A turnstile for initiation of DNA replication.

The progress of a cell through its growth cycle is a multifaceted process; so far we have seen only a glimpse of the complex interplay between the macromolecules performing and regulating the different steps involved. In most organisms, control mechanisms ensure that all chromosomal DNA sequences are replicated once, and only once, between two cell divisions. This enables each division to produce two daughter cells with a genetic content identical to that of their mother. Although the biochemical synthetic processes involved in replicating DNA have been described in detail, our knowledge of the regulatory mechanisms of DNA replication remains scant. In recent experiments with Escherichia coli, new light has been shed on these elusive control mechanisms, and evidence has emerged that may signal an end to our ignorance about this important biological problem.