DNA replication, like all mechanisms, must have a way to terminate itself. This avoids situations where too much DNA is present in a cell (known as aneuploidy) or alternatively where cells are replicated too frequently (leading to tumour formation). This article will focus on termination of replication in both prokaryotes and eukaryotes, such as E.coli and humans respectively. Luckily, not much is known about the subject, so there isn't too much to learn!
*All key proteins or enzymes will be in bold, so they're easier to pick out.
**Please click the images to enlarge them.
DNA in E.coli is present in a double stranded, circular form and all replication is initiated at a single point on the DNA, called the origin of replication. Replication is initiated by the DnaB helicase which opens up the DNA and allows the fork to progress; this helicase comes into play later during termination (figure 1).
Replication in E.coli is bi-directional; the replication fork travels in both directions along the circle. In theory, replication should occur at the same speed on both sides of the circle, meaning that both forks should meet exactly in the centre at the bottom of the circle. Unfortunately it doesn’t happen quite as simply as that.
Replication occurs at different speeds on each side, this is due to other enzymes being present on the DNA. They may be creating mRNA or repairing DNA for example, but they inhibit the fork until they have finished their task. This would lead to one fork getting further around the circle than the other, which isn’t wanted. To counteract this, both forks are trapped at the very bottom of the circle so they can be terminated in a very specific region (Figures 2 and 3).
Present on the DNA near to the bottom of the circle are 6 terminator sequences (3 on each side). These sequences act as landing pads for specific proteins called Tus proteins (figure 4). Tus proteins effectively act as one way gates for the replication forks; they allow one fork through in one direction but don’t allow the progression of the other fork attempting to move through it in the opposite direction (figure 5). This is accomplished by the Tus proteins being in orientated in a specific way; facing the region where the forks will eventually become trapped.
Tus proteins manage to trap the replication forks by one face of the protein interacting with the DnaB helicases in such a way that prevents them from continuing to open up the DNA. The effect of this is that both forks become trapped in a relatively short region between the closest two Tus proteins which are in opposite orientations (figure 5).
In this region, the original DNA will be catenated (linked) due to the effect on DnaB helicase and will need to be cut so that both new circles can be finished. These two linked circular strands of DNA, each comprising one parental/one newly-synthesised strand (due to semi-conservative replication) have to be separated by the enzyme topoisomerase IV (figure 6). It is not exactly known how, but all of the enzymes required for DNA replication drop off the DNA in a controlled fashion, allowing the DNA to be cut.
Not much is known about the process of termination in eukaryotes such as ourselves. As of yet, no such proteins similar to or linked to prokaryotic Tus proteins or terminator sequences have been found in the human genome. This may be due to the fact that there are many origins of replication on a eukaryotic genome and the DNA is also not circular, meaning that the replication forks don't need to be trapped anywhere as in prokaryotic termination.
One way in which termination in prokaryotes is known to be different in eukaryotes is the catenation. After replication has taken place on the linear eukaryotic chromosome the two new daughter helices are both completely finished and do not need to be chopped apart by a topoisomerase. Whereas we know that the two new daughter circles of DNA are linked and do need to be chopped apart.
A current theory suggests that that the linear DNA is replicated until it encounters another origin of replication and termination occurs by ligation of the separate fragments using an enzyme named DNA ligase.
This ligation of fragments causes a problem once the end of a linear chromosome is reached. Replication cannot be terminated just by finishing adding the last few bases as bases need to be added adjacent to an RNA primer. This RNA primer cannot be added though as it would need to be put in place after where the chromosome finished due to replication running in an anti-parallel fashion. To allow the primer to be added, the existing parental DNA is extended by an enzyme named telomerase. This extension is called a telomere and is what allows the last fragment to be created and then ligated with the previous fragment. To understand this concept better and to look at some diagrams of this, please see this link #mce_temp_url#, it is another Fastbleep article written by myself which goes through the topic of replication.
These books are both excellent resources, so get hold of one if you can!
Alberts, B et al; 2010. Essential cell biology, 3rd ed. Garland science, Taylor and Francis group. New York and London.
Brown, T; 2011. Introduction to genetics: A molecular approach. Garland science, Taylor and Francis group. New York and London.
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