<P> The rate of DNA replication in a living cell was first measured as the rate of phage T4 DNA elongation in phage - infected E. coli . During the period of exponential DNA increase at 37 ° C, the rate was 749 nucleotides per second . The mutation rate per base pair per replication during phage T4 DNA synthesis is 1.7 per 10 . </P> <P> DNA replication, like all biological polymerization processes, proceeds in three enzymatically catalyzed and coordinated steps: initiation, elongation and termination . </P> <P> For a cell to divide, it must first replicate its DNA . This process is initiated at particular points in the DNA, known as "origins", which are targeted by initiator proteins . In E. coli this protein is DnaA; in yeast, this is the origin recognition complex . Sequences used by initiator proteins tend to be "AT - rich" (rich in adenine and thymine bases), because A-T base pairs have two hydrogen bonds (rather than the three formed in a C-G pair) and thus are easier to strand separate . Once the origin has been located, these initiators recruit other proteins and form the pre-replication complex, which unzips the double - stranded DNA . </P> <P> DNA polymerase has 5' - 3' activity . All known DNA replication systems require a free 3' hydroxyl group before synthesis can be initiated (note: the DNA template is read in 3' to 5' direction whereas a new strand is synthesized in the 5' to 3' direction--this is often confused). Four distinct mechanisms for DNA synthesis are recognized: </P>

When does dna replication take place in somatic cells