Why GC’s are Rare
DNA bases spontaneously lose amine groups.
Deaminated C turns to U, which is read as T
Repair mechanisms turn U’s back to C’s
C’s that precede G’s are often methylated to control gene expression and to mark new strand.
Methylated C’s turn straight to T’s when deaminated, hence cannot be repaired.
Net result: CG -> TG at a high rate.