1. B. A deletion of genomic DNA encompassing only a single exon results in the clinically severe Duchenne muscular dystrophy. A larger deletion encompassing four exons results in the clinically milder Becker muscular dystrophy. In both cases the gene is transcribed into mRNA, and the exons flanking the deletion are spliced together. 1. If a single exon is deleted and a nonintegral set of codons is missing, the borders of neighboring exons may not match, causing the translational reading frame to shift. As a result, incorrect amino acids are inserted into the growing polypeptide chain until an abnormal stop codon is reached, causing premature termination of the protein. The truncated protein may be unstable, may fail to be localized in the membrane, or may fail to bind to glycoproteins. Functional dystrophin is then almost totally absent. A muscle biopsy of Duchenne dystrophy (right) shows no detectable immunoreactive dystrophin. 2. If the deletion is larger but an integral number of codons are deleted, the reading frame can be maintained in the mRNA. This produces a dystrophin molecule with an internal deletion but intact ends. Although the protein is smaller than normal and may be present in less than normal amounts, some muscle function is preserved. Immunoperoxidase staining of dystrophin is minimally reduced in a muscle biopsy of Becker dystrophy (right).
Source: Diseases of the Nerve and Motor Unit, Principles of Neural Science, Fifth Editon
Citation: Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, Mack S. Principles of Neural Science, Fifth Editon; 2012 Available at: Accessed: December 31, 2017
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