Intra- and intermolecular proofreading

a | Intramolecular proofreading is carried out by the same DNA polymerase polypeptide that created the mistake.  This is done within the same binding event, so the same polymerase (green circle) changes from a ‘polymerization’ mode into an ‘exonuclease’ mode, without dissociating from the DNA.  
b | Intermolecular proofreading is carried out by a different DNA polymerase molecule (3) to the one that made the mistake (2).  This can be an identical molecule when there is frequent dissociation and association.
c | Intermolecular proofreading can also be carried out by another molecule, such as an autonomous 3'–5' proofreading exonuclease (3). This exonuclease would bind the DNA, excise the mistake, then the original (2) (or another (4)) DNA polymerase molecule could continue DNA synthesis.