Mutations are permanent changes in an organism's DNA or chromosomes. Mutations can occur during DNA replication because mistakes are made when nucleotides are added to a DNA strand. Mutations can also occur because chromosomes break or pieces of chromosomes are swapped with non-homologous chromosomes. Mutations can also occur because mutagens like chemicals and radiation cause chemical changes in the nucleotides of the DNA. Mutations can be harmful and cause disease and disorders; however, not all mutations have negative effects. Mutations create genetic diversity in a population, and are sometimes beneficial.