Single- and double-labeling methods to assess nonrandom template DNA strand segregation. (a and b) Two pairs of chromosomes are represented for clarity. Newly synthesized DNA strands are shown in dashed, colored lines. After one round of cell division, all cells are labeled with BrdU for the single-labeling method (a) or CldU for the double-labeling method (b). It is only after the second division that NRTS can be observed. If cells randomly distribute their template DNA, the two resulting daughter cells will inherit BrdU (a)- or CldU (b)-labeled strands. However, if all template DNA chromosomes are segregated to one daughter cell, the other daughter cell inherits the BrdU (a)- or CldU (b)-labeled newly synthesized DNA strands (after the second round of division, all daughter cells are labeled with IdU in the double-labeling method).