Duplication of a DNA molecule. How does DNA duplication occur in the mitotic cycle? Explain what

Replication (doubling) of DNA. DNA is found on chromosomes and is replicated before each chromosome duplication and cell division. J. Watson and F. Crick proposed a DNA doubling scheme, according to which helical double-stranded DNA first unwinds (unwinds) along its axis. At the same time hydrogen bonds between the nitrogenous bases break and the chains diverge. At the same time, complementary nitrogenous bases of the nucleotides of the second chain are attached to the nucleotides of each chain, where thymine stands opposite adenine, adenine stands opposite thymine, cytosine stands against guanine, etc., which are linked into new polynucleotide chains with the help of DNA polymerase enzymes. As a result, two new daughter DNA molecules are formed from one. Each daughter molecule, inheriting the structure of one chain of the parent molecule, strictly retains the specificity of the information contained in it. Since one of the two chains of the molecule serves as the template for replication, this type of DNA synthesis is called semi-conservative autoreproduction.

Further research showed that the replication of bacterial and other DNA molecules begins at a specific starting point. Several such starting points have been found in the chromosomes of eukaryotes. The DNA strands at the point of replication initiation are separated under the influence of a special helicase protein (Fig. 19). Single-stranded DNA sections appear, which become templates for replication-attraction of complementary nucleotides. These single-chain regions bind to special proteins that stabilize them (prevent their complementary interaction). A special enzyme topoisomerase (called DNA gyrase in prokaryotes) promotes cleavage of the DNA helix in the region of the replication fork.

Replication on the mother chain, going from the starting point in the direction 5"->3", occurs in the form of a solid line. This chain is called the leading chain. Synthesis on the second chain 3"->5" occurs in separate fragments in opposite direction(also 5" - "3"). This chain is called retarded. Fragments are small sections of DNA (E. coli have about 2000 nucleotides, eukaryotes have about 200). They are named after the Japanese scientist R. Okazaki who discovered them. After synthesis is completed, the Okazaki fragments are combined using the enzyme ligase into a common polynucleotide chain. In eukaryotes, DNA replication and the joining of its various replication sites occur during the S phase of interphase. After this phase is completed, each chromosome has two DNA molecules, which become two identical chromatids.

A structure capable of replication (chromosome, plasmid, viral genome) is called a replicon.

Self-duplication of DNA molecules is the basis for the stability of genetic information of a given species and ensures the material continuity of the hereditary substance of the cell.

DNA is a reliable store of genetic information. But it must not only be kept safe, but also passed on to offspring. The survival of the species depends on this. After all, parents must pass on to their children everything that they have achieved in the course of evolution. It records everything: from the number of limbs to the color of the eyes. Of course, microorganisms have much less of this information, but it also needs to be transmitted. To do this, the cell divides. So that genetic information goes to both daughter cells, it needs to be doubled, this process is called “DNA replication.” It occurs before cell division, no matter which one. It could be a bacterium that has decided to multiply. Or it could be new skin growing at the cut site. Deoxyribo doubling process nucleic acid must be clearly regulated and completed before cell division begins.

Where does doubling occur?

DNA replication occurs directly in the nucleus (in eukaryotes) or in the cytoplasm (in prokaryotes). Nucleic acid consists of nucleotides - adenine, thymine, cytosine and guanine. Both chains of the molecule are built according to the principle of complementarity: adenine in one chain corresponds to thymine, and guanine to cytosine. The doubling of the molecule must take place in such a way that the principle of complementarity is preserved in the daughter helices.

Start of replication - initiation

Deoxyribonucleic acid is a double-stranded helix. DNA replication occurs by adding daughter strands along each parent strand. For this synthesis to become possible, the spirals must be “unraveled” and the chains separated from each other. This role is played by helicase - it unwinds the helix of deoxyribonucleic acid, rotating with high speed. The beginning of DNA duplication cannot begin from any place; such a complex process requires a specific part of the molecule - the replication initiation site. Once the starting point for duplication has been determined and helicase has begun its work of unraveling the helix, the DNA strands move apart to form a replication fork. DNA polymerases sit on them. It is they who will synthesize the daughter chains.

Elongation

In one molecule of deoxyribonucleic acid, from 5 to 50 replication forks can form. The synthesis of daughter chains occurs simultaneously in several parts of the molecule. But it is not easy to complete the construction of complementary nucleotides. The nucleic acid chains are antiparallel to each other. The different directions of the parental chains affect duplication; this determines the complex mechanism of DNA replication. One of the chains is continuously completed by the child and is called the leading one. This is correct, because it is very convenient for polymerase to attach a free nucleotide to the 3’-OH end of the previous one. This synthesis occurs continuously, in contrast to the process on the second chain.

Lagging chain, O'Kazaki fragments

Difficulties arise with the other chain, because there the 5’ end is free, to which it is impossible to attach a free nucleotide. Then DNA polymerase acts from the other side. In order to complete the daughter chain, a primer is created that is complementary to the parent chain. It is formed at the replication fork itself. This is where the synthesis of a small piece begins, but along the “correct” path - the addition of nucleotides occurs at the 3’ end. Thus, the completion of the chain at the second daughter helix occurs discontinuously and has the direction opposite to the movement of the replication fork. These fragments were called O'Kazaki fragments and are about 100 nucleotides long. After the fragment has been built up to the previous finished piece, the primers are cut out by a special enzyme, and the cut site is filled with the missing nucleotides.

Termination

Doubling is completed when both chains have completed their daughter chains, and all O’Kazaki fragments are sewn together. In eukaryotes, DNA replication ends when replication forks meet each other. But in prokaryotes this molecule is circular, and the process of doubling occurs without first breaking the chain. It turns out that all deoxyribonucleic acid is one large replicon. And duplication ends when the replication forks meet on the opposite side of the ring. After replication is complete, both strands of the parent deoxyribonucleic acid must be linked back together, after which both molecules are twisted to form supercoils. Next, both DNA molecules are methylated at adenine in the -GATC- region. This does not separate the chains or interfere with their complementarity. This is necessary for the folding of molecules into chromosomes, as well as for the regulation of gene reading.

Replication speed and accuracy

The second stage of DNA doubling (elongation) occurs at a speed of about 700 nucleotides per second. If we remember that there are 10 pairs of monomers per turn of nucleic acid, it turns out that during “unwinding” the molecule rotates at a frequency of 70 revolutions per second. For comparison: the cooler rotation speed is system unit computer is approximately 500 revolutions per second. But despite high rates, DNA polymerase almost never makes mistakes. After all, she simply selects complementary nucleotides. But even if it makes a mistake, DNA polymerase recognizes it, takes a step back, tears off the incorrect monomer and replaces it with the correct one. The mechanism of DNA replication is very complex, but we were able to understand the main points. It is important to understand its significance for both microorganisms and multicellular creatures.

Reproduction is the main property that distinguishes living organisms from nonliving ones. Absolutely all species of living organisms are capable of reproducing their own kind, otherwise the species would disappear very quickly. The methods of reproduction of different creatures are very different from each other, but the basis of all these processes is cell division, and it is based on the mechanism of DNA reduplication.

Cell division does not necessarily accompany the process of reproduction of an organism. Growth and regeneration also depend on cells. But in single-celled creatures, which include bacteria and protozoa, cell division is the main reproductive process.

Multicellular organisms live much longer than unicellular ones, and their life span exceeds the life span of the cells of which they are composed, sometimes by a huge number of times.

How does DNA reduplication occur?

Doubling the DNA helix is ​​the most important process during cell division. The spiral is divided into two similar ones, and each chain of chromosomes is absolutely identical to the parent. That is why the process is called reduplication. Two identical “halves” of the helix are called chromatids.

Between the bases of the DNA helix (these are adenine-thymine and guanine-cytosine) there are complementary hydrogen bonds, and during reduplication special enzymes break them. Complementary bonds are those when a pair can only connect to each other. If we are talking about the bases of the DNA helix, then guanine and cytosine, for example, form a complementary pair. The DNA strand splits into two parts, after which another complementary nucleotide is attached to each nucleotide. Thus, it turns out that two new spirals are formed, completely identical.

Mitosis is the process of cell division

Typically, cells divide through mitosis. This process includes several phases, and nuclear fission is the very first of them. After the nucleus has divided, the cytoplasm also divides. Associated with this process is the concept of life cycle cells: this is the time that elapses from the moment a cell separates from its parent until it itself divides.

Mitosis begins with reduplication. After this process, the nucleus shell is destroyed, and for some time the nucleus does not exist in the cell at all. At this time, the chromosomes are twisted as much as possible and can be clearly seen under a microscope. The two new helices then separate and move toward the poles of the cell. When the spirals reach their goal - each approaching its cellular pole - they unwind. At the same time, core shells begin to form around them. While this process is being completed, division of the cytoplasm has already begun. The last phase of mitosis occurs when two completely identical cells separate from one another.

Doubling with the formation of two identical copies is a completely normal phenomenon. Often such doubling is called replication. The latter can occur on different levels organization of matter - from DNA itself to chromosomes and whole cells. In this case, if the process went smoothly, two identical units are obtained. Replication is a jewel-precise doubling.

Like nowhere else exactly

DNA replication is considered the most interesting and basic for all other species. This is a process that occurs over several stages, at each of which accuracy is critical, because inaccuracy will provoke the synthesis of a completely incorrect protein, unsuitable for use in the cell and in the body as a whole.

The beginning began

Cell replication begins with the consequence of chromosome doubling. But replication is the cornerstone of the whole process. It consists of three stages: firstly, initiation, secondly, elongation, thirdly, termination. The work of enzymes begins from special points - replicators and only from them. Starting in the wrong place would distort the whole process. The catalyst enzyme activates special proteins that form the pre-replication complex necessary for DNA duplication. Before reproduction, the DNA is cut into two parts by special enzymes.

Complementary chain

During elongation, a complementary maternal chain is built on the matrix. That is, one that can then form a complete molecule. The process ends with termination, which also occurs at a certain point. There is a special unit - the replicon. This is the DNA fragment that is built up at a time. DNA replication is the basis; without it, chromosome replication is impossible. The latter occurs when the cell prepares to divide.

Protein as a sign

Chromosome duplication begins literally a few hours after DNA duplication occurs. In order for a chromosome to be rebuilt, not only new sets are needed, but also proteins that are part of the chromosomal apparatus, and their synthesis takes time. Accelerated replication is a sign of cancer. If too much protein is detected, characteristic of intensive, the doctor begins to sound the alarm.

Enough space

Is in the process of replication interesting feature- in the space of the nucleus, the points of synthesis of new DNA are located quite evenly, thus, there are no distortions that could provoke mutual influence. There are quite a lot of dots, usually one and a half to two dozen.

How exactly the chromosomes are organized depends on whether the cell is undergoing mitosis or meiosis. In the first case, the resulting cell will have a normal set, in the second - a half set. After all, the remaining half will be brought by the cell of the second partner. If in meiosis it suddenly turns out to be a complete set, it will either be non-viable, or a sick child will be conceived. However, such children are still most often not born; a miscarriage occurs at an early stage of pregnancy, which the mother may mistake for menstruation or ovulation

Before each cell division, with absolutely exact adherence to the nucleotide sequence, self-duplication (reduplication) of the DNA molecule occurs. Reduplication begins when the DNA double helix temporarily unwinds. This occurs under the action of the enzyme DNA polymerase in an environment that contains free nucleotides. Each single chain, according to the principle of chemical affinity (A - T, G - C), attracts to its nucleotide residues and secures free nucleotides located in the cell with hydrogen bonds. Thus, each polynucleotide chain acts as a template for a new complementary chain. The result is two DNA molecules, of each of them one half comes from the parent molecule, and the other is newly synthesized, i.e. two new DNA molecules represent exact copy the original molecule.

Squirrels

Squirrels - mandatory component all cells. In the life of all organisms, proteins are of paramount importance. Protein contains carbon, hydrogen, nitrogen, and some proteins also contain sulfur. Amino acids play the role of monomers in proteins. Each amino acid has a carboxyl group (-COOH) and an amino group (-NH 2). The presence of acidic and basic groups in one molecule determines their high reactivity. Between amino acids that come together, a bond called peptide, and the resulting combination of several amino acids is called peptide. Connection from large number amino acids are called polypeptide.

Proteins contain 20 amino acids that differ from each other in their structure. Different proteins are formed by combining amino acids in different sequences. The enormous diversity of living things is largely determined by differences in the composition of the proteins they have.

There are four levels of organization in the structure of protein molecules:

Primary structure - a polypeptide chain of amino acids linked in a certain sequence by covalent (strong) peptide bonds.

Secondary structure - a polypeptide chain twisted in the form of a spiral. In it, weak hydrogen bonds arise between adjacent turns. Together they provide a fairly strong structure.

Tertiary the structure is a bizarre, but specific configuration for each protein - a globule. It is held by weak hydrophobic bonds or cohesive forces between nonpolar radicals, which are found in many amino acids. Due to their abundance, they provide sufficient stability of the protein macromolecule and its mobility. The tertiary structure of proteins is also maintained covalent S-S bonds arising between radicals of the sulfur-containing amino acid cysteine ​​that are distant from each other.

Due to the connection of several protein molecules with each other, it is formed quaternary structure. If the peptide chains are arranged in the form of a ball, then such proteins are called globular. If polypeptide chains are arranged in bundles of threads, they are called fibrillar proteins.

Violation of the natural structure of a protein is called denaturation. It can occur under the influence high temperature, chemicals, radiation, etc. Denaturation can be reversible (partial destruction of the quaternary structure) and irreversible (destruction of all structures).

FUNCTIONS:

The biological functions of proteins in a cell are extremely diverse. They are largely due to the complexity and diversity of the forms and composition of the proteins themselves.

1 Construction function - organelles are built.

2 Catalytic - protein enzymes (amylase, converts starch into glucose)