DNA polymerase III: - Prokaryotes only- Elongates leading strand by adding deoxynucleotides to the 3' end (5' → 3' synthesis).- Elongates lagging strand until it reaches primer of preceding fragment.- 3' → 5' exonuclease activity "proofreads" each added nucleotide.- Drugs blocking DNA replication often have modified 3' OH, thereby preventing addition of the next nucleotide.

DNA polymerase I:- Prokaryotic only- Degrades RNA primer; replaces it with DNA.- Same functions as DNA polymerase III, also excises RNA primer with 5' → 3' exonuclease.