Actual coverage will always differ somewhat from what is printed here. Collections of strains are kept. Can be repaired, but if not will cause inconsistent base insertions during replication. Such mutants are called revertants .
Most mutations can revert with some frequency, even if slight.
Certain chemicals called intercalating agents can slip into DNA double helix between base pairs, induce mutations that result in extra bases being added. Resulting genetic code now has extra base, will be frame shifted at some point. Acridine dyes are good intercalating agents, cause frameshift mutations. Deletion mutations Can result from ionizing radiation, other treatments that cause double-stranded breaks. Deletion mutants cannot revert.