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p.728 left column 2nd paragraph:"To be duplicated, a DNA double helix must open to allow the DNA synthesis machinery to copy each DNA strand. These opening sites, called replication origins, are recognized by specific proteins, and DNA synthesis progresses from these sites in a bidirectional manner (Fig. 1a). In Escherichia coli, DNA replication starts from a single, sequence-specific element, and the speed of the two replication forks (60 kb/min) keeps pace with a rapid cell cycle (less than 30 min). The human genome is 700-fold larger than the E. coli genome, but the replication fork speed is 20-fold slower (2–3 kb/min). Thus, it would take at least 20 days to achieve a single division if there was one origin per chromosome." |