What is DNA reduplication? DNA Reduplication Process

A DNA molecule is a structure in the chromosome. One chromosome contains one such molecule, consisting of two strands. Reduplication of DNA is the transfer of information after the self-reproduction of the filaments from one molecule to another. It is inherent in both DNA and RNA. This article deals with the process of DNA reduplication.

General information and types of DNA synthesis

It is known that the filaments in the molecule are twisted. However, when the process of DNA reduplication begins, they despiralize, then move aside, and a new copy is synthesized on each. At the end, two absolutely identical molecules appear, in each of which there is a mother and daughter filament. This synthesis was called semiconservative. DNA molecules move away, while remaining in a single centromere, and finally diverge only when this centromere begins the process of division.

Another type of synthesis was called reparative. It, unlike the previous one, is not associated with any cellular stage, but begins when DNA damage occurs. If they are too extensive, then the cell eventually dies. However, if the damage is local, you can restore it. Depending on the problem, a single or two DNA chains are immediately recovered. This, as it is also called, unscheduled synthesis does not take a long time and does not require large energy inputs.
But when DNA reduplication occurs, a lot of energy is consumed, the material, the duration of it stretches for hours.
Reduplication is divided into three periods:

• Initiation;
• Elongation;
• Termination.

Consider this sequence of DNA reduplication in more detail.

Initiation

In human DNA there are several tens of millions of nucleotide pairs (in animals there are only one hundred and nine). DNA reduplication begins in many places in the chain for the following reasons. Approximately at the same time, transcription occurs in RNA, but it stops at the time of DNA synthesis in some separate places. Therefore, before such a process, a sufficient amount of substance accumulates in the cytoplasm of the cell in order to maintain the expression of genes and that the vital activity of the cell is not disturbed. In view of this, the process should proceed as quickly as possible. Translation during this period is carried out, and transcription is not conducted. As research has shown, reduplication of DNA occurs immediately in several thousand points - small areas with a certain sequence of nucleotides. They are joined by special initiator proteins, to which, in turn, other enzymes of DNA reduplication are added.

A fragment of DNA, where a synthesis occurs, is called a replicon. It starts from the point of origin and ends when the enzyme completes replication. Replicon is self-contained, and also provides the entire process with its own security.
The process can not start from all points at once, somewhere it starts earlier, somewhere - later; Can flow in one or two opposite directions. Events occur in the following order, when:

• Replication fork;
• RNA primer.

Replicative plug

This part is a process in which the synthesis of deoxyribonucleic yarns takes place on disconnected DNA strands. Forks form the so-called reduplication eye. The process is preceded by a number of actions:

• Release from binding to histones in the nucleosome - DNA reduplication enzymes such as methylation, acetylation and phosphorylation produce chemical reactions that result in the proteins losing their positive charge, which facilitates their release;
• Despiralization is unwinding, which is necessary for the further release of threads;
• Breaking of hydrogen bonds between DNA strands;
• Their divergence in different directions of the molecule;
• Fixation, occurring with the help of SSB proteins.

RNA primer

The synthesis is carried out by an enzyme called DNA polymerase. However, he can not start it on his own, therefore other enzymes, RNA polymerases, called RNA primers, do this. They are synthesized parallel to deoxyribonucleic fibers according to the complementary principle. Thus, the initiation ends with the synthesis of two RNA primers on two DNA strands that have been torn and separated in different directions.

Elongation

This period begins with the addition of a nucleotide and a 3 'end of the RNA primer, which is already carried out by the said DNA polymerase. To the first, she attaches the second, third nucleotide, and so on. The foundations of a new thread are connected with the parent chain by hydrogen bonds. It is believed that the synthesis of the filament goes in the direction of 5 '- 3'.
Where it occurs towards the replication fork, the synthesis proceeds continuously and lengthens at the same time. Therefore, such a thread is called leading or leading. On it, RNA primers are no longer formed.

However, on the opposite maternal strand, the DNA nucleotides continue to attach to the RNA primer, and the deoxyribonucleic chain is synthesized in the opposite direction from the redupplication fork. In this case, it is called retarded or lagging.

On the lagging filament, the synthesis takes place fragmentarily, where, at the end of one section, synthesis begins on another site nearby with the help of the same RNA primer. Thus, there are two fragments on the retarded chain, which are connected by DNA and RNA. They were called fragments of Okazaki.

Then everything repeats itself. Then another thread of the helix is disintegrated, the bonds of hydrogen break, the threads diverge, the leading chain is lengthened, the next fragment of the RNA-primer is synthesized on the lagging one, followed by the Okaucas fragment. After this, on a retarded strand, the RNA primers are destroyed, and the DNA fragments combine into one. So on this chain occurs simultaneously:

• The formation of new RNA primers;
• Synthesis of fragments of Okaucasi;
• Destruction of RNA primers;
• Reunion into one single chain.

Termination

The process continues until two replicative forks meet, or one of them approaches the end of the molecule. After meeting the forks, the daughter strands of DNA are connected by an enzyme. In the case, if the plug has moved to the end of the molecule, DNA reduplication ends with the help of special enzymes.

Correction

In this process an important role is assigned to control (or correction) of reduplication. All four kinds of nucleotides enter the synthesis site, and by trial pairing, the DNA polymerase selects those that are needed.

The desired nucleotide must be able to form as many hydrogen bonds as there is an analogous nucleotide on the matrix strand of DNA. In addition, there must be a certain constant distance between the sugar-phosphate backbones corresponding to the three rings in the two bases. If the nucleotide does not meet these requirements, the connection will not occur.
The control is performed before it is included in the chain and before the subsequent nucleotide is included. After this, a bond is formed in the skeleton of the sugar phosphate.

Mutational variability

The mechanism of DNA reduplication, despite a high percentage of accuracy, always has disruptions in threads, called mainly "gene mutations." Approximately a thousand nucleotide pairs account for one error, which is called contagiar reduplication.

It happens for various reasons. For example, with a high or too low concentration of nucleotides, deamination of cytosine, the presence of mutagens in the synthesis region, and more. In some cases, errors can be corrected by reparation processes, in others the correction becomes impossible.

If the damage has touched an inactive location, the error will not have serious consequences when the DNA reduplication process takes place. The sequence of the nucleotide of a particular gene can manifest itself with a mismatch error. Then the situation is different, and the negative result can be both the death of this cell, and the death of the whole organism. It should also be taken into account that gene mutations are based on mutational variability, which makes the gene pool more plastic.

Methylation

At the time of synthesis or immediately after it, the chains are methylated. It is believed that a person needs this process in order to form chromosomes and regulate the transcription of genes. In bacteria, this process serves to protect DNA from cutting it with enzymes.