Intron splice sites, detecting

The majority of these somatic mutations are heterozygous and located in the intracytoplasmic (IC) domain, which contains the death domain, as summarized in Table 2. The literature shows that 89.5% of the mutations are detected in exons 5.3% in exon 2, 3.9% in exon 3, 3.9% in exon 4, 7.9% in exon 6, 2.6% in exon 7, 1.3% in exon 8, and 64.5% in exon 9. In contrast, only 10.5% of the mutations are reported to occur in introns. All mutations in introns reportedly occur at 5 or 3 splice sites and cause probably aberrant splicing, resulting in premature termination... 

Detection of intron/exon splice sites Introns in eukaryotes cause discontinuation of the reading frame. If the analysis is not focused on a cDNA sequence, these introns must be spliced out and the exons joined to form the sequence that actually codes for the protein. Intron/exon splice sites can be predicted based on their common features. Most introns begin with the nucleotides GT and end with the nucleotides AG. There is a branch sequence near the downstream end of each intron involved in the splicing event. 

The molecular defect has recently been linked [7] to a single base substitution, an A—transversion, in the penultimate 3 nucleotide of the third intron of the Apo E gene. This leads to a loss of the correct 3 splice site, thus giving rise to two abnormally spliced mRNA forms. The smaUer form contains 53 nucleotides and the larger one, the entire third intron of the gene. Since both mRNA species contain chain termination codons within the intronic sequence, only short Apo E peptides not detectable by standard gel electrophoretic techniques are produced. Apo E deficiency is, therefore, the result of a molecular error which gives rise to shorter, nonfunctional forms of Apo E. In contrast to Apo B, where the mechanism is posttranslational, here it is clearly pretranslational. 

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