading to the replacement of one nucleotide by another,deletions leading to the loss of one of the nucleotides,insertions, leading to the addition of an extra nucleotide to the sequence.

Rice. 23. Gene (point) mutations

According to the mechanism of action on the protein, gene mutations are divided into:synonymous, which (as a result of the degeneracy of the genetic code) do not lead to a change in the amino acid composition of the protein product,missense mutations, which lead to the replacement of one amino acid by another and can affect the structure of the synthesized protein, although often they are insignificant,nonsense mutations, leading to the replacement of the coding codon with a stop codon,mutations leading to splicing disorder:

Rice. 24. Mutation schemes

Also, according to the mechanism of action on the protein, mutations are isolated, leading to frame shift readings such as insertions and deletions. Such mutations, like nonsense mutations, although they occur at one point in the gene, often affect the entire structure of the protein, which can lead to a complete change in its structure. when a segment of a chromosome rotates 180 degrees Rice. 28. Translocation

Rice. 29. Chromosome before and after duplication

Genomic mutations

Finally, genomic mutations affect the entire genome, that is, the number of chromosomes changes. Polyploidy is distinguished - an increase in the ploidy of the cell, and aneuploidy, that is, a change in the number of chromosomes, for example, trisomy (the presence of an additional homologue in one of the chromosomes) and monosomy (the absence of a homolog in the chromosome).

Video related to DNA

DNA REPLICATION, RNA CODING, PROTEIN SYNTHESIS

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