Induction of chromosome replication during maturation of amphibian oocytes.

After fertilization amphibian embryos replicate their chromosomes faster than bacteria replicate their much smaller chromosomes. During oogenesis, materials are accumulated to sustain these rapid cycles of chromosome replication. Thus growth is uncoupled from nuclear division. Most of the machinery for DNA replication and chromatin assembly is present in the oocyte, which lacks only the ability to initiate on a DNA duplex. After maturation and activation a cell cycle clock is triggered which specifies initiation of DNA replication on endogenous chromosomes, injected nuclei or injected plasmid DNA. The ability to reinitiate replication of a replicated molecule is tightly coupled to the cell cycle clock. Each egg can replicate an amount of DNA equivalent to 500 diploid nuclei in only five hours. However, each egg can assemble an amount of purified DNA equivalent to 12 000 diploid nuclei into regularly spaced nucleosomes in only one hour. The molecular basis of these extraordinary rates of DNA replication and chromatin assembly is considered.