3' to 5' Exonuclease question

[NathanielZhu]

The lectures notes of a molecular biology course recorded from Berkley (one of those free online courses where they record the classes) says that "Polymerization is a race between the 3' to 5' exonuclease and polymerase" (probably polymerase III).

What does that mean?
How can it be a race since the DNA Polymerase is a holoenzyme/molecular machine/replisome which pretty much means that all the subunits are connected? That's like if I were to tie a string onto two matchbox cars - if I push one, they other moves along with it. DNA Polymerase + the 3' to 5' exonuclease is the same way so how is it a race?

[JackBean]

I guess they mean the proof-reading function?

[Duc]

Hi,

This is my first time in this forum.
As I read about DNA Polymerase III in "molecular biology of the cell" the enzyme has both functions of polymerization 5'-3' and exonuclease 3'-5'. They locate in two active site of this enzyme. When a nucleotide is mismatched at polymerase site, the enzyme will stop and the mismatched nucleotide will be removed at exonuclease site. That's also called proof-reading function of DNA Pol III.