The mechanism again does not involve, as depicted here, complete rupture of bonds prior to bond formation with the huge energy barrier that would imply. New bonds form as old bonds are broken.

Not only do common intermediates involving e.g.,. formation of a phosphoanhydride linkage at the end of the 3’ DNA strand, appear to be implicated, but so does weak bonding of the cosubstrate to the enzyme (Lehman, I.R. (1974) Science, 186, 790-797).