Why do nucleotides only get added to the 3' end during DNA synthesis

just a basic question :) thanks so much!-Because DNA polymerase can only use the 3 OH as a substrate.That is the part of the nucleotide that fits in its active site.All life evolved from an ancestor that produced this specific enzyme.The 3 OH attaches to the first phosphate group on the nucleotide triphosphates breaking the pyrophosphate molecule free.This is a high energy reaction and is irreversible because the pyrophosphate breaks down into inorganic phosphates.......