Cyclins are proteins that accumulate throughout the life of a cell. Once there is enough cyclins, it tells the cell to advance to the next stage. A CDK (Cyclin Dependent Kinase), is a chemical that speeds up actions within a cell. They're found in all living organisms. Once the cyclins accumulate to a high amount, they latch and work with the CDK's. CDK's, like the Mitosis Promoting Factor, when they accumulate a lot can actually work on the cell (break up the nucleus, work on the microtubules). Together, the CDK's push the cell into the next phase in the cell cycle.