English: The chromosome is prepared for DNA duplication in G1, when prereplicative complexes are assembled at replication origins (red). Transformation of these complexes to preinitiation complexes and activation of the origin in S phase results in the unwinding of the DNA helix and initiation of DNA replication. Two replication forks move out from each origin until the entire chromosome is duplicated. Segregation of the duplicated chromosomes in M phase then results in two daughter cells with identical chromosomes. The activation of replication origins also causes disassembly of the prereplicative complex. Because new prereplicative complexes cannot be formed at origins until the following G1, each origin can be activated only once in each cell cycle.^[1]