Cell Cycle Control 
known cells are exquisitely interdependent enti- 
ties. For example, the replication of DNA is de- 
pendent on proteins, but the proteins are en- 
coded by DNA, and so on. It is clear, however. 
that the primitive cyclin kinase has evolved in 
present-day cells into a very complicated family 
of enzymes that regulate all stages of the cell divi- 
sion cycle. 
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