The genetic code is replicated during mitosis. If the code is replicated incorrectly, such as a code that should be “AGCT” becomes “ACCT,” by mistake of DNA replication protein machinery, this is called a mutation. There are several types of DNA mutations:
Missense mutation: a base pair substitution that causes the amino acid to change. Any amino acid change could cause the protein identity to change, causing a change or loss in its function.
Nonsense mutation: a base pair substitution that stops protein translation prematurely, causing a protein sequence to be shorter than it should be.
Insertion: a nucleotide or base pair is incorrectly added in as an extra base pair. Could change the amino acid code of the protein.
Deletion: a nucleotide or base pair is incorrectly deleted, or just left out, of a sequence it should be in. Could change the amino acid code of the protein.
Frameshift: one or more base pairs is added or deleted, causing a the reading frame to change. This changes how the codons, changing the amino acid sequence and thus the protein.
Duplication: a gene is copied more times than it should be. This can cause an increased size of the chromosome the gene lies on, as well as a positional shift on the genome of downstream genes.
Repeat expansion: short DNA sequences that exist as repeats (such as ACGACG) are repeated more than they should be (such as ACGACG become ACGACGACGACG). This can lengthen the gene, and cause an additional, amino acid to be abnormally incorporated into the protein, potentially changing the protein function.
Several of these mutation types can cause a change in the protein sequence, and thus the protein’s properties and its functions. These types of mutations can cause the protein to be destroyed by other proteins that sense this kind of mutation. If this occurs, the cell may not have this particular protein and may be unable to perform an essential function, leading to disease. Another potential outcome is that the protein acts abnormally and in a way that negatively affects cell functions, and thus causes disease. There are several mechanisms the cell can use to detect and fix mutations, but when these fail disease can arise.