Dynamin, the GTPase required for clathrin-mediated endocytosis, is recruited to 
clathrin-coated pits in two sequential phases. The first is associated with 
coated pit maturation; the second, with fission of the membrane neck of a coated 
pit. Using gene-edited cells that express dynamin2-EGFP instead of dynamin2 and 
live-cell TIRF imaging with single-molecule EGFP sensitivity and high temporal 
resolution, we detected the arrival of dynamin at coated pits and defined 
dynamin dimers as the preferred assembly unit. We also used live-cell 
spinning-disk confocal microscopy calibrated by single-molecule EGFP detection 
to determine the number of dynamins recruited to the coated pits. A large 
fraction of budding coated pits recruit between 26 and 40 dynamins (between 1 
and 1.5 helical turns of a dynamin collar) during the recruitment phase 
associated with neck fission; 26 are enough for coated vesicle release in cells 
partially depleted of dynamin by RNA interference. We discuss how these results 
restrict models for the mechanism of dynamin-mediated membrane scission.