Nature of Mutations
Mutations can originate in a number of ways. In one common mechanism during DNA replication, a base may pair incorrectly with the newly forming strand, or extra bases may be added. Or, sections of DNA strands may be deleted, moved to other chromosomes. In any case, the consequences are similar-genetic information is changed. If a protein is constructed from this information, its molecular structure may be faulty and the function changed or absent. For example, the muscle weakness of Duchenne muscular dystrophy may result from a mutation in the gene encoding the protein dystrophin . The mutation may be a missing or changed nucleotide base or absence of the entire dystrophin gene. In each case, lack of dystrophin, which normally supports muscle cell membranes during contraction, causes the cells to collapse. The muscles weaken and atrophy.
Fortunately, cells detect damage in their DNA molecules and use repair enzymes to clip out defective nucleotides in a single DNA strand and fill the resulting gap with nucleotides complementary to those on the remaining strand of DNA. This restores the original structure of the double-stranded DNA molecule.