Figure 3.

Detection of very late DNA replication for autosomal loci. (A) Replication of two inter-Alu sequences located on chromosomes 4 and 12 is highly restricted and very late in the cell cycle in lymphoblasts. The high cyclin B1 fraction contains on average 13% of the replication signal for these loci. (B) In phytohemagglutinin-stimulated lymphocytes, replication of the same autosomal inter-Alu sequences described in A is also highly restricted and very late in the cell cycle. In this case, the high cyclin B1 fraction contains 26% of the replication signal for these autosomal sequences. (C) Replication timing for a heterogeneous population of inter-Alu sequences is spread throughout the cell cycle in lymphoblasts, with about 1% occurring within the high cyclin B1 fraction. The percentage of the total inter-Alu replication detected by phosphorimager analysis is shown for each cell-cycle fraction. The same membrane was stripped of the labeled primer and reprobed with a specific late replicating inter-Alu sequence found on the X chromosome (H-8); the results demonstrated the specificity of this approach. An alternative quantification method in which radioactive deoxyribonucleoside triphosphates were incorporated during the PCR gave similar results.