Eukaryotes have multiple cyclins, each of which activates a different cyclin dependent kinase responsible for regulating a different part of the cell cycle. During the cell cycle, the cell produces different cyclins at different time points, while the level of cyclin dependent kinases remain the same throughout the cycle, and are only activated in the presence of a cyclin.
The necessity of changing levels of cyclin requires that cyclins are both manufactured and degraded by the cell during the cell cycle. The degradation of cyclins is carried out through a ubiquitin pathway. Ubiquitin is a small protein found in almost all tissues in eukaryotes. At certain points during the cell cycle, ubiquitin is signaled to bond with a cyclin, and with the addition of several enzymes, the cyclin degrades (Hershko, 1999: 1571). With that degradation of cyclin, the cyclin dependent kinase responsible for that phase of the cell cycle deactivates and the next cyclin dependent kinase is activated by the production of a different cyclin. The process continues until the end of the cell cycle, when all cyclins have been sequentially degraded.