Rad51 is the primary eukaryotic recombinase responsible for initiating DNA strand exchange during homologous recombination. Although the subject of intense study for over a decade, many molecular details of the reactions promoted by Rad51 and related recombinases remain unknown. Using total internal reflection fluorescence microscopy, we directly visualized the behavior of individual Rad51 complexes on double-stranded DNA (dsDNA) molecules suspended in an extended configuration above a lipid bilayer. Here we show that complexes of Rad51 can bind to and slide freely along the helical axis of dsDNA. Sliding is bidirectional, does not require ATP hydrolysis, and displays properties consistent with a 1D random walk driven solely by thermal diffusion. The proteins move freely on the DNA for long periods of time; however, sliding terminates and the proteins become immobile upon encountering the free end of a linear dsDNA molecule. This study provides previously uncharacterized insights into the behaviors of human Rad51, which may apply to other members of the RecA-like family of recombinases.DNA repair ͉ homologous recombination ͉ total internal reflection fluorescence microscopy T he repair of double-stranded DNA breaks by homologous recombination is essential for maintaining genome integrity in most organisms (1-3). The importance of homologous recombination is highlighted by the finding that Rad51 null mutations are lethal in mice (4). Furthermore, defects in this repair pathway are associated with a variety of human cancers (5, 6). In eukaryotes, the broken ends of chromosomes are processed by 5Ј to 3Ј exonucleases to yield long single-stranded DNA overhangs (2, 3). Rad51, a DNA-dependent ATPase, assembles onto these overhangs, forming a nucleoprotein filament that is a key intermediate in homologous recombination (1,2,7,8). The primary functions of this filament are to locate homologous sequence that can be used as a template to repair the damaged DNA strand and to initiate strand exchange (1, 7).The structure and function of the complexes formed by Rad51 and the other RecA-like recombinases are conserved throughout evolution (8, 9). In their active states, Rad51 and related recombinases form a helical filament on DNA that induces a 50% extension of the bound DNA molecule (8). The extended nucleoprotein filament is correlated with DNA recombination activity; however, these proteins also form inactive filaments with shortened pitches (Ϸ65-85 Å versus Ϸ90-130 Å) (10). Rad51 and related recombinases also form octameric rings with a central pore large enough to accommodate a double-stranded (dsDNA) molecule (11-16). These ring-like recombinase structures do not appear to be the form of the protein that is active during the strand exchange phase of homologous recombination. Although the biological role of these rings remains unknown, it has been suggested that they may function as DNA ''pumps,'' allowing the proteins to move along DNA (12, 13).Here we have developed a unique total internal reflection fluorescence micr...