In molecular genetics, the processes of DNA, RNA and protein synthesis are divided into three stages for convenience of description: initiation, elongation and termination. These stages describe different mechanisms for different synthesized molecules, however, they always mean the beginning, the course of the process, and the end. Replication termination is the end of the synthesis of DNA molecules.
The biological role of termination
Initiation and termination are the initial and final boundaries of the growth of the synthesized chain, which is carried out at the elongation stage. The completion of the process usually occurs where the biological expediency of further synthesis ends (for example, at the site of the end of the replicon or transcripton). At the same time, termination performs 2 important functions:
- does not allow synthesis to go beyond a specific section of the matrix chain;
- releases the biosynthetic product.
Thus, for example, in the process of transcription (synthesis of RNA based on a DNA template), termination does not allow the process to cross the border of a particular gene or operon. ATotherwise, the semantic content of messenger RNA would be violated. In the case of DNA synthesis, termination keeps the process within a single replicon.
So, termination is one of the mechanisms for maintaining the isolation and orderliness of the biosynthesis of various sections of matrix molecules. In addition, the release of the product allows the latter to perform its functions, and also returns the system to its original state (detachment of enzyme complexes, restoration of the spatial structure of the matrix, etc.).
What is the termination of DNA synthesis
DNA synthesis occurs during replication, the process of doubling the genetic material in a cell. In this case, the original DNA unwinds, and each of its chains serves as a template for a new (daughter) one. As a result, two full-fledged DNA molecules are formed in place of one double-stranded helix. The termination (end) of this process in prokaryotes and eukaryotes occurs differently due to some differences in the mechanisms of chromosome replication and the nucleoid of nuclear-free cells.
How replication works
A whole complex of proteins is involved in replication. The main function is performed by the synthesis enzyme, DNA polymerase, which catalyzes the formation of phosphodiester bonds between the nucleotides of the growing chain (the latter are selected according to the principle of complementarity). To start working, DNA polymerase requires a primer - a primer that is synthesized by DNA primase.
This event is preceded by the unwinding of DNA and the separation of its chains,each of which serves as a matrix for synthesis. Since the latter can only occur from the 5' to the 3' end, one strand becomes leading (synthesis occurs in the forward direction and continuously), and the other becomes lagging behind (the process is carried out in the opposite direction and fragmentary). The gap between fragments is subsequently repaired by DNA ligase.
The unwinding of the double helix is carried out by the DNA helicase enzyme. This process forms a Y-shaped structure called the replication fork. The resulting single-stranded regions are stabilized by the so-called SSB proteins.
Termination is the stoppage of DNA synthesis, which occurs either as a result of the meeting of replication forks, or when the end of the chromosome is reached.
Mechanism of termination in prokaryotes
The completion of replication in prokaryotes occurs at the appropriate point in the genome (termination site) and is determined by two factors:
- replication fork meeting;
- ter-sites.
The meeting of the forks occurs if the DNA molecule has a closed circular shape, which is characteristic of most prokaryotes. As a result of continuous synthesis, the 3' and 5' ends of each chain are connected. For unidirectional replication, the match point is the same as the origin site (OriC). In this case, the synthesized chain, as it were, goes around the ring molecule, returning to the starting point and meeting with the 5'-end of itself. With bidirectional replication (synthesis proceeds simultaneously in two directions from the OriC point), the meetingforks and the connection of the ends occurs in the middle of the ring molecule.
Coupling of rings is carried out by DNA ligase. This forms a structure called a cathekan. By introducing a single strand break, DNA gyrase breaks the rings and the replication process is completed.
Ter-sites also take part in replication. They are located 100 base pairs beyond the point where the forks meet. These regions contain a short sequence (23 bp) to which the protein product of the tus gene binds, blocking the further advancement of the replication fork.
Replication termination in eukaryotic cell
And the last moment. In eukaryotes, one chromosome contains several points of replication initiation, and termination occurs in two cases:
- when forks moving in opposite directions collide;
- if the end of the chromosome is reached.
At the end of the process, the separated DNA molecules bind to chromosomal proteins and are orderly distributed among daughter cells.
