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Mutations, or changes in the DNA sequence, will often lead to changes in the amino acid sequence of the resulting polypeptide. If the structure of the protein changes, then the function will also change.
Mutations can come from several sources, including environmental mutagens and replication errors. In colloquial terms, mutations are not considered good. However, not all changes are harmful. Some may be neutral, while others may be beneficial. It is these beneficial mutations that can lead to evolution of species (see Chapter 20).
Mutations fall into three main categories (see Figure 6.4): 
  1. Base pair substitutions: When a gene is altered in one nucleotide in the sequence, then only one amino acid in the protein may be altered (but remember the degeneracy of the code and the wobble rules: a nucleotide change may not result in a change in the amino acid). This single amino acid difference may result in a fatal disease, alter the function of the protein (thus creating a new variation or trait: see Chapter 19), or it may not affect the function of the protein at all. However, if a codon specifying an amino acid is changed to a termination codon, the polypeptide will not be translated beyond that point. This truncated transcript will, in all likelihood, not function properly.
  2. Frame shift mutations: If one or two nucleotides are either added to or deleted from the DNA sequence, the rest of the transcript will be affected as the codons will be shifted. The resulting polypeptide will have the incorrect amino acid sequence from the point of the mutation, and hence it will have a different function.
  3. Transposable element: Often referred to as “jumping genes,” these pieces of DNA can insert themselves into genes and thereby disrupt the sequence and expression of the gene.

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