Final answer:
The enzyme responsible for proofreading a growing DNA strand and for replacing mismatched nucleotides is DNA polymerase.
Step-by-step explanation:
During DNA replication, this enzyme performs a crucial proofreading function. It reads each newly added base to ensure it has paired correctly with the base in the template strand. If a mismatch is detected, DNA polymerase's 3' exonuclease activity allows it to remove the incorrect nucleotide. Then, the correct nucleotide can be added, minimizing errors in the replicated DNA strand.
There are several repair mechanisms to catch errors that might slip past this initial proofreading. One such mechanism is mismatch repair, which occurs after replication. Specific enzymes recognize a mispaired nucleotide and excise it; then, DNA polymerase comes back to fill the gap with the correct base.