A DNA molecule is a structure on the chromosome. One chromosome contains one such molecule, consisting of two strands. DNA reduction is the transfer of information after the self-reproduction of strands from one molecule to another. It is inherent in both DNA and RNA. This article discusses the process of DNA replication.
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 step aside, and a new copy is synthesized on each. Upon completion, two absolutely identical molecules appear, in each of which there is 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 the division process begins at this centromere.
Another type of synthesis is called reparative. He, 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, 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 spent, its duration stretches for hours.
Reduction is divided into three periods:
- initiation;
- elongation;
- termination.
Let us consider in more detail this DNA reduction sequence.
Initiation
In human DNA, there are several tens of millions of pairs of nucleotides (in animals there are only one hundred and nine). DNA reduction begins in many places in the chain for the following reasons. Around the same time, transcription occurs in RNA, but it stops in some separate places during DNA synthesis. Therefore, before such a process, a sufficient amount of the substance accumulates in the cytoplasm of the cell in order to support gene expression and so 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, but transcription is not conducted. As studies have shown, DNA reduplication occurs immediately in several thousand points - small areas with a specific nucleotide sequence. They are joined by special initiator proteins, which in turn are joined by other DNA reduplication enzymes.
The DNA fragment where the synthesis takes place is called a replicon. It starts from the start point and ends when the enzyme completes replication. The replicon is autonomous, and also provides 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 they are formed:
- replication fork;
- RNA seed.
Replicative fork
This part is a process in which deoxyribonucleic strands are synthesized on disconnected DNA strands. The forks at the same time form the so-called peephole of reduction. The process is preceded by a number of actions:
- liberation from binding to histones in the nucleosome - such DNA reduction enzymes as methylation, acetylation and phosphorylation produce chemical reactions, as a result of which the proteins lose their positive charge, which contributes to their release;
- despiralization is the unwinding that is necessary for the further release of the threads;
- breaking of hydrogen bonds between DNA strands;
- their divergence in different directions of the molecule;
- fixation using SSB proteins.
Seed RNA
The synthesis is carried out by an enzyme called DNA polymerase. However, he cannot start it on his own, which is why other enzymes, RNA polymerases, which are also called RNA seeds, do this. They are synthesized in parallel with deoxyribonucleic filaments according to the complementary principle. Thus, the initiation ends with the synthesis of two RNA seeds on two strands of DNA that have been broken and displaced in opposite directions.
Elongation
This period begins with the addition of the nucleotide and the 3 'end of the RNA seed, which is carried out by the already mentioned DNA polymerase. She attaches the second, third nucleotide to the first, and so on. The bases of the new strand are connected to the mother chain by
hydrogen bonds. It is believed that the synthesis of the thread goes in the direction of 5 '- 3'.
Where it occurs towards the replication fork, synthesis proceeds continuously and lengthens at the same time. Therefore, such a thread is called leading or leading. On it, RNA seeds are no longer formed.
However, on the opposite maternal strand, the DNA nucleotides continue to attach to the RNA seed, and the deoxyribonucleic acid 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 site, synthesis begins at another site nearby using the same RNA seed. Thus, on the retarded chain, there are two fragments that are connected by DNA and RNA. They are called Okazaki fragments.
Then everything repeats. Then another coil of the spiral unravels, hydrogen bonds break, the strands diverge to the sides, the leading chain lengthens, the next fragment of the RNA seed is synthesized on the lagging one, followed by the Okazaki fragment. After that, on the retarded thread, the RNA seeds are destroyed, and the DNA fragments are combined into one. So on this circuit occurs simultaneously:
- the formation of new RNA seeds;
- synthesis of Okazaki fragments;
- destruction of RNA seeds;
- reunion in one single chain.
Termination
The process continues until the two replicative forks meet, or one of them approaches the end of the molecule. After the forks meet, the daughter DNA strands are joined by an enzyme. If 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 arrive at the site of synthesis, and DNA polymerase selects those that are necessary by trial pairing.
The desired nucleotide must be able to form as many hydrogen bonds as a similar nucleotide on the template DNA strand. In addition, there must be a fixed constant distance between the sugar phosphate cores corresponding to the three rings in two bases. If the nucleotide does not meet these requirements, the connection will not occur.
Control is carried out before including it in the chain and before including the subsequent nucleotide. After this, a bond forms in the saccharophosphate backbone.
Mutational variability
The DNA reduplication mechanism, despite a high percentage of accuracy, always has abnormalities in the strands, which are mainly called “gene mutations”. About one thousand nucleotide pairs have one error, called invariant reduction.
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 field of synthesis, and more. In some cases, errors can be corrected by reparation processes; in others, correction becomes impossible.
If the damage touched an inactive place, the error will not have serious consequences when the process of DNA replication occurs. The nucleotide sequence of a particular gene may occur with a mismatch error. Then the situation is different, and the death of this cell, as well as the death of the whole organism, can become a negative result. It should also be noted that gene mutations are based on mutational variation, which makes the gene pool more plastic.
Methylation
At the time of synthesis or immediately after it, methylation of chains occurs. It is believed that in humans this process is needed in order to form chromosomes and regulate gene transcription. In bacteria, this process serves to protect DNA from being cut by enzymes.