Every time a cell divides, it must accurately copy all the DNA it contains, then divide the duplicated chromosomes equally into daughter cells in mitosis or into four gametic cells in meiosis. The process of copying DNA then dividing the duplicated chromosomes, while very accurate, is not perfect. Errors in the number of chromosomes can lead to major problems in the fetus. Errors that result in multiple sets of chromosomes are also fatal in mammals. However, multiple sets of chromosomes are not lethal in plants and in other animals, such as fish. In fact, multiple sets of chromosomes are common among plants and have been used by plant breeders to produce larger fruits. Errors in copying DNA can result in nucleotide substitutions. These substitutions can have results ranging from no change to very slight changes in the amino acids of a protein to major deleterious changes, or even to an advantageous change. Errors in copying DNA that result in insertion or deletion of nucleotides result in major changes to amino acid sequences because insertions and deletions totally change the amino acids from that point on. Finally, some diseases are due to triplet repeat expansion.

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