DNA base flipping

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.