A nuclease-derived fragment of metaphase DNA and its relationship to the replicon.

When isolated Chinese hamster cells (CHO) metaphase chromosomes are treated with nuclease Bal-31, the DNA is reduced to a size class that is resistant to further degradation. This size class resembles the distribution of replicon sizes in this particular cell line in both average size and size range. Tests based on molecular weight (MW) analysis were devised to locate the origin of replication within the Bal-31 segments. The evidence indicates that replication origins are positioned at or near the center of these segments. The tests were made possible by the additional discovery that BrdU-substituted DNA is highly susceptible to Bal-31 nuclease attack while still contained in the isolated metaphase chromosome.