DNA molecule - it is located in chromosome structure.One chromosome contains a single molecule consisting of two strands.DNA replication - transfer information after it self-replication filaments from one molecule to another.It is inherent in both DNA and RNA.This article discusses the process of DNA replication.
General and types of DNA synthesis
known that twisted yarn in the molecule.However, when the process starts DNA replication, they dispiralized, then steps aside, and on each new copy is synthesized.Upon completion of the two appear absolutely identical molecules in each of which there is a parent and subsidiary strands.This synthesis is called semi-conservative.DNA molecules are moved, while remaining in a single centromere, and finally diverge only when this begins the process of dividing the centromere.
Another type is called reparative synthesis.It, unlike the previous one, is not associated with any cell stage, but starts when DNA damage occurs.If they are too wide-ranging nature, the cell eventually dies.However, if the damage is local, then you can restore them.Depending on the problem to be restored or two once separate strands of DNA.This, as it is called, unscheduled synthesis does not take a long time and requires a lot of energy.
But when DNA replication occurs, then spent a lot of energy, the material is stretched to the length of his watch.
Reduplication is divided into three periods:
- initiation;
- elongation;
- termination.
Let us consider the sequence of DNA replication.
Initiation
in human DNA - tens of millions of base pairs (the animals they number a hundred and nine).DNA replication begins at many places the chain for the following reasons.Around the same time in the RNA transcription occurs, but the synthesis of DNA, it is suspended in some selected areas.Therefore, before this process in the cell cytoplasm accumulates sufficient substance to support gene expression and cellular activity that has not been broken.In view of this process should take place as soon as possible.Broadcasting carried out in this period, and transcription is not conducted.Studies have shown that DNA replication occurs in several thousands of points - small areas with a specific nucleotide sequence.They are joined by special initiator proteins, which in turn joined by other enzymes of DNA replication.
DNA fragment is synthesized which is called replication.It starts from the origin and ends when completing replication enzyme.Autonomous replicon, as well as supplying the entire process of its own software.
process can not start from all points at once, somewhere it begins earlier, somewhere - later;It can take place in one or in two opposite directions.Events occur in the following order when the image:
- replication forks;
- RNA primer.
replication fork
This part is a process in which strands of DNA on a disconnected the synthesis of deoxyribonucleic threads.Plugs thus form a so-called eye of replication.The process is preceded by a number of actions:
- exemption due to the histones in the nucleosome - DNA replication enzymes such as the methylation, acetylation and phosphorylation produce chemical reactions that result in the proteins lose their positive charge, which contributes to their release;
- despiralization - is unwinding, which is necessary for the further release of filaments;
- gap hydrogen bonds between the DNA strands;
- their divergence in the different sides of the molecule;
- fixation occurring proteins using SSB.
RNA primer
Synthesis carries an enzyme called DNA polymerase.However, to start his own he can not, so do other enzymes - RNA polymerase, which is also called RNA primers.They are synthesized in parallel strands of deoxyribonucleic complementary principle.Thus, the initiation of RNA synthesis of the two ends, two primers and departed torn apart strands of DNA.
Elongation
This period begins with the attachment of a nucleotide and the 3 'end of the RNA-primer, which carries the already mentioned DNA polymerase.It attaches to first second, third nucleotide, and so on.Foundations of the new thread connected with the parent chain of hydrogen bonds.It is believed that the synthesis of the thread is in the direction 5 '- 3'.
Where it occurs in the side of the replication fork, the synthesis takes place continuously and at the same time lengthens.Therefore, this thread is called leading or leadership.She RNA primer is no longer formed.
However, on the opposite strand of the parent DNA nucleotides continue to join the RNA primer, and deoxyribonucleic chain is synthesized in the opposite direction of the replication fork.It is in this case called retarded or lagging.
on lagging strand synthesis occurs in fragments, where at the end of one section begins synthesis on another site nearby with the help of the same RNA primer.Thus, there are two delayed chain fragment are joined DNA and RNA.They are called Okazaki fragments.
Further, all repeated.Then spliced another spiral turn, breaks the bonds of hydrogen, the thread to the sides, leading the chain is lengthened to lagging synthesized following snippet of RNA primer, and then - Okazaki fragments.Thereafter the retarded RNA primer strands are destroyed and the DNA fragments are joined into one.So this circuit occurs simultaneously:
- formation of new RNA primer;
- synthesis of Okazaki fragments;
- destruction of RNA primers;
- reunification into a single chain.
Termination
process continues as long as the two do not meet the replication fork, or one of them has come to an end of the molecule.After the meeting, forks daughter DNA strands are joined by the enzyme.In case, if the plug is moved to the end of the molecule, DNA replication finishes with special enzymes.
Correction
In this process, an important role for the control (or correction) replication.To place synthesis receives all four types of nucleotides, and the probe by the DNA polymerase pairing selects those that are needed.
desired nucleotide to be able to form hydrogen bonds as well as similar nucleotide on the template strand of DNA.In addition, between the sugar-phosphate backbone must be a certain constant distance corresponding to the three rings in the two bases.If the nucleotide does not meet these requirements, the connection will not occur.
control is carried out before its inclusion in the circuit before turning and subsequent nucleotide.After that connection is formed in the core saharofosfata.
mutational variability
mechanism of DNA replication, despite the high percentage of accuracy, is always in violation of filaments, called mostly "gene mutations."About a thousand nucleotide pairs there is one mistake that konvariantnaya called reduplication.
It happens for various reasons.For example, at high or too low concentrations of nucleotides deamination of cytosine, presence of mutagens in the synthesis of both.In some cases, the error can be corrected by repair processes in other correction becomes impossible.
If the damage affected the inactive space, an error will not have serious consequences when the process of DNA replication.Nucleotide sequence of a gene can manifest error mating.Then it is not the case, and the result can be negative as the death of the cells, and the death of the whole organism.It should also be borne in mind that gene mutations based on mutational variability, which makes the gene pool of plasticity.
methylation
At the time of synthesis, or immediately after it occurs methylation chains.It is believed that in humans, this process is needed to form the chromosomes and regulate gene transcription.In bacteria, this process serves to protect DNA from cutting its enzymes.
