Exposure to mutagens often causes alterations in the molecular structure of nucleotides, ultimately causing substitutions, insertions, and deletions in the DNA sequence.
Mutations can arise in cells of all types as a result of a variety of factors, including chance. In fact, some of the mutations are the result of spontaneous events during replication; hence, spontaneous mutations. Slippage of the DNA template strand and subsequent insertion of an extra nucleotide is one example of a spontaneous mutation; excess flexibility of the DNA strand and the subsequent mispairing of bases is another.
Environmental exposure to certain chemicals, ultraviolet radiation, or other external factors can also cause DNA to change. These external agents of genetic change are called mutagens. Exposure to mutagens often causes alterations in the molecular structure of nucleotides, ultimately causing substitutions, insertions, and deletions in the DNA sequence.
Insertion or deletion of one or more nucleotides during replication can also lead to another type of mutation known as a frameshift mutation. The outcome of a frameshift mutation is complete alteration of the amino acid sequence of a protein. This alteration occurs during translation because ribosomes read the mRNA strand in terms of codons, or groups of three nucleotides. These groups are called the reading frame. Thus, if the number of bases removed from or inserted into a segment of DNA is not a multiple of three (Figure 4a), the reading frame transcribed to the mRNA will be completely changed. Consequently, once it encounters the mutation, the ribosome will read the mRNA sequence differently, which can result in the production of an entirely different sequence of amino acids in the growing polypeptide chain.
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