The genomes of small-intestinal stem cells appear to be extremely well protected: despite dividing thousands of times, they do not lose their proliferative potential and rarely acquire oncogenic mutations. Why are they so resistant to replication-induced errors? One hypothesis is that, when each stem cell divides, template DNA strands are segregated into the stem cell daughter whereas newly synthesized DNA enters the transit cell daughter -replication-induced errors would thus be passed on to cells destined to differentiate rather than to those that propagate indefinitely. This hypothesis has proven extremely difficult to test, but Chris Potten and co-workers have now performed the definitive experiment by sequentially labelling stem cell DNA with tritiated thymidine (3H) and bromodeoxyuridine (see p. 2381). They demonstrate that 3H-labelled small-intestinal stem cells can be labelled with bromodeoxyuridine but that, whereas the 3H is retained in stem cells, the bromodeoxyuridine is lost after...

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