Mapping replication origins by nascent DNA strand length.

The mapping of replication origins by nascent DNA strand length determination is a very sensitive generally applicable method that identifies even single-copy origins in mammalian chromosomes. A major advantage of this procedure is that there is no need for synchronization of cells or treatment with metabolic agents, which allows the origin to be studied under physiological conditions. This technique is based upon the amplification of specific sequence markers on nascent DNA strands that initiated replication within the region of the putative origin. Therefore, this method requires detailed sequence information of the locus to be analyzed. As a first step, nascent DNA of proliferating cells is pulse-labeled with BrdU followed by size fractionation and purification with anti-BrdU antibodies. The position of putative origins can then be determined via identification of the shortest nascent strands that can be amplified by PCR and hybridized to probes homologous to the amplified segments. Here, we give a detailed description of the theory behind the method and a full recipe for its application. Advantages and limitations of the procedure are discussed.