Eukaryotic origins of replication. The image shows a single molecule of DNA containing two origins of replication. The origins were identified by labeling new replicated DNA in human cells first with one thymine analog (iodo-deoxyuridine, I-dU) and then another (chloro-deoxyuridine, Cl-dU). DNA molecules from these cells were then extended on a microscope slide and labeled with antibodies to I-dU (green) and Cl-dU (red) to visualize the DNA. This method allows the detection of replication origins as well as determination of the rate of DNA synthesis.