RARE MUTATIONS IN LOWER ANIMALS. “Mutation accumulation experiments with bacteria and single-celled eukaryotes typically find that the rate of single base mutations is of the order of 10(-10) – 10(-9) per base pair per replication. Given that the typical genome sizes in these organisms are of the order of 10(6) – 10(7) base pairs, these rates correspond to only one point mutation in every few hundred to several thousand cell divisions.”
https://www.nature.com/articles/nrg3564