FIGURE 13–11 Point Mutations These diagrams show how changes in a single nucleotide can affect the amino acid sequence of proteins. Analyze Data Which type of mutations affects only a single amino acid in a protein? Which can affect more than one?

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Gene Mutations Gene mutations that involve changes in one or a few nucleotides are known as point mutations because they occur at a single point in the DNA sequence. Point mutations include substitutions, insertions, and deletions. They generally occur during replication. If a gene in one cell is altered, the alteration can be passed on to every cell that develops from the original one. Refer to Figure 13–11 as you read about the different forms of point mutations.

▸ Substitutions In a substitution, one base is changed to a different base. Substitutions usually affect no more than a single amino acid, and sometimes they have no effect at all. For example, if a mutation changed one codon of mRNA from CCC to CCA, the codon would still specify the amino acid proline. But a change in the first base of the codon—changing CCC to ACC—would replace proline with the amino acid threonine.

▸ Insertions and Deletions Insertions and deletions are point mutations in which one base is inserted or removed from the DNA sequence. The effects of these changes can be dramatic. Remember that the genetic code is read three bases at a time. If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings shift in every codon that follows the mutation.

Insertions and deletions are also called frameshift mutations because they shift the “reading frame” of the genetic message. By shifting the reading frame, frameshift mutations can change every amino acid that follows the point of the mutation. They can alter a protein so much that it is unable to perform its normal functions.

In Your Notebook Use a cause/effect diagram to describe the different types of gene mutations.

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