In 1953—the year that Watson and Crick published those findings “of considerable biological interest”—Howard and Pelc (1953) showed that DNA synthesis occurred in a discrete phase of the cell cycle. They had labeled DNA with radioactive phosphate. But better spatial localization of replication required a lower energy and thus more highly localized radioactive probe. This was tritiated thymidine, which Taylor et al. (1957) used to show that DNA replication was restricted to one sister chromatid and thus semi-conservative. The biochemical proof of the same principle came only in the following year, from Meselson and Stahl (1958).
Into this flurry of activity came Lima-de-Faria (1959), who showed that heterochromatin replicated later than euchromatin. Heterochromatin was first identified as a darkly staining, condensed material whose significance was unclear. Lima-de-Faria injected tritium-labeled thymidine into grasshopper abdomens and then looked at developing spermatocytes. As the spermatocytes developed in a...
