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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.
The process of semiconservative replication for the site of DNA replication is a fork-like DNA structure, the replication fork, where the DNA helix is open, or unwound, exposing unpaired DNA nucleotides for recognition and base pairing for the incorporation of free nucleotides into double-stranded DNA. [3]
During DNA replication, the double helix is unwound and the complementary strands are separated by the enzyme DNA helicase, creating what is known as the DNA replication fork. Following this fork, DNA primase and DNA polymerase begin to act in order to create a new complementary strand.
Figure 1: A basic schematic of Polδ function at the DNA replication fork. The Polδ complex (p125, p66, p50 and p12) associates with replication fork. Single-stranded DNA are coated with replication protein A (RPA) (light pink). Polα bound to a primase, initiates lagging strand synthesis (blue line), herein an RNA primer is first elongated by ...
Being the primary holoenzyme involved in replication activity, the DNA Pol III holoenzyme also has proofreading capabilities that corrects replication mistakes by means of exonuclease activity reading 3'→5' and synthesizing 5'→3'. DNA Pol III is a component of the replisome, which is located at the replication fork.
DNA is a duplex formed by two anti-parallel strands. Following Meselson-Stahl, the process of DNA replication is semi-conservative, whereby during replication the original DNA duplex is separated into two daughter strands (referred to as the leading and lagging strand templates). Each daughter strand becomes part of a new DNA duplex.
Loading of these factors completes the active replication fork and initiates synthesis of new DNA. Complete replication fork assembly and activation only occurs on a small subset of replication origins. All eukaryotes possess many more replication origins than strictly needed during one cycle of DNA replication. [5]
At each replication fork, the primosome is utilized once on the leading strand of DNA and repeatedly, initiating each Okazaki fragment, on the lagging DNA strand. Initially the complex formed by PriA, PriB, and PriC binds to DNA. Then the DnaB-DnaC helicase complex attaches along with DnaT. This structure is referred to as the pre-primosome.