Sequencing Okazaki fragments in human cells. Homo sapiens

Despite intense investigation, human replication origins and termini remain elusive. Existing data have shown strong discrepancies. Here, we present a new methodology to isolate and sequence Okazaki fragments (OK-Seq) to reveal the proportions of rightward- (R) and leftward- (L) moving forks throughout the genome. Zones of predominant initiation (termination) can then be detected as upward (downward) slopes in replication fork directionality (RFD = R–L) profiles. This allowed us to determine RFD genome-wide in asynchronous cultures of two human cell lines. We infer the location of replication initiation and termination events at high resolution and paint a novel, comprehensive landscape of human genome replication in these two cell types.