What is one reason that there are very few errors in DNA replication?

Answer: The reason is because DNA polymerase which is the enzyme that catalyzes the synthesis of DNA has a 3'->5' exonuclease activity that double-checks each nucleotide after it is added.

Explanation: The 3'->5' exonuclease activity of DNA polymerase allows the enzyme to double-check and remove a newly added wrong nucleotide. It is highly specific for mismatched base pairs. When the DNA polymerase adds a wrong nucleotide to the growing strand, translocation of the enzyme to the position where the next nucleotide is to be added is halted, but the exonuclease activity of the DNA polymerase removes the incorrectly paired nucleotide so that replication can continue. The process of double-checking a newly added nucleotide and removal of any incorrectly paired nucleotide by the DNA polymerase is called proofreading.