linc-RNA's

linc-RNA
(Long, Intergenic, Non-Coding-RNA)

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Only about 3% of human DNA consists of codons and their complementary strands. Another few percent deal with μ-RNA. What might be the remainder? Was it ever transcribed?

The most commonly considered possibility was that this was "junk" DNA - perhaps "fossil" DNA left over by inactivated, integrated viral genomes that had accumulated over the generations and millenia. This idea was recently shot down upon the sequencing of genomes. Were it "junk" then there would be no value in conserving the sequences and mutations would build up over time. True there are some such sections of DNA that do seem to be unconserved, but the vast majority of the non-expressed [in polypeptides, that is] DNA was highly conserved and thus there must be a strong need for that conservation of sequence.

then upon the sequenquencing of two closely related species - chimps and humans, it was found that nearly all of the cistronic DNA was identical, but that species differences rested almost exclusively in the all this residual DNA that was highly conserved within each species.

Then some congenital diseases were ascribed to these non-expressed sections - muscular dystrophy and micro-cephaly, for two examples. These non-cistronic sequences thus rapidly accumulated recognition as being control elements - stimulating or damping-down the expression of the cistronic genes.

Soon there appeared considerable similarity in structure among these control elements: all were much longer than μ-RNAs - and even longer than tRNAs. But tRNAs (and rRNAs) are also derive from non-expressed genes. Anyway, these new-found control RNAs had some similarity to tRNAs as they formed hairpins - meaning that they possessed sections of palendromic sequences. And they were found between expressed cistronic DNA. Thus they were named "long, intergenic, non-coding RNA, or "lincRNA" for short.

Current models of action indicate that while proteins bind to the loose ends of the tRNA and the distinctive anti-codon is in the bend of the hairpin, the lincRNAs are the reverse - the proteins bind to the bend while the loose ends intertwine with chromosomal DNA and thus are able in various ways to regulate the efficiences of expression.