DNA polymerase only synthesises DNA 5′→3′: this
allows mismatched pairs to be chewed back. A 3′→5′
polymerase could not do this as easily, because the chewed end would
lack the required triphosphate group. Priming is the most likely time
for mismatch, so DNA polymerase refuses to bind ssDNA. RNA primers are
readily erasable, unlike DNA ones. DNApol is both polymerase and
exonuclease, allowing lesions to be corrected microseconds after they
are formed.
Since DNA is synthesised only in the 5′ to 3′
direction, one of the strands at a replication fork is necessarily
replicated in a discontinuous fashion, forming short Okazaki fragments,
which must be corrected to remove RNA and link the fragments
together.