... the6–9 bp-long B-DNA, at the 5¢-phosphate side of the nick,changes to A-DNA after formation of the DNA–ligasecomplex, similarly to the motif at the 3¢-OH primer end ofthe dsDNA bound to DNA ... respect to the enzyme, becauseDNA bends here. At the nick, however, only the5¢-phosphate of the nicked strand is enzyme-bound(Fig. 3, ds nicked DNA). During the contraction, orB -to- A DNA helix ... propose that the B -to- A DNA helix transition serves asa dynamic test to determine the state of the DNA substrate,and is used by DNA ligase to comply with its fidelityrequirements.(A) To test for...