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Changes in genetic material are called mutations. They can be neutral in effect, disease producing, or rarely protective. Polymorphisms are variations in the DNA sequence that occur at a frequency of more than 1%. These mutations are less frequent in coding regions of the gene and are much more commonly found in introns or between genes. Different types of DNA variations include restriction fragment-length polymorphisms (RFLPs) (5), single nucleotide polymorphisms (SNPs) (6), variable number of tandem repeat polymorphisms (7), and microsatellite markers. SNPs are the most common of these, occurring as often as once in every 100 to 300 base pairs. Typically, polymorphisms do not produce disease, although in some cases, a polymorphism can increase a person’s disease susceptibility. Polymorphisms are helpful for following the inheritance of a nearby gene through families, as well as for studying the genetic susceptibility to certain complex diseases (8).

Many different types of disease-producing mutations have been identified (9). A change in a single base pair is referred to as a point mutation (Mevacor). This can be either a missense mutation, in which the base change results in a codon that codes for a different amino acid, or a nonsense mutation, in which the base pair change results in substitution of a stop codon, coding for premature termination of protein translation (but usually causing RNA degradation: Mevacor). Mutations can also alter the initiation codon, and then the initiation of translation does not occur where it should. If a nucleotide changes at or near the intron–exon junction, the splicing mechanism that removes the introns from the coding sequence can be altered. Sometimes, a point mutation occurs such that the resulting codon codes for the same amino acid; this is called a silent mutation (Mevacor). Mutations that change the length of the gene are called insertions and deletions. If the number of nucleotides added or deleted is not a multiple of three, a frameshift mutation occurs. This means that the reading frame for the DNA sequence following the mutation is incorrect, and therefore all the subsequent codons are altered : Mevacor:. This generally results in a premature stop codon.