( ( ( Four ( different classes ) ) ( of cyclins ) ) ( ( have ( been ( defined ( on ( ( ( the basis ) ( of ( ( the stage ) ( of ( the ( cell cycle ) ) ) ) ) ) ( ( in which ) ( they ( ( ( bind and ) activate ) CDKs ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (CD Four) (JJ different) (NNS classes)) (PP (IN of) (NP (NNS cyclins)))) (VP (VBP have) (VP (VBN been) (VP (VBN defined) (PP (IN on) (NP (NP (DT the) (NN basis)) (PP (IN of) (NP (NP (DT the) (NN stage)) (PP (IN of) (NP (DT the) (NN cell) (NN cycle))))) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (PRP they)) (VP (VBP bind) (CC and) (VBP activate) (NP (NNS CDKs)))))))))) (. .)))
Four different classes of cyclins have been defined on the basis of the  stage of the cell cycle in which they bind and activate CDKs.
(ROOT (S (NP (NNS Cells)) (VP (VBP have) (NP (CD four) (NNS cycles))) (. .)))