( ( Of ( ( the ( four phases ) ) ( depicted ( in ( the Figure ) ) ) ) ) ( , ( ( the ( two ( critical steps ) ) ) ( ( are ( ( ( ( ( ( DNA replication ) , ) ( which ( occurs ( during S-phase ) ) ) ) , ) and ) ( ( ( the ( physical process ) ) ( of ( cell division ) ) ) ( which ( ( occurs ( during M-phase ) ) ( -LRB- ( ( for mitosis ) -RRB- ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN Of) (NP (NP (DT the) (CD four) (NNS phases)) (VP (VBN depicted) (PP (IN in) (NP (DT the) (NN Figure)))))) (, ,) (NP (DT the) (CD two) (JJ critical) (NNS steps)) (VP (VBP are) (NP (NP (NP (NN DNA) (NN replication)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ occurs) (PP (IN during) (NP (NNP S-phase))))))) (, ,) (CC and) (NP (NP (DT the) (JJ physical) (NN process)) (PP (IN of) (NP (NN cell) (NN division))) (SBAR (WHNP (WDT which)) (S (VP (VBZ occurs) (PP (IN during) (NP (NNP M-phase))) (PRN (-LRB- -LRB-) (PP (IN for) (NP (NNS mitosis))) (-RRB- -RRB-)))))))) (. .)))
Of the four phases depicted in the Figure, the two critical steps are DNA  replication, which occurs during S-phase, and the physical process of cell  division which occurs during M-phase (for mitosis).
(ROOT (S (NP (NNS Mitosis)) (ADVP (RB actually)) (VP (VBZ occurs) (PP (IN in) (NP (CD four) (NNS phases)))) (. .)))
Mitosis actually occurs in four phases.