DNA Replication in Eukaryotes
DNA replication and cell cycle
DNA replication in eukaryotic cells is limited to S-phase of the cell cycle (see Physical Basis of Heredity 3.  Genetics, Biochemistry and Dynamics of Cell Division). During the S-phase, not only the DNA should be replicated, but also the associated chromosomal histone proteins should be duplicated. The initiation of DNA replication is a prominent landmark in the cell cycle and is, therefore, an important point for exercising control. Further, while in E. coli and other prokaryotes, a solitary origin for DNA replication is found, in eukaryotes, DNA replication needs to be initiated at multiple origins (upto several thousands) in a coordinated manner, though not identically at all origins. This leads to great variation in the duration of S-phase (S phase is 3.4 minutes in early embryonic cells, but 10 hours in cultured cells in Drosophila melanogaster), even though the rate of DNA synthesis does not differ. It is possible that different specific initiator proteins are used at different origins at different times during S-phase. For instance, in yeast, a protein ABF₁ (a transcription factor; see Expression of Gene : Protein Synthesis 2.  Transcription in Prokaryotes and Eukaryotes) binds many origins, but not all origins of replication. However, a set of origins may be organized in clusters to be used synchronously.