DNA is a reliable storehouse of genetic information. But it needs not only to be kept safe, but also to the offspring. The survival of the species depends on this. After all, parents must give the children all 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, this information is much less for microorganisms, but it must also be transmitted. For this, the cell is divided. To get the genetic information for both daughter cells, it needs to be doubled, this process is called "DNA replication". It occurs before cell division, no matter which one. It can be a bacterium that has decided to multiply. Or it could be the growth of a new skin in place of a cut. The process of doubling deoxyribonucleic acid should be clearly regulated and completed before the beginning of cell division.
Where there is a doubling
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 on the principle of complementarity: adenine in one chain corresponds to thymine, and guanine - cytosine. The doubling of the molecule must pass in such a way that the principle of complementarity is preserved in the daughter spirals.
The origin of replication is initiation
Deoxyribonucleic acid is a double-stranded helix. DNA replication occurs by the child completing circuits for each parent chain. This synthesis became possible, spirals should "unravel", and chains to separate from each other. The role of helicase – she spins the spiral of deoxyribonucleic acid, revolving with great speed. Start doubling DNA cannot start from any place, such a complex process requires a certain area of the molecule – site of initiation of replication. After it was determined the starting point of doubling, and helicase started work on unravelling the spiral, the DNA sequences diverge to the sides, forming a replication fork. They sit DNA polymerase. They will synthesize a subsidiary of the chain.
In one molecule of deoxyribonucleic acid, from 5 to 50 replicative plugs can be formed. Synthesis of daughter chains occurs simultaneously in several parts of the molecule. But it is not easy to complete complementary nucleotides. Nucleic acid strands are antiparallel to each other. The different orientation of the parental chains affects the doubling, this caused a complex mechanism of DNA replication. One of the chains is completed by the child continuously and is called the leading one. It is correct, because it is very convenient for a polymerase to attach a free nucleotide to the 3'-OH end of the previous one. Such synthesis proceeds continuously, in contrast to the process on the second chain.
The retarded chain, fragments of O'Kazaki
With the other chain, there are complications, because there is a free 5'-end, to which it is impossible to attach a free nucleotide. Then the DNA polymerase acts on the other side. In order to complete the child chain, a primer is created, complimentary to the parent chain. It forms at the most replicative fork. From it, the synthesis of a small piece begins, but already along the "right" path - the attachment of nucleotides occurs to the 3'-end. Thus, the completion of the chain in the second daughter spiral occurs intermittently and has a direction opposite to the movement of the replicative fork. These fragments were called O'Kazaki fragments, they are about 100 nucleotides in length. After the fragment was completed to the previous finished piece, the primers are cut with a special enzyme, the place of the cut is filled with missing nucleotides.
Doubling is completed when both chains finished building myself a child, and all the pieces O the Cossacks sewn together. In eukaryotes DNA replication ends when the replication forks meet each other. As in prokaryotes, this molecule is circular, and the process of doubling without breaking the circuit. It turns out that all deoxyribonucleic acid is one large replicon. And doubling ends when the replication forks meet on the opposite side of the ring. After replication, the two parental chains of deoxyribonucleic acid must be linked back, after which both molecules are twisted to form superspiral. Next, the methylation of both DNA molecules in the adenine in the site –GATZ-. It does not divide the circuit and does not interfere with their complementarity. It is necessary for the folding of molecules into chromosomes, and to regulate the reading of genes.
Speed and accuracy of replication
The second stage of DNA doubling (elongation) passes at a rate of about 700 nucleotides per second. If we recall that 10 pairs of monomers are required for one nucleic acid turn, then it turns out that during the "unraveling" the molecule rotates at a frequency of 70 revolutions per second. For comparison: the speed of rotation of the cooler in the computer's system unit is about 500 revolutions per second. But despite high rates, DNA polymerase is almost never wrong. After all, she just chooses complimentary nucleotides. But even if she makes a mistake, DNA polymerase recognizes it, takes a step back, tears off the wrong monomer and replaces it with the right one. The mechanism of DNA replication is very complicated, but we could disassemble the main points. It is important to understand its significance for both microorganisms and multicellular creatures.