DNA fibre autoradiography has been used to study the replication of chromosomal DNA from chicken cells in tissue culture at 37 °C. The S-phase is estimated to be about 7.5 h. Replication proceeds in the manner first described by Huberman & Riggs. Tandemly arranged units replicate bi-directionally from fork-like growing points, the times of initiation of individual replicating sections being staggered. Evidence against the existence of defined termini is presented. The rate of replication in these chicken cells is some 25-30 µm/h one-way. The mean interval between adjacent initiation points for replication is 63 µm, with a range of some 25-145 µm. Although initiation intervals vary in length, neighbouring intervals tend to be similar in length.

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