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During the process of DNA replication, errors occasionally occur in the polymerization of the second strand (these error rates are generally extremely low, 1 error in every 10-100 million bases).These errors, called mutations, can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Processes which increase the rate of changes in DNA are called “mutagenic”: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure. Chemical damage to DNA occurs naturally as well, and cells use DNA repair mechanisms to repair mismatches and breaks in DNA — nevertheless, the repair sometimes fails to return the DNA to its original sequence.
In organisms which use chromosomal crossover to exchange DNA and shuffle genes, errors in alignment during meiosis can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment, which makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence — duplications, inversions or deletions of entire regions, or the accidental exchanging of whole parts between different chromosomes (called “translocation”).
Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism
