[Biophysics] DNA replication errors

[hansfcox at hotmail.com]

I have had a question in my mind for years now, having to do with DNA
replication and the Pauli-Exclusion Principle of quantum physics.
During DNA replication, how are bonding electrons' spins paired in the
new bonds?  They must have oppositely aligned spins to occupy the same
bonding orbital, but how does that come about if they had parallel
spins before, as I assume could happen sometimes?  I have heard of
"spin-flip" transitions in the context of astronomy, but no nothing
about them.

What causes DNA replication errors?  Is it the failure of
electron-spins to become oppositely aligned, foiling the correct bonds'
formation?  If this is a question with an obvious answer, please accept
my apologies; but I want to know.