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DNA base flipping, also known as nucleotide flipping, is a mechanism in which a single nucleotide base, or nucleobase, in the DNA structure is rotated out from the DNA backbone by 180 degrees. This occurs when an enzyme needs access to the base to perform work on it, such as when it needs to be replaced with another base during DNA repair. It was first observed in 1994 using X-ray crystallography to view a methyltransferase enzyme performing work on a cytosine base. Since then, it has been shown to be used by many different enzymes in many biological processes such as DNA methylation, various DNA repair mechanisms, RNA transcription and DNA replication. DNA base flipping occurs by breaking the hydrogen bonds between the bases and unstacking the base from its neighbors. This can occur by an active process, where an enzyme binds to the DNA and then actively rotates the base, or a passive process, where the base first rotates out spontaneously, then is recognized and bound by an enzyme. It can be detected using X-ray crystallography, NMR spectroscopy, fluorescence spectroscopy, or hybridization probes. ==Discovery== Base flipping was first observed in 1994 when researchers Klimasauskas, Kumar, Roberts, and Cheng used X-ray crystallography to view an intermediate step in the chemical reaction of a methyltransferase bound to DNA. The methyltransferase they used was the C5-cytosine methyltransferase from ''Haemophilus haemolyticus'' (M. HhaI). This enzyme recognizes a specific sequence of the DNA (5'-GCGC-3') and methylates the first cytosine base of the sequence at its C5 location.〔 Upon crystallization of the M. HhaI-DNA complex, they saw the target cytosine base was rotated completely out of the double helix and was positioned in the active site of the M. HhaI. It was held in place by numerous interactions between the M. HhaI and DNA.〔 The authors theorized that base flipping was a mechanism used by many other enzymes, such as helicases, recombination enzymes, RNA polymerases, DNA polymerases, and Type II topoisomerases.〔 Much research has been done in the years subsequent to this discovery and it has been found that base flipping is a mechanism used in many of the biological processes the authors suggest. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「DNA base flipping」の詳細全文を読む スポンサード リンク
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