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Types of Mutations: Substitution, Insertions, Deletion and Frameshifts
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After discussing substitution in detail, let us consider an insertion type of mutation in detail. Insertion mutation is the insertion or addition of nucleotide base pairs into a sequence of DNA, thereby making it longer than the usual length. For example, if a DNA sequence reads CAGC and a T is inserted between G and C during the copying of the sequence, it would then read CAGTC. This is a mutation due to insertion. This mutation could be detrimental if it occurs in a gene, or the region of a DNA sequence coding for a protein, resulting in the formation of a nonfunctional protein.
Certain inherited human disorders occur from insertion of several copies of the same nucleotide triplets. Some of the diseases caused from trinucleotide repeat are Huntington's disease and the fragile X syndrome. These disorders are caused by the genetic inheritance wherein insertions of 3 to 4 nucleotides repeated over and over. In human a locus on X chromosome consists of such stretch of nucleotide in which CGG triplet is repeated (CGGCGGCGGCGG, etc.). If these repeats are in a noncoding region of the gene then it may not cause any harmful phenotype. Even 100 or more repeats cause no harm. But, these long stretches of nucleotide repeat may grow longer from one generation to the other may be up to 4000 repeats, which causes a constriction region on the X chromosome making it quite fragile. Males inheriting such X chromosome from their mothers possess quite a number of harmful phenotypic effects such as mental retardation. Males with the syndrome rarely become fathers. However, females inheriting such a fragile X chromosome also from their mothers only mildly get affected.
In another disorder Polyglutamine Diseases, the trinucleotide repeat CAG adds a string of glutamines to the protein encoded. These types of mutation are implicated in several disorders related to central nervous system that including:
• Huntington's disease: Here the protein known as Huntingtin carries the extra string of glutamines. The abnormal protein resulted from insertion increases the p53 protein level in the brain cells, thereby causing death due to apoptosis.

• Certain cases of Parkinson's disease wherein the extra string of glutamines is inserted to the encoded protein ataxin-2.
• Certain forms of muscular dystrophy appearing in adults are due to the formation of trinucleotide or tetranucleotide repeats, e.g. (CTG)n and (CCTG)n, where n may be up to thousands. Due to the formation of the large RNA transcript interference of the alternative splicing of other transcripts occurs.[/align]
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RE: Types of Mutations: Substitution, Insertions, Deletion and Frameshifts - by debram - 09-18-2014, 02:34 PM



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