DNA is a reliable repository of genetic information. But it is necessary not only to keep it safe, but also to pass it on to progeny. From this depends on the survival rate. After all, parents must pass on to children all that they have achieved in the course of evolution. It contains 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 transferred. For this cell is divided. For genetic information to go 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 may be a bacterium that has decided to multiply. Or it may be the growth of new skin at the site of the cut. The process of doubling of deoxyribonucleic acid must be clearly regulated and completed before the start of cell division.
Where is the 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 constructed according to the complementarity principle: thymine corresponds to adenine in one chain, and cytosine corresponds to guanine. The doubling of the molecule must proceed in such a way that the principle of complementarity is also preserved in the daughter helixes.
Start replication - 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 can form from 5 to 50 replicative forks. Synthesis of daughter chains occurs simultaneously in several regions of the molecule. But it is not easy to complement complementary nucleotides. The nucleic acid chains are anti-parallel to each other. Different orientation of the parental chain affects the doubling, this led to a complex mechanism of DNA replication. One of the chains is completed by the subsidiary continuously and is called the leading one. This is correct, since polymerase is very convenient to attach a free nucleotide to the 3’-OH end of the previous one. Such synthesis goes on continuously, unlike the process on the second chain.
Lag Chain, O’Kazaki Fragments
With the other chain, difficulties arise, because there is a free 5'-end to which it is impossible to attach a free nucleotide. Then DNA polymerase acts on the other side. In order to complete the child chain, a primer is created that complements the parental chain. It is formed at the most replicative forks. The synthesis of a small piece begins with it, but already along the “right” path - the attachment of nucleotides occurs at the 3’-end. Thus, the completion of the chain in the second daughter helix occurs intermittently and has the 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 has been completed to the previous finished piece, the primers are cut out with a special enzyme, the place of the cutout 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 doubling DNA (elongation) takes place at a speed of about 700 nucleotides per second. If we recall that there are 10 monomer pairs per spin of nucleic acid, it turns out that during the "unwinding" the molecule rotates at a frequency of 70 revolutions per second. For comparison: the speed of rotation of the cooler in the computer system unit is approximately 500 revolutions per second. But despite the high rates, DNA polymerase almost never makes a mistake. After all, she just picks up the complementary 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 were able to make out the main points. It is important to understand its significance for both microorganisms and multicellular creatures.