Chapter 11, Problem 27ESP

DNA polymerases in all organisms add only 5′ nucleotides to the 3′ end of a growing DNA strand, never to the 5′ end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3′ to 5′ direction.

1. (a) Sketch the reaction that DNA polymerase would have to catalyze if DNA synthesis occurred in the 3′ to 5′ direction.
2. (b) Consider the information in your sketch and speculate as to why proofreading would be problematic.