Replication fork
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a: template, b: leading strand, c: lagging strand, d: replication fork, e: primer, f: Okazaki fragments
The replication fork is a structure that forms within the nucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching "prongs", each one made up of a single strand of DNA, that are called the leading and lagging strands. DNA polymerase creates new partners for the two strands by adding nucleotides.
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[edit] Replication
When replicating, the original DNA splits in two, forming two "prongs" which resemble a fork (hence the name "replication fork"). DNA has a ladder-like structure; imagine a ladder broken in half vertically, along the steps. Each half of the ladder now requires a new half to match it. Because DNA polymerase can only synthesize a new DNA strand in a 5' to 3' manner, the process of replication goes differently for the two strands comprising the DNA double helix.
[edit] Leading strand
The leading strand is the template strand of the DNA double helix that is oriented in a 3' to 5' manner. Thus, the new, antiparallel strand being produced will be oriented in the 5' to 3' manner.
On the leading strand, a polymerase "reads" the template DNA and adds nucleotides to the 3' end of the newly made DNA strand continuously. This polymerase is DNA polymerase III (DNA Pol III) in prokaryotes and presumably Pol ε[1][2] in eukaryotes.
[edit] Lagging strand
The lagging strand is that template strand of the DNA double helix that is oriented in a 5' to 3' manner. The newly made strand will be antiparallel, oriented in the 3' to 5' direction. Because of its orientation, the newly made strand is extending opposite to the working orientation of DNA polymerase III which is in a 5' to 3' manner. As a result, replication of the lagging strand is more complicated than of the leading strand.
On the lagging strand, primase "reads" the DNA and adds RNA to it in short, separated segments. In eukaryotes, primase is intrinsic to Pol α.[3] DNA polymerase III or Pol δ lengthens the primed segments, forming Okazaki fragments. Primer removal in eukaryotes is also performed by Pol δ.[4] In prokaryotes, DNA polymerase I "reads" the fragments, removes the RNA using its flap endonuclease domain, and replaces the RNA nucleotides with DNA nucleotides (this is necessary because RNA and DNA use slightly different kinds of nucleotides). DNA ligase joins the fragments together.
[edit] See also
- DNA replication#The replication fork for context
[edit] References
- ^ Pursell, Z.F. et al. (2007). "Yeast DNA Polymerase ε Participates in Leading-Strand DNA Replication". Science 317: 127. doi:. PMID 17615360.
- ^ Scott D McCulloch; Thomas A Kunkel (01/2008). "The fidelity of DNA synthesis by eukaryotic replicative and translesion synthesis polymerases". Cell Research 18: 148. doi:. PMID 18166979.
- ^ Elizabeth R. Barry; Stephen D. Bell (12/2006). "DNA Replication in the Archaea". Microbiology and Molecular Biology Reviews 70: 876–887. doi:. PMID 17158702.
- ^ Distinguishing the pathways of primer removal during Eukaryotic Okazaki fragment maturation Contributor Author Rossi, Marie Louise. Date Accessioned: 2009-02-23T17:05:09Z. Date Available: 2009-02-23T17:05:09Z. Date Issued: 2009-02-23T17:05:09Z. Identifier Uri: http://hdl.handle.net/1802/6537. Description: Dr. Robert A. Bambara, Faculty Advisor. Thesis (PhD) - School of Medicine and Dentistry, University of Rochester. UR only until January 2010. UR only until January 2010.
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