Flap endonuclease
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Flap endonucleases (FENs, also known as 5' nucleases in older references) are a class of nucleolytic enzymes that act as both 5'-3' exonucleases and structure specific endonucleases on specialised DNA structures that occur during the biological processes of DNA replication, DNA repair and DNA recombination. Flap endonucleases have been identified in eukaryotes, prokaryotes, archea and some viruses.
The endonuclease activity of FENs was initially identifed as acting on a DNA duplex which has a single stranded 5' overhang on one of the strands (termed a "5' flap", hence the name flap endonuclease)[1]. FENs catalyse hydrolytic cleavage of the phosphodiester bond at the junction of single and double stranded DNA
Protein structure models based on X-ray crystallography data suggest that FENs have a flexible arch created by two α-helices through which the single 5' strand of the 5' flap structure can thread through [2].
Flap endonucleases have been used in biotechnology, for example the Taqman PCR assay [3] and the Invader single nucleotide polymorphism (SNP) assay [4].