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 clear geographical sequence, replication events occurring at different times could be ordered.

“The evidence,” wrote Lima-de-Faria, “[was] clear. The tritium was incorporated into heterochromatin later than into euchromatin.” Heterochromatin in rye leaves also replicated at a different time, and late replication of heterochromatin was confirmed in detail by Taylor (1960).

The biological importance of this finding “is an important issue,” says Danesh Moazed (Harvard Medical School, Boston, MA). “But over the years nothing has come of it.” The late replication may help set up the heterochromatic state, but equally “it may be a side effect of the DNA being less accessible,” says Moazed.

Miller and Nasmyth (1984) showed that passage through

graphic
S phase was required for heterochromatin to be established in budding yeast. But Kirchmaier and Rine (2001) and Li and Gartenberg (2001), while confirming the need for an S-phase event, found that DNA replication was not required.

Whatever the outcome, Lima-de-Faria's initial result remains unassailable. It also led others to consider DNA replication not as a simple, monolithic process, but as something that was complex and potentially regulated.

Howard, A., and S.R. Pelc. 1953. Heredity. 6:261–273.

Kirchmaier, A.L., and J. Rine. 2001. Science. 291:646–650.

Li, Y.-C., et al. 2001. Science. 291:650–653.

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Miller, A.M., and K.A. Nasmyth. 1984. Nature. 312:247–251.

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