In molecular genetics, the processes of DNA, RNA and protein synthesis for convenience of description are divided into three stages: initiation, elongation and termination. These stages describe different mechanisms for different synthesized molecules, but they always mean the beginning, the course of the process, and completion. 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 build-up of the synthesized chain, which is carried out at the stage of elongation. The completion of the process usually occurs where the biological feasibility of further synthesis ends (for example, at the end of the replicon or transcripton). At the same time, termination performs 2 important functions:
- does not allow the synthesis to go beyond a specific section of the matrix chain;
- releases the biosynthesis product.
So, for example, in the process of transcription (RNA synthesis based on a DNA matrix), termination does not allow the process to cross the boundary of a specific gene or operon. Otherwise, the semantic content of messenger RNA would be violated. In the case of DNA synthesis, termination keeps the process within one replicon.
So, termination is one of the mechanisms for maintaining the isolation and ordering of the biosynthesis of various sections of matrix molecules. In addition, the release of the product allows the latter to fulfill 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 DNA synthesis termination
DNA synthesis occurs during replication - the process of doubling the genetic material in a cell. In this case, the original DNA is unraveling, and each of its chains serves as a matrix for the new (daughter) one. As a result, two complete DNA molecules are formed in place of one double-stranded helix. Termination (termination) of this process in prokaryotes and eukaryotes occurs in different ways due to some differences in the mechanisms of replication of chromosomes and nucleoid of nuclear-free cells.
How replication is done
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 nucleotides of an expanding chain (the latter are selected by the principle of complementarity). To start the work of DNA polymerase, a seed is required - 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 occur only from the 5` to 3`-end, one chain becomes the leading one (synthesis occurs in the forward direction and continuously), and the other becomes lagging (the process is carried out in the opposite direction and fragmentarily). The gap between the fragments is subsequently eliminated by DNA ligase.
The unwinding of the double helix is carried out by the DNA helicase enzyme. During this process, a Y-shaped structure called a replication fork is formed. The resulting single-stranded regions are stabilized by the so-called SSB proteins.
Termination is the termination of DNA synthesis that occurs either as a result of the meeting of replication forks, or when the end of the chromosome is reached.
The mechanism of termination in prokaryotes
The completion of replication in prokaryotes occurs at the corresponding point in the genome (termination site) and is determined by two factors:
- meeting replication forks;
- ter sites.
The meeting of the forks occurs if the DNA molecule has a closed ring shape, which is characteristic of most prokaryotes. As a result of continuous synthesis, the 3` and 5`-ends of each chain are joined. In unidirectional replication, the alignment point coincides with the initiation site (OriC). In this case, the synthesized chain, as it were, bends around the ring molecule, returning to the starting point and meeting with the 5`-end of itself. In bidirectional replication (synthesis occurs simultaneously in two directions from the OriC point), the forks meet and the ends join in the middle of the ring molecule.
Ring coupling is carried out by DNA ligase. In this case, a structure called a cathecan is formed. By introducing a single-stranded break, DNA gyrase disconnects the rings, and the replication process is completed.
Ter sites also participate in replication. They are located 100 nucleotide pairs farther from the point of encounter of the forks. These sites contain a short sequence (23 bp) with which the protein product of the tus gene binds, blocking the further progression of the replication fork.
Termination of Replication in a Eukaryotic Cell
And the last moment. In eukaryotes, one chromosome contains several points of replication initiation, and termination occurs in two cases:
- in the collision of forks moving in opposite directions;
- in case of reaching the end of the chromosome.
At the end of the process, the separated DNA molecules bind to chromosomal proteins and are orderedly distributed among daughter cells.