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a: template, b: leading strand, c: lagging strand, d: replication fork, e: primer, f: Okazaki fragments Many enzymes are involved in the DNA replication fork. The replication fork is a structure that forms within the long helical DNA during DNA replication.
After around 20 nucleotides, elongation is taken over by Pol ε on the leading strand and Pol δ on the lagging strand. [103] Polymerase δ (Pol δ): Highly processive and has proofreading, 3'->5' exonuclease activity. In vivo, it is the main polymerase involved in both lagging strand and leading strand synthesis. [104]
The leading strand is continuously synthesized and is elongated during this process to expose the template that is used for the lagging strand (Okazaki fragments). During the process of DNA replication, DNA and RNA primers are removed from the lagging strand of DNA to allow Okazaki fragments to bind to.
As a summary, a typical DNA rolling circle replication has five steps: [2] Circular dsDNA will be "nicked". The 3' end is elongated using "unnicked" DNA as leading strand (template); 5' end is displaced. Displaced DNA is a lagging strand and is made double stranded via a series of Okazaki fragments. Replication of both "unnicked" and displaced ...
DNA Pol III uses one set of its core subunits to synthesize the leading strand continuously, while the other set of core subunits cycles from one Okazaki fragment to the next on the looped lagging strand. Leading strand synthesis begins with the synthesis of a short RNA primer at the replication origin by the enzyme Primase (DnaG protein).
In terms of structure, the replisome is composed of two replicative polymerase complexes, one of which synthesizes the leading strand, while the other synthesizes the lagging strand. The replisome is composed of a number of proteins including helicase , RFC , PCNA , gyrase / topoisomerase , SSB / RPA , primase , DNA polymerase III , RNAse H ...
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However, branched DNA can occur if a third strand of DNA is introduced and contains adjoining regions able to hybridize with the frayed regions of the pre-existing double-strand. Although the simplest example of branched DNA involves only three strands of DNA, complexes involving additional strands and multiple branches are also possible. [ 69 ]