The messenger RNAs containing the thirteen protein coding sequences of the human mitochondrial genome have frequently been regarded as a single functional category, alike in arrangement and hence in mode of expression. The "generic" mitochondrial mRNA is perceived as having (i) an arrangement within the polycistronic unit that permits its liberation following mt-tRNA processing, (ii) no 5' cap structure or introns, (iii) essentially no untranslated regions, and (iv) a poly(A) tail of approximately fifty nucleotides that is required in part to complete the termination codon. Closer inspection reveals that only two molecules fit this pattern. This article examines the extent to which human mitochondrial mRNA species differ from one another.
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