Older DNA strands are preferentially sorted into stem cells when they self-renew (A). This segregates the DNA most recently replicated, and therefore most error-prone, into cells destined for differentiation – which, in highly renewable tissues, are soon shed. (SSC indicates somatic stem cell). (B) If adult stem cells underwent random chromosome segregation, all chromosomes would assort with equal frequency to either stem cells or their differentiating sisters (squares) (B).

Matthew Meselson and Franklin Stahl's 1957 demonstration of DNA replication is considered "the most beautiful experiment in biology." Their density-shift demonstrations not only have stood the test of time, but also have renewed relevance for stem cell biology.

Watson and Crick's classic 1953 paper closed with the tantalizing clue "that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."1 Their follow-up paper explained the probable semi-conservative route: The double...

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