( ( ( ( ( The nucleotides ) ( are ( ( strung together ) ( in ( pairs ( on ( two ( long chains ) ) ) ) ) ) ) ) and ) ( it ( is ( ( this pairing ) ( that ( forms ( the ( DNA ( double helix ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NNS nucleotides)) (VP (VBP are) (VP (VBG strung) (ADVP (RB together)) (PP (IN in) (NP (NP (NNS pairs)) (PP (IN on) (NP (CD two) (JJ long) (NNS chains)))))))) (CC and) (S (NP (PRP it)) (VP (VBZ is) (NP (NP (DT this) (NN pairing)) (SBAR (WHNP (WDT that)) (S (VP (VBZ forms) (NP (DT the) (NNP DNA) (JJ double) (NN helix)))))))) (. .)))
The nucleotides are strung together in pairs on two long chains and it is this pairing that forms the DNA double helix.
(ROOT (S (NP (NNP Dna)) (VP (VBZ consists) (PP (IN of) (NP (NP (CD two) (JJ long) (NNS chains)) (PP (IN of) (NP (NNS nucleotides)))))) (. .)))