DNA molecules move through nanopores under an applied voltage, a phenomenon that is fundamental to DNA sequencing and other applications in nanobiotechnology. Despite many experimental and computational studies, the detailed mechanism of DNA translocation remains unclear. Here we employed long-time atomistic molecular dynamics simulations in conjunction with translocation experiments to elucidate the unzipping dynamics. Simulations reveal that the translocation of a single DNA molecule is mediated by a collective dynamics of multiple base pairs that cooperatively interact with the surface of the nanopore. The unzipping dynamics exhibits intermittent bursts, leading to the translocation of DNA bases with a step size of 0.34 nm, which is half of the double-stranded DNA pitch. This finding resolves the long-standing debate on whether the translocation step size of DNA is 0.34 nm or 0.68 nm. Our results uncover the atomistic details of the translocation mechanism and provide insight into the design of nanopore-based devices for DNA analysis and manipulation.