A complete copy of the genome must be made every time when a cell divides. DNA replication has to be extremely accurate in order to avoid the introduction of mutations into the genome copies. Some mutations do, however, occur either as errors in replication. It may be due to effects of chemical and physical mutagenes that directly alter the chemical structure of DNA. DNA repair enzymes. These enzymes correct many of these errors. Those that escape the repair processes become permanent feature of the lineage descending from the original mutated genome. These events, along with genome rearrangement resulting from recombination, underlie molecular evolution, the driving force behind the evolution of living organisms.