Cyclins are the molecules that allow for the progression of the cell cycle. The cell cycle is the process by which cells replicate their DNA and divide. Eukaryotes have two different kinds of cells, somatic cells and sex cells. Somatic cells make up most of the cells of the body. There are four phases of the cell cycle of somatic cells: Gap 1 (G1), where cells increase in size; Synthesis (S), where the cell’s DNA is replicated; Gap 2 (G2), where the cell continues to grow; and Mitosis (M), where the cell divides into two identical cells with the same genetic information. Cells can also enter a fifth phase of the cell cycle known as the resting phase (G0), which occurs during Gap 1 phase. The resting phase can play a variety of roles in an organism and is particularly important in the study of sea urchin (Arbacia punctulata) eggs that Tim Hunt used for his research with cyclins.
In order to ensure that the cell cycle in somatic cells produces the correct products, the cell cycle has several checkpoints. At those checkpoints, different proteins carry out a number of biochemical reactions to regulate the process. This is where cyclin comes in. Many of the proteins that regulate the cell cycle are cyclin dependent kinases: enzymes activated by the presence of a cyclin.