Cyclins are a group of proteins that control the progression of the cell cycle by activating particular enzymes—cyclin dependent kinases. Those kinases act on different protein structures that play large roles in the cell cycle. For example, one kinase in humans activates the proteins that carry out DNA replication. Without the cyclin that activates the kinase, DNA replication could not occur and cells would be unable to proliferate.
There are many types of cyclins, all of which activate different kinases that play different roles in the cell cycle. Organisms vary in their numbers and types of cyclins, which are distinguished by their protein structures.. Proteins are made of chains of amino acids that fold in specific ways to fit with other molecules – like a lock and key. The structure of a cyclin is exactly matched to the structure of the kinase it activates, which ensures that the right kinase is activated at the correct time during the cell cycle and that the cell cycle progresses in the correct order.
Proteins have four levels of organization – primary, secondary, tertiary, and quaternary. The primary structure of a protein is its amino acid sequence. The amino acid sequence of different cyclins varies; however, there is a conserved 100 amino acid chain known as the ‘cyclin box’,in all cyclins (Branden, 1999: 108). The secondary, tertiary, and quaternary structure of proteins refer to different levels of protein folding, which create the three dimensional structure of the protein.