Molecular Biology

Molecular Genetics

intron

Because the exon segments of archival DNA are transcribed into messenger RNAs and translated into proteins, the earlier view of genes held that introns were solely ‘junk DNA’ or DNA 'deserts' because they do not contain ORFs to code for proteins. However, it has recently been recognized that some exons code for micro RNAs and represent a source of epigenetic coding.

Some segments that were formerly designated introns contain information for protein, and others code for RNA products and are thus not "junk" DNA. Other introns posses translatable nucleotide sequences that, in the absence of splicing, can generate production of novel peptides (maturases) fused to the peptide encoded by the N-terminal exons. In fungi, these peptide maturases, appear to function in intron removal. Their encoding in introns results in homeostatic regulation of their production. Maturase genes are interspersed within other genes.“

Group II introns are a novel class of RNAs (ribozymes) best known for their self-splicing reaction. Under certain in vitro conditions, the introns can excise themselves from precursor mRNAs and ligate together their flanking exons, without the aid of protein. The splicing mechanism is essentially identical to splicing of nuclear pre-mRNA introns (pre-mRNA splicing), and this similarity has led to the widespread belief that group II introns were the ancestors of spliceosomal introns, which make up 25% of the human genome.

Some group II introns have a second remarkable property: they encode reverse transcriptase (RT) ORFs and are active mobile elements. Such mobile group II introns can insert into defined sites at high efficiencies (called retrohoming), or can invade unrelated sites at low frequencies (retrotransposition).”

Ribozyme-mediated revision of RNA and DNA -- Long et al. 112 (3): 312 -- Journal of Clinical Investigation: "Group I introns are ribozymes that carry out two transesterification reactions in order to excise themselves from a precursor transcript. "

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