( ( ( crossing over ) ( ( A process ) ( ( in which ) ( ( homologous chromosomes ) ( ( exchange material ) ( through ( ( the ( ( breakage and ) reunion ) ) ( of ( two chromatids ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VBG crossing) (PRT (RP over)) (NP (NP (DT A) (NN process)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (RB homologous) (NNS chromosomes)) (VP (VBP exchange) (NP (NN material)) (PP (IN through) (NP (NP (DT the) (NN breakage) (CC and) (NN reunion)) (PP (IN of) (NP (CD two) (NNS chromatids)))))))))) (. .)))
crossing over A process in which homologous chromosomes exchange material through the breakage and reunion of two chromatids.
(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)))))))) (. .)))