The regulation of DNA replication at a subchromosomal level in mammalian cells has been investigated. DNA fiber autoradiographs were prepared from mouse L-929 cells pulse labeled with (3H)thymidine. Initiation events and subsequent chain growth occurring over short stretches (up to three replication units in length) of chromosomal DNA were analyzed. The results show that adjacent units usually initiate replication synchronously and that this synchrony is related to the proximity of initiation sites. In addition, adjacent units are of similar size and the rates of replication fork progression within units and on adjacent units are similar. The rate of fork progression increases with increasing replication unit size. Finally, no evidence for fixed termination sites for the units has been found. These observations suggest that despite large variations in size of replication units, timing of initiation events, and rates of fork progression found in chromosomal DNA as a whole, these processes are closely regulated within subchromosomal clusters of active replication units.

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