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Molecular Basis of Heredity: Part 3. Genetic Variation

Author(s): Raye L. Alford, PhD

he Molecular Basis of DNA Sequence Variations (I)

Mutations are changes in the DNA sequence of a gene. Because of the redundancy of the genetic code, mutations can, but may not necessarily alter the sequence of the protein encoded by a gene. Several types of changes can occur in a DNA sequence. Some simple examples are shown in the slide. Substitution mutations are the replacement of one nucleotide for another. Deletions are the loss of one or more nucleotides. Insertions are the addition of one or more nucleotides. The inserted nucleotides can come from a variety of sources including foreign DNA from viruses, transposable elements from other parts of the genome, or duplications of nearby or adjacent DNA sequence. Frameshift mutations alter the reading frame of an mRNA in such a way that the amino acid sequence downstream from the mutation is altered. Frameshift mutations are often caused by small insertions or deletions in genes and typically result in the appearance of a stop codon somewhere downstream from the mutation. Rearrangement mutations can be inversions or other complicated alterations of the normal sequence of a gene.