DNA Replication and Replication Fork
Generally, in molecular biology, Replication of DNA, is one of the important biological processes for producing two identical replicas of the DNA from one original DNA.
The process of replication of DNA usually occurs in all living organisms which acts as the most important and essential function for the cell division during the growth and repair process of the damaged tissues, it also ensures that each new cell receives its own copy of DNA.
So, it is said that all living cells have a distinctive property of cell division, which makes the replication of DNA essential. Replication fork is considered as one of the regions of DNA which is formed during the process of replication.
What is Replication Fork?
DNA replication is similar to that of all the biological polymerization processes which proceeds with the help of three catalysation reactions by enzymes and also in other co ordinated steps such as initiation, elongation and termination.
Replication fork usually occurs during the process of elongation. Replication fork is generally a structure which forms within the long helical DNA during the process of replication.
This replication fork is usually created by the helicases, which acts by breaking the hydrogen bonds between the two strands of the DNA which holds the helices together.
The resultant of this activity by the helicases forms a structure which has two branching prongs, where each one is made up of a single strand.
It is important to know the basics of replication of DNA, before understanding the replication fork.
Mechanism of DNA Replication
We all now that DNA is a double stranded structure with two complementary strands. Where two linear strands run opposite to each other in a twisted manner.
During the process of replication these two strands are splitted and the separation id initiated at the replication fork during the step of elongation.
The replication fork is a structure which is formed during the long helical DNA during the process of DNA replication.
It is activated by helicases, which helps in breaking the hydrogen bonds, which holds the two strands of the helix.
The resulting structure has two branching’s which is known as prongs, where each one is made up of single strand of DNA.
These two strands serve as a template for the leading and lagging strands, which will be created as DNA polymerases which matches the complementary nucleotide with the templates, then the templates are as usual properly referred to as the leading strand template and the; lagging strand template.
DNA is being read by the DNA polymerase in the 3’ to 5’ direction which means that a new strand is being synthesised in the 5’ to 3’ direction.
Since the leading and the lagging strand templates which are oriented in the opposite directions at the replication fork, a major issue is in synthesis of the lagging strand whose direction of synthesis is opposite to the direction of the replication forks growth.
Leading Strand in Replication Fork
The leading strand is one of the strands where the new DNA is synthesised along the same direction of the growing replication fork. This method of DNA replication occurs in a continuous manner.
Lagging Strand in Replication Fork
The lagging strand is one other type of strand where the new DNA is produced in the direction opposite to that of the growth of replication fork.
Because of this type of orientation, lagging strand replication is considered more complicated on comparing with the leading strand.
In the same manner the DNA polymerase in this strand is seen to lag behind, on the other strand Whereas the lagging strand is produced in a short and separated segment.
In Lagging strands template an enzyme primase extends the primed segments, which forms the ookazagi fragments.
Following this the RNA primers are removed and then it is replaced with the DNA and their fragments which are then joined by the enzyme DNA ligase.
Dynamics at the Replication Fork
In all the cases, the enzyme helicase composes of six polypeptides which wraps around one of the strands of the DNA, that is being replicated.
The two polymerases are then bound to the enzyme helicase hexamer. In eukaryotes this helicase wraps around the leading strand and in prokaryotes it is winded around the lagging strand.
Due to this helicase unwraps the DNA, at the replication fork, The DNA is now forced to rotate and this process results in building up the twists in the DNA ahead.
This building up of forms a torsional resistance which results in eventually halting the process of replication of fork. Topoisomerases are the enzymes which temporarily breaks the strands of DNA, by relieving the tension which are caused by unwinding the double strands of DNA helix.
Topoisomerases which also includes DNA gyrase which is achieved by adding negative supercoils to the DNA helix.
Bare single stranded DNA has the capability to fold back on itself that tends to form a secondary structure., these structures further interfere with the movement involved by DNA polymerases.
So, in order to prevent this single strand binding proteins are bind to the DNA until the second strand is produced, which prevents the formation of secondary structures.
Double stranded DNA is usually coiled by the histones during replication which play an important role in regulating the gene expressions, so that the replicated DNA must be coiled around the histones at the same place as the original DNA.
In order to ensure this histone proteins, dis assembles the chromatin before it has been replicated and further it replaces the histones in an appropriate place. The steps involved in reassemble must be appropriate.
Replication Fork Citations
- Template-switching during replication fork repair in bacteria. DNA Repair (Amst) . 2017 Aug;56:118-128.
- Replication fork dynamics. Cold Spring Harb Perspect Biol . 2014 Jan 1;6(1):a010157.
- Replication fork pausing at protein barriers on chromosomes. FEBS Lett . 2019 Jul;593(13):1449-1458.
- Replication fork reversal in eukaryotes: from dead end to dynamic response. Nat Rev Mol Cell Biol . 2015 Apr;16(4):207-20.
- PRIMPOL-Mediated Adaptive Response Suppresses Replication Fork Reversal in BRCA-Deficient Cells. Mol Cell . 2020 Feb 6;77(3):461-474.e9.
- Regulation of replication fork speed: Mechanisms and impact on genomic stability. DNA Repair (Amst) . 2019 Sep;81:102654.