In vitro HeLa cell DNA synthesis similarity to in vivo replication

An in vitro DNA synthesizing system, consisting of a HeLa cell lysate which incorporated dNTPs into an acid-insoluble, DNase-labile product, was optimized for incorporation per nucleus. Synthesis depended on the presence of all four dNTPs and was linear for about 15 minutes, then slowed and finally stopped after one to two hours at 37 °C. The DNA synthesized in vitro was found to be preferentially attached by covalent linkage to sites which had just been replicated in vivo. DNA fiber autoradiography of DNA labeled in vitro suggests that synthesis occurs by the replicon mechanism proposed for in vivo replication, but at a fork movement rate 50 to 60% of that in vivo.When analyzed on alkaline sucrose gradients, dNTPs appeared to be incorporated by a semidiscontinuous mechanism, with label after brief pulses (10 to 20 s) distributed about equally between a peak of Okazaki fragments and a very heterogeneous distribution of longer DNA strands. Okazaki fragments, which can be initiated in vitro, sedimented in a broad peak averaging 180 nucleotides in length.