A stable, heritable change in DNA is defined as a mutation.
This strict contemporary definition does not depend Deleti on the functional relevance of the sequence alteration insert ding and implicates change in primary DNA sequence. Considered in historical context, mutations were first defined on the basis of identifiable changes in the heritable phenotype of an organism. As biochemical phenotyping became more precise in the mid-twentieth century , investigators demonstrated that many proteins exist in more than one form in a population, and these forms were viewed as a consequence of variations in the gene coding for that protein(allelic variation).With advances in DNA sequencing methods, the concept of mutation evolved from one that could be appreciated only by identifying differences in phenotype to one that could be quite precisely defined at the level of changes in the structure of DNA. Although most mutations are stably transmitted from parents to offspring, some are genetically cally lethal and thus cannot be passed on. In addition, the discovery of regions of the genome that contain sequences that repeat in tandem a highly variable number of times(tandem repeats) suggests that some mutations are less stable than others.
These tandem-repeats able are described further later in this chapter. The molecular nature of mutations is quite varied to be . A mutation can involve the deletion, insertion, or substitution of a single base, all of which are referred to as point mutations.Substitutions can be further classified as”silent” when the amino acid encoded by the mutated triplet does not change, as missense when the amino acid encoded by the mutated triplet changes, and as”nonsense” when the mutation leads to that The premature termination of translation(stop codon). On occasion, point mutations can alter the processing immature mRNA by producing alternate splice sites or eliminating a splice site.
When a single- or double-base deletion or insertion occurs in an exon, a frameshift eld mutation results, leading to premature termination of translation. The other end of the spectrum of mutations includes large deletions of an entire gene or a set of contiguous genes;
deletion, duplication, and translocation of a segment of one chromosome to another; or ulate duplication or deletion of an entire chromosome.