Abstract
A highly sensitive procedure was developed for the identification of the origin of bidirectional DNA synthesis in single-copy replicons of mammalian cells. The method, which does not require cell synchronization or permeabilization, entails the absolute quantification, by a competitive PCR procedure in newly synthesized DNA samples, of the abundance of neighboring DNA fragments distributed along a given genomic region. This procedure was utilized for mapping the start site of DNA replication in a 13.7-kb region of human chromosome 19 coding for lamin B2, which is replicated immediately after the onset of S phase in HL-60 cells. Within this region, DNA replication initiates in a 474-bp area corresponding to the 3' noncoding end of the lamin B2 gene and the nontranscribed spacer between this gene and the 5' end of another highly transcribed one. This localization was obtained both in aphidicolin-synchronized and in exponentially growing HL-60 cells.