Mutation is an alteration in the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes can be of either DNA or RNA. Mutations result from errors during DNA replication, mitosis and meiosis or other types of damage to DNA which then may undergo error-prone repair or cause an error during other forms of repair or else may cause an error during replication. Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements.