(Related to Fig. 2 ): Examples of DNA fiber images collected with the DNA combing, spreading, and hybrid spreading techniques. (A) Images collected with DNA combing. The numbers 1, 2, and 3 denote the examples of red–green–red, red–green, and green-only fibers shown in Fig. 2 A. (B) Images collected with DNA spreading. The numbers 1, 2, and 3 denote the examples of red–green–red, red–green, and green-only fibers shown in Fig. 2 E. (C) Images collected with the hybrid spreading technique. The numbers 1, 2, and 3 denote the examples of red–green–red, red–green, and green-only fibers shown in Fig. 2 G. In all cases, when we score red–green–red or red–green fibers, the red signal is continuous and extends through the green tracts, confirming that our labeling originates from two separate replication cycles. The reason why we do not see yellow tracts when the CldU and IdU overlap is that the IdU signal (green) is stronger than the CldU signal (red) due to the different efficiencies of the two antibodies (see also Materials and methods).