Circular RNA

Circular RNA (or circRNA) is a type of RNA which, unlike the better known linear RNA, forms a covalently closed continuous loop, i.e., in circular RNA the 3' and 5' ends normally present in an RNA molecule have been joined together. This feature confers numerous properties to circular RNAs, many of which have only recently been identified.

Many circular RNAs arise from otherwise protein-coding genes, but circular RNAs produced in the cell have not been shown to code for proteins. They have therefore been categorized as noncoding RNA. Some circular RNAs have recently shown potential as gene regulators. Like many other alternative noncoding isoforms, the biological function of most circular RNAs are unclear.

Because circular RNAs do not have 5' or 3' ends, they are resistant to exonuclease-mediated degradation and are presumably more stable than most linear RNAs in cells.

In contrast to genes in bacteria, eukaryotic genes are split by non-coding sequences known as introns. In eukaryotes, as a gene is transcribed from DNA into a messenger RNA (mRNA) transcript, intervening introns are removed, leaving only exons in the mature mRNA, which can subsequently be translated to produce the protein product. The spliceosome, a protein-RNA complex located in the nucleus, catalyzes splicing in the following manner:

Alternative splicing is a phenomenon through which one RNA transcript can yield different protein products based on which segments are considered "introns" and which are considered "exons" during each splicing event. Although not specific for humans, it is a partial explanation for the fact that human and other, much more simple species (such as nematodes) have a similar number of genes (in the range of 20 - 25 thousand). One of the most striking examples of alternative splicing is in the Drosophila DSCAM gene. This single gene can give rise to approximately 30 thousand distinct alternatively spliced isoforms.

Exon scrambling, also called exon shuffling, describes an event in which exons are spliced in a "non-canonical" (atypical) order. There are three ways in which exon scrambling can occur:

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