<Table> <Tr> <Td> Nobel Laureate Paul Nurse </Td> <Td> Nobel Laureate Tim Hunt </Td> </Tr> </Table> <Tr> <Td> Nobel Laureate Paul Nurse </Td> <Td> Nobel Laureate Tim Hunt </Td> </Tr> <P> Two key classes of regulatory molecules, cyclins and cyclin - dependent kinases (CDKs), determine a cell's progress through the cell cycle . Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules . Many of the genes encoding cyclins and CDKs are conserved among all eukaryotes, but in general more complex organisms have more elaborate cell cycle control systems that incorporate more individual components . Many of the relevant genes were first identified by studying yeast, especially Saccharomyces cerevisiae; genetic nomenclature in yeast dubs many of these genes cdc (for "cell division cycle") followed by an identifying number, e.g. cdc25 or cdc20 . </P> <P> Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin . When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle . Different cyclin - CDK combinations determine the downstream proteins targeted . CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals . </P>

What regulates the timing of the cell cycle in eukaryotes