In biology, genetic modification is a modification of the nucleotide sequence of the genome of an organ, a virus, or extrachromosomal DNA. Viral genomes can be either DNA or RNA. Conversion occurs as a result of defects during DNA replication, mitosis, and meiosis or other types of DNA damage (such as pyrimidine dimers that may be caused by exposure to radiation or carcinogens), in which case there may be defects in repair (especially microhomology-Mediated end joining) either caused an error during other repair steps or may cause an error during replication (translesion synthesis). Mutations can also be caused by the insertion or deletion of DNA fragments due to cellular genes.

Mutations may or may not produce significant changes in the physical phenotype. Mutations play a role in the sequence of normal and abnormal environmental processes including: evolution, cancer, and the development of the immune system, including differentiation. Mutations are the ultimate source of genetic diversity, providing immature materials through which the ability to emerge such as natural selection can.

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