A DNA molecule is a structure found on a chromosome. One chromosome contains one such molecule consisting of two strands. DNA reduplication is the transfer of information after self-reproduction of threads from one molecule to another. It is inherent in both DNA and RNA. This article discusses the process of DNA reduplication.
General information and types of DNA synthesis
It is known that the threads in the molecule are twisted. However, when the process of DNA reduplication begins, they despiralize, then move to the sides, and a new copy is synthesized on each. Upon completion, two absolutely identical molecules appear, each of which contains a mother and daughter thread. This synthesis is called semi-conservative. DNA molecules move away, while remaining in a single centromere, and finally diverge only when this centromere begins to divide.
Another type of synthesis is called reparative. He, unlike the previous one,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 localized, then it can be repaired. Depending on the problem, a single or two strands of DNA are subject to restoration. This, as it is also called, unscheduled synthesis does not take a long time and does not require large energy costs.
But when DNA reduplication occurs, a lot of energy, material is consumed, its duration stretches for hours.
Reduplication is divided for three periods:
- initiation;
- elongation;
- termination.
Let's take a closer look at this DNA reduplication sequence.
Initiation
In human DNA there are several tens of millions of base pairs (in animals there are only one hundred and nine). DNA reduplication starts at many places in the chain for the following reasons. Around the same time, transcription occurs in RNA, but it is suspended in some separate places during DNA synthesis. Therefore, before such a process, a sufficient amount of a substance accumulates in the cytoplasm of the cell in order to maintain gene expression and so that the vital activity of the cell is not disturbed. In view of this, the process should be carried out as quickly as possible. Broadcast during this period is carried out, and transcription is not conducted. Studies have shown that DNA reduplication occurs at once in several thousand points - small areas with a certainsequence of nucleotides. They are joined by special initiator proteins, which in turn are joined by other enzymes of DNA replication.
The DNA fragment where synthesis occurs is called the replicon. It starts from the start point and ends when the enzyme completes replication. The replicon is autonomous, and also supplies the entire process with its own support.
The process may 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 generated:
- replication fork;
- RNA primer.
Replication fork
This part is the process by which deoxyribonucleic strands are synthesized on the detached strands of DNA. The forks form the so-called reduplication eye. The process is preceded by a series of actions:
- release from binding to histones in the nucleosome - DNA reduplication enzymes such as methylation, acetylation and phosphorylation produce chemical reactions that cause proteins to lose their positive charge, which facilitates their release;
- despiralization is the unwinding that is necessary to further release the threads;
- breaking hydrogen bonds between DNA strands;
- their divergence in different directions of the molecule;
- fixation by SSB proteins.
RNA primer
Synthesis carries outan enzyme called DNA polymerase. However, he cannot start it on his own, so other enzymes do this - RNA polymerases, which are also called RNA primers. They are synthesized in parallel with deoxyribonucleic strands according to the complementary principle. Thus, the initiation ends with the synthesis of two RNA primers on two DNA strands that are broken and detached in different directions.
Elongation
This period begins with the addition of a nucleotide and the 3' end of the RNA primer, which is carried out by the already mentioned DNA polymerase. To the first, she attaches the second, third nucleotide, and so on. The bases of the new strand are connected to the parent chain by hydrogen bonds. It is believed that filament synthesis proceeds in the 5'-3' direction.
Where it occurs towards the replication fork, synthesis proceeds continuously and elongates as it does so. Therefore, such a thread is called leading or leading. RNA primers no longer form on it.
However, on the opposite maternal strand, DNA nucleotides continue to attach to the RNA primer, and the deoxyribonucleic chain is synthesized in the opposite direction from the reduplication fork. In this case, it is called lagging or lagging.
On the lagging strand, synthesis occurs fragmentarily, where, at the end of one section, synthesis begins at another site nearby using the same RNA primer. Thus, there are two fragments on the lagging strand that are connected by DNA and RNA. They are called Okazaki fragments.
Then everything repeats. Then another turn of the helix unwinds, the hydrogen bonds break, the strands diverge to the sides, the leading strand lengthens, the next fragment of the RNA primer is synthesized on the lagging one, after which the Okazaki fragment. After that, on the lagging strand, the RNA primers are destroyed, and the DNA fragments are combined into one. So on this circuit happens simultaneously:
- formation of new RNA primers;
- synthesis of Okazaki fragments;
- destruction of RNA primers;
- reunification into one single chain.
Termination
The process continues until two replication forks meet, or one of them reaches the end of the molecule. After the forks meet, the daughter strands of DNA are connected by an enzyme. In the event that the fork 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 given to the control (or correction) of reduplication. All four types of nucleotides are supplied to the site of synthesis, and by trial pairing, DNA polymerase selects those that are needed.
The desired nucleotide must be able to form as many hydrogen bonds as the same nucleotide on the DNA template strand. In addition, there must be a certain constant distance between the sugar-phosphate backbones, corresponding to three rings in two bases. If the nucleotide does not meet these requirements, the connection will not occur.
Control is carried out before its inclusion in the chain and beforeinclusion of the next nucleotide. After that, a bond is formed in the backbone of the sugar phosphate.
Mutational variation
The mechanism of DNA replication, despite the high percentage of accuracy, always has disturbances in the threads, mainly called "gene mutations". Approximately one thousand base pairs have one error, which is called convariant reduplication.
It happens for various reasons. For example, at a high or too low concentration of nucleotides, deamination of cytosine, the presence of mutagens in the synthesis area, and more. In some cases, errors can be corrected by reparation processes, in others, correction becomes impossible.
If the damage has touched an inactive place, the error will not have serious consequences when the DNA reduplication process occurs. The nucleotide sequence of a particular gene may appear with a mismatch. Then the situation is different, and both the death of this cell and the death of the whole organism can become a negative result. 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, chain methylation occurs. It is believed that in humans, this process is necessary in order to form chromosomes and regulate gene transcription. In bacteria, this process serves to protect the DNA from being cut by enzymes.
