Researchers at Caltech discovered in 1971 that the circular mitochondrial DNA from growing cells included a short segment of three strands which they called a displacement loop.[1] They found the third strand was a replicated segment of the heavy strand (or H-strand) of the molecule, which it displaced, and was hydrogen bonded to the light strand (or L-strand). Since then, it has been shown that the third strand is the initial segment generated by a replication of the heavy strand that has been arrested shortly after initiation and is often maintained for some period in that state.[2] The D-loop occurs in the main non-coding area of the mitochondrial DNA molecule, a segment called the control region or D-loop region.
Replication of the mitochondrial DNA can occur in two different ways, both starting in the D-loop region.[3] One way continues replication of the heavy strand through a substantial part (e.g. two-thirds) of the circular molecule, and then replication of the light strand begins. The more recently reported mode starts at a different origin within the D-loop region and uses coupled-strand replication with simultaneous synthesis of both strands.[3][4]
Certain bases within the D-loop region are conserved, but large parts are highly variable and the region has proven to be useful for the study of the evolutionary history of vertebrates.[5] The region contains promoters for the transcription of RNA from the two strands of mitochondrial DNA immediately adjacent to the D-loop structure that is associated with initiation of DNA replication.[6] D-loop sequences are also of interest in the study of cancers.[7]
The function of the D-loop is not yet clear, but recent research suggests that it participates in the organization of the mitochondrial nucleoid.[8][9]