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13 Changes in DNA Replication during Animal Development

Janet L. Carminati, Terry L. Orr-Weaver


Throughout animal growth and development, the cell cycle is modified in response to developmental signals. These alterations in the cell cycle influence the control of S phase and the properties of DNA replication (outlined in Table 1). The production of haploid gametes requires a modified cell cycle, meiosis, in which two rounds of chromosome segregation follow a single S phase. Thus, S phase must be prevented between the two meiotic divisions. In the early embryos of many animals, a rapid cell cycle occurs in which S phase oscillates with mitosis without gap phases. This S-M cycle necessitates unique controls for the entry into S phase. Later in embryogenesis in Drosophila and Xenopus, a G1 phase is added to the cell cycle, resulting in another developmental alteration of the onset of S phase. There are numerous examples of tissues that become polytene as a consequence of a modified cell cycle with only an S phase and a gap phase.

These developmental changes in the cell cycle require special controls for entry into S phase. The regulation of the onset of S phase has been extensively investigated in the normal cell cycle with G1-S-G2-M phases and is reviewed by Nasymth (this volume) and Weisshart and Fanning (this volume). Cyclin-dependent kinases (cyclin-cdk complexes), including cyclins E, D, and A complexed with cdk2, 4, and 6, are all known to play a role in S-phase regulation of higher eukaryotes (Sherr 1993Sherr 1994). G1 cyclin kinases are thought to phosphorylate the retinoblastoma gene product, pRb,...

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