The number of replication origin sequences in the genome does not change during the lifespan of an organism, but the number of active origins does vary according to the developmental stage. In the early embryo, the greater number of active origins may be supported by the higher density of origin recognition proteins and by the less constrained genomic regulation. In later stages, more concerned with differentiation than proliferation, it is thought that epigenetic marking restrains most potential origins and restricts the speed of DNA replication and cell division, with the rate of replication fork movement remaining more or less constant. In the August 8 Cell, Mauro Anglana and colleagues at the Institut Curie show that the speed of replication fork movement varies depending on the size of the nucleotide pool, and it is this which determines the activity of origins within a cell and hence the speed of cell...

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