Figure 1. Chromosome fates and labelling in the stem-cell lineages depending on the mode of chromosome segregation (CS).

Solid lines represent parental template DNA strands and dotted lines represent newly synthesized strands. (a) Nonrandom chromosome segregation (CS) implies an asymmetric mode of cell division generating stem and non-stem daughter cells. The 'immortal' DNA template (*) inherited by a stem cell at generation g−1, is again inherited by the stem progeny at generations g and g+1. (b) In the case of nonrandom CS, and no matter how long the labelling period (1 to n generations), only one strand of each chromosome can be labelled (red) in stem cells as they systematically inherit the chromatid containing the initially unlabelled 'immortal' strand (blue*). (c) In the case of nonrandom CS, stem cells replicating their DNA in the absence of label are unlabelled from the first generation. (d) On the contrary, because random CS implies that both templates of generation g−1 can be transmitted to generation g+1 with equal probability, the proportion of labelled chromosomes steadily increases with each generation of stem cells produced in the presence of label (e), and steadily decreases with each generation of stem cells produced during the chase period (f).