Answer: During DNA Replication
Step-by-step explanation:
DNA Polymerase is the enzyme that uses a single stranded DNA template strand to produce a piece of dsDNA. This is used to replicate the DNA according to the semi-conservative model of replication.
During replication DNA polymerase makes approximately one mistake per 10^5 nucleotides. Thankfully the DNA polymerase has proof reading capabilities and has the ability to excise the mistake using it's 3' - 5' exonuclease domain.