( ( Chromatids ( of ( homologous chromosomes ) ) ) ( ( may ( ( exchange parts ) ( in ( ( a process ) ( called ( crossing over ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NNS Chromatids)) (PP (IN of) (NP (JJ homologous) (NNS chromosomes)))) (VP (MD may) (VP (VB exchange) (NP (NNS parts)) (PP (IN in) (NP (NP (DT a) (NN process)) (VP (VBN called) (S (VP (VBG crossing) (PRT (RP over))))))))) (. .)))
Chromatids of homologous chromosomes may exchange parts in a process called crossing over.
(ROOT (S (NP (NP (NNP Exchange)) (PP (IN of) (NP (NN dna) (NNS segments)))) (VP (VBZ happens) (PP (IN between) (NP (NP (JJ non-sister) (NNS chromatids)) (PP (IN of) (NP (NP (RB homologous) (NNS chromosomes)) (PP (IN during) (NP (NN crossing-over)))))))) (. .)))