REPLICATION

From Big Medical Encyclopedia

REPLICATION (late lat. replicatio repetition; synonym reduplication) — process of biosynthesis of molecules of deoxyribonucleic acid, as a result to-rogo two affiliated, completely identical maternal are formed of one molecule. River. deoxyribonucleic acid (see) provides transfer of a full complex of hereditary genetic information from generation to generation (see. Heredity ). Property of molecules DNA to reduplitsirovatsya is shown also in a reproduction chromosomes (see) the higher organisms.

According to the model offered by J. Watson and T. Shout, molecule DNA represents the double helix constructed of chains of deoxyribonucleotides complementary each other. In the course of R. hydrogen bindings between couples of nucleotides are terminated and dezoksinukleozid-triphosphates join them new, complementary to the corresponding deoxyribonucleotides. Process of connection of nucleotides in a polynucleotide chain happens to eliminating of a pyrophosphate. DNA replication has character of semi-conservative process, i.e. each affiliated double helix includes one the maternal and one again synthesized polynucleotide chain.

Fig. 1. Scheme of replication of molecule DNA: the semi-conservative nature of process — on each of polynucleotide chains of parent molecule DNA as on a matrix, the affiliated chain (remark) is under construction is well visible. The arrow specified the direction of the movement of a so-called fork of replication, the dotted line designated hydrogen bindings between nitrogen bases; And — adenine, T — thymine — guanine, C — tsitozin.

The river is complex multistage process with participation of a large number of enzymes and other proteins. Matrix DNA, existence of dezoksiribonukleozidtrifosfat of all four nitrogen bases is necessary for R.'s implementation: two purines — adenine (A) and guanine (G) and two pirimpdin — thymine (T) and a tsitozin (Ts), and also ions of Mg 2+ . Replication happens by the movement along molecule DNA of a so-called fork of replication, or a replicative fork (fig. 1).

Formation of a fork of replication is preceded by interaction with molecule DNA of the special protein (the DNA-untwisting protein) which is eliminating superspiral rounds and locally untwisting a double helix of DNA. Assume that DNK-raskruchnvayushchy protein is built in a dezoksiribozofosfatny chain. This reaction is reversible: after release of the DNA-untwisting protein the helical structure of DNA is recovered. Local discrepancy of complementary chains of DNA is provided by proteins destabilizers of a double helix of DNA. Joining DNA, proteins destabilizers reduce heat stability of its molecule, edges are got by ability to melt at a temperature at 40 ° below the usual temperature of melting.

Fig. 2. Diagrammatic representation of a fork of replication. Shooters specified the direction in which the DNA polymerase conducts synthesis concerning a matrix; 3' and 5' — OH groups of desoxyribose of molecules DNA; it is visible that the lower (leading) chain of DNA grows continuously, and upper (late) — falteringly, with formation of short polynucleotide fragments (fragments of Okazaki).

Growth of a new chain of DNA in a fork of replication is catalyzed by enzyme a DNA polymerase (see. Polymerases ). In cells of the higher organisms and bacteria several forms of DNA polymerases are revealed, but R. of a genome at eukaryotes is carried out only under the influence of a DNA polymerase and, and at bacteria — DNA polymerases of III. Komplementarny chains of a double helix of DNA of an antiparallelna on orientation of carbon atoms desoxyriboses (see). Therefore, at the movement of a fork of replication one affiliated chain shall accrue in the direction 5' —> 3', and another — in the direction 3' —> 5'. At the same time all DNA polymerases are capable to attach new nucleotides only to Z '-to hydroxylic group of desoxyribose of the growing chain of DNA, thereby providing synthesis only in the direction 5' —> 3', i.e. is formed by the first 5' - the end of a new chain. The DNA polymerase besides is not capable to initiate synthesis of new chains on a one-chained DNA matrix. It was established that both of these difficulties are overcome by synthesis on one chain of short polynucleotide fragments with polarity 5' —> 3' in the direction opposite to the movement of a fork of replication (fig. 2). At the same time R.'s initiation each new fragment of DNA is carried out by enzyme a RNA polymerase (a so-called primaz), with participation to-rogo on a DNA matrix the initiator (priming) — the short site of RNA is synthesized (so-called. The RNA primer), to 3 '-hydroxylic group of a ribose to-rogo a DNA polymerase begins to attach deoxyribonucleotides. Afterwards the RNA primer is removed ekzonukleazy (see. Nucleases ), and the formed gap is closed by a DNA polymerase. At Escherichia coli 5' —> Z '-ekzonukleaznoy the DNA polymerase of I has activity.

Separate polynucleotide fragments are sewed among themselves by DNA-ligase enzyme (KF 6.5.1.1; 6.5.1.2). At the same time one of two chains of DNA grows continuously (the leading thread), and another — falteringly (the late thread). Fragments of discontinuous synthesis of DNA call fragments of Okazaki (Okazaki) by name the Japanese scientific R. Okazaki which opened them. At bacteria fragments of Okazaki have length apprx. 1000 nucleotide couples, and their RNA primer — 50 — 200 nucleotide couples. At the higher organisms fragments of Okazaki consist approximately of 150 — 200 nucleotide couples, and their RNA primer — of 10 — 20 couples.

Having attached the next nucleotide to the growing chain of DNA, the DNA polymerase «verifies» it with the partner on a chain matrix, and in case of discrepancy to couple (And — T or G — C) the same polymerase shows 3' —> 5 '-ekzonukleaznuyu activity, deleting mistakenly attached nucleotide. Thus the correction providing high precision of process of R. of molecules DNA that defines safety of hereditary information among generations of cells and organisms is carried out.

In the cells breeding in the way mitosis (see), and at bacteria DNA replication happens between acts of cell fission; in meiosis (see) DNA once before two following one after another divisions reduplitsirutsya that leads to a reduction (reduction) twice amounts of DNA (as well as chromosome numbers) on a cell. This piece of interphase is called the period of synthesis of DNA or the S-period.

The river begins (is initiated) in certain sites of molecule DNA (on F. Jacob's terminology — replicators), primary structure to-rykh is characterized with a high content of couples And — T and existence of so-called return repetitions (palindromes). From an origin locus either one, or two forks of replication (in the latter case they move to the opposite sides) move, providing elongation (lengthening) of again synthesized sites of molecule DNA. Termination (termination) of R. happens or at merge of two forks of replication moving towards each other or in special points of termination R.

Otrezok of molecule DNA who is replicated as a result of one act of initiation call unit of replication or replpkony. In a genome of bacteria, as a rule, there is only one site of initiation of R. connected with a cellular membrane. Ring molecule DNA of a genome of a bacterium is replicated as one replicon. In a genome of eukaryotes of R. it is carried out polireplikonno, i.e. R.'s initiation happens at the same time in many points on length of molecules DNA. It is established that on molecules DNA of a genome of eukaryotes there is a large number of potential origin loci of R. located at distance of 1 — 4 micron from each other. Depending on that how many potential origin loci are involved in R., the size of replicon can change. E.g., at DNA replication in the crushed eggs of a drosophila where cell fissions one follows very quickly another, R. joins every second or third potential point of R. and the amount of replication is equal to 9 — 12 microns; during the doubling of DNA of somatic cells of eukaryotes 1 of 10 or even from 100 potential origin loci of R. participates in R. on average and the size of replicons increases to 30 — 300 microns.

Time necessary for R.'s end of one replicon, is defined by the speed of the movement of a fork of replication. In cells of Escherichia coli at t°37 ° the fork of replication moves with a speed of 25 microns/min. that corresponds to accession about 1200 nucleotides in a second in each of affiliated branches of a fork of replication. The river of the ring molecule DNA having length of 1500 microns is carried out by two forks of replication in 30 min. In cells of hematothermal animals and the person the average speed of the movement of a fork of replication makes 0,5 microns/min. and since the majority of replicons has razkhmer apprx. 200 microns, their R.'s end demands 3 hours and more. R.'s speed of molecules DNA in cells of plants and cold-bloodeds is lower, than in cells of hematothermal animals, and depends on ambient temperature. More low speeds of DNA replication at eukaryotes in comparison with bacteria are explained by presence at their kernels of proteins, to-rye in a complex with DNA create nuclear chromatin (see).

DNA replication of viruses is generally similar to DNA replication of the highest animals and bacteria; it is carried out by enzymes of a cell of chorion. In nek-ry cases (viruses of herpes) the RNA primer is found as a part of virionny DNA. At oncogenous DNK-soder-zhashchikh of viruses (a papov viruses) DNA can integrate into a genome of a cell then replication of virus DNA happens together with DNA of a cell.

Replication of the majority of RNA-containing of viruses is carried out by virus-specific enzymes — RNK-polimerazamn RNA-dependent (replikaza), to-rye complete complementary thread on a virionny RNA matrix, forming so-called replicative forms of RNA.

At oncogenous RNA-containing of viruses (see. Retrovirusa ) The river is carried out by RNA-dependent enzyme a DNA polymerase (the return transcriptase, a revertaza), to-ry synthesizes the DNA copy of a virus genome capable to be built in a genome of a cell (see. Viruses ).

At bacteria and eukaryotes, as a rule, in each cycle of cell fission all DNA and at the same time only once shall be replicated. It means that there have to be regulatory systems controlling R.'s initiation and distinguishing parent and affiliated molecules. The mechanism of such regulation is not clear yet.

In certain cases (is normal also at pathology) there can be repeated R. of all genome without the subsequent cell division (it leads to emergence of polyploid kernels) or R. of separate parts of a genome without R. of all genome, a so-called ekstrareplikation (e.g., amplification of DNA of a ribosomal gene in an oogenesis of nek-ry animals). Cases of underreplication of a part of DNA of a genome in cells of eukaryotes are described. The ego concerns only DNA of heterochromatin, in Krom there are no genes necessary for life support of a cell.

Similarity of the enzymes P. and basic processes happening in a fork of replication at prokariot and eukaryotes testifies to high evolutionary stability and rigid genetic control of process of DNA replication. Disturbances of normal process of R. influence division and can lead to death of cells.



Bibliography: Boston To. and Sumner of E. Hromosom of an eukaryotic cell, the lane with English, page 248, M., 1981; Kornberg A. Synthesis of DNA, lane e English, M., 1977; Watson D. Molecular biology of a gene, the lane with English, M., 1978; DNA synthesis, ed. by J. Moli-neux a. M. Kohiyama, N. Y. — L., 1978; Jacob F., Brenner S. a. With u-z i n F. On the regulation of DNA replication in bacteria, Cold Spr. Harb. Symp. quant. Biol., v. 28, p. 329, 1963.


H. A. Lyapunova.

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