The cyclins involved in the cell cycle are actually quite diverse in higher mammals. When Rb is phosphorylated it releases a transcription factor that regulates expression of genes encoding proteins critical for DNA replication. It is thought that cyclins focus direct the activity of the kinases on the proper substrates. They disappear rapidly after their job is done. Yeast that could grow normally at a permissive temperature were found to arrest at a higher temperature.
Oncogenes are genes encoding proteins that can induce such dysregulation.
Cells must be signaled by the environment to enter cell cycle. The most prominent mitotic event in a mammalian life occurs after fertilization. This prevents other sperm from successfully penetrating the cell. Next, the oocyte induces the release of vesicles just below the membrane of the oocyte. This release results in a change in the outer coating of the cell - and prevents other sperm from penetrating.