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Animation: Mutation Types

by Pearson
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As living organisms reproduce and grow, they are constantly replicating DNA with a high degree of precision. However, inevitably, about once in every billion nucleotides that are replicated, an error occurs in the replication process, changing the sequence of nucleotides. Chemicals called mutagens can increase the chance of such mistakes. These errors are called mutations. There are two main classes of mutations: base-substitution mutations and frameshift mutations. Base-substitution mutations occur when a single nucleotide replaces another in the DNA sequence. Base-substitution mutations can be further divided into silent mutations, missense mutations, and nonsense mutations. If the base substitution produces no change in the amino acid sequence of the resulting protein, it is a silent mutation. Due to the overlapping nature of the genetic code, about one third of all base substitutions lead to silent mutations. Since there is no change in the amino acid sequence of the protein, silent mutations affect the genotype of the organism but not the phenotype. If a base-substitution mutation cases a single amino acid to change in the protein, the mutation is a missense mutation. Depending on the nature of the amino acid substitution, the missense mutation can be harmful, neutral, or even beneficial in rare cases. If the base substitution causes a codon to change from one coding for an amino acid to a stop codon, it is a nonsense mutation. In general, nonsense mutations are harmful since they lead to the premature termination of the protein. If nucleotides are inserted into or removed from the DNA sequence, the resulting mutation is known as a frameshift mutation. Because the ribosome translates messenger RNA into protein by reading the mRNA in three-nucleotide codons, if one or two nucleotides are removed or inserted, the result is a shift in the reading frame of the codons. This leads to a change in nearly every codon and nearly every amino acid that follows the site of the frameshift mutation. Frameshift mutations usually lead to a nonfunctional protein. If one or more bases are added to a DNA sequence causing a shift in the reading frame of the resulting codons, the mutation is a frameshift insertion.
As living organisms reproduce and grow, they are constantly replicating DNA with a high degree of precision. However, inevitably, about once in every billion nucleotides that are replicated, an error occurs in the replication process, changing the sequence of nucleotides. Chemicals called mutagens can increase the chance of such mistakes. These errors are called mutations. There are two main classes of mutations: base-substitution mutations and frameshift mutations. Base-substitution mutations occur when a single nucleotide replaces another in the DNA sequence. Base-substitution mutations can be further divided into silent mutations, missense mutations, and nonsense mutations. If the base substitution produces no change in the amino acid sequence of the resulting protein, it is a silent mutation. Due to the overlapping nature of the genetic code, about one third of all base substitutions lead to silent mutations. Since there is no change in the amino acid sequence of the protein, silent mutations affect the genotype of the organism but not the phenotype. If a base-substitution mutation cases a single amino acid to change in the protein, the mutation is a missense mutation. Depending on the nature of the amino acid substitution, the missense mutation can be harmful, neutral, or even beneficial in rare cases. If the base substitution causes a codon to change from one coding for an amino acid to a stop codon, it is a nonsense mutation. In general, nonsense mutations are harmful since they lead to the premature termination of the protein. If nucleotides are inserted into or removed from the DNA sequence, the resulting mutation is known as a frameshift mutation. Because the ribosome translates messenger RNA into protein by reading the mRNA in three-nucleotide codons, if one or two nucleotides are removed or inserted, the result is a shift in the reading frame of the codons. This leads to a change in nearly every codon and nearly every amino acid that follows the site of the frameshift mutation. Frameshift mutations usually lead to a nonfunctional protein. If one or more bases are added to a DNA sequence causing a shift in the reading frame of the resulting codons, the mutation is a frameshift insertion.