also of interest in the study of cancers. The function of the D-loop is not yet clear, but recent research suggests that it participates in the organization of the mitochondrial nucleoid. In telomeres In 1999 it was reported that telomeres, which cap the end of chromosomes, terminate in a lariat-like structure termed a T-loop (Telomere-loop). This is a loop of both strands of the chromosome which are joined to an earlier point in the double-stranded DNA by the 3' strand end invading the strand pair to form a D-loop. The joint is stabilized by the shelterin protein POT1. The T-loop, which is completed by the D-loop splice, protects the end of the chromosome from damage. In DNA repair When a double-stranded DNA molecule has suffered a break in both strands, one repair mechanism available in diploid eukaryotic cells is homologous recombination repair. This makes use of the intact chromosome homologous to the broken one as a template to bring the two double-stranded pieces into correct alignment for rejoining. Early in this process, one strand of one piece is matched to a strand of the intact chromosome and that strand is used to form a D-loop at that point, displacing the intact chromosome's other strand. Various ligation and synthesis steps follow to effect the rejoining. In humans, the protein RAD51 is central to the homologous search and formation of the D-loop. In the bacterium Escherichia coli, a similar function is performed by the protein RecA. Meiotic recombination During meiosis, repair of double-strand damages, particularly double-strand breaks, occurs by the recombination process outlined in the accompanying diagram. As shown in the diagram, a D-loop plays a central role in meiotic recombinational repair of such damages. During this process, Rad51 and Dmc1 recombinases bind the 3’ single-strand DNA (ssDNA) tails to form helical nucleoprotein filaments that perform a search for intact homologous double-stranded DNA (dsDNA). Once the homologous sequence is found, the recombinases facilitate invasion of the ssDNA end into the homologous dsDNA to form a D-loop. After strand exchange, homologous recombination intermediates are processed by either of two distinct pathways (see diagram) to form the final recombinant chromosomes. See also D-loop replication mtDNA control region References DNA Telomeres