Until recently, circular RNAs were considered as by-products or products of transcription. Interest in them has grown with the discovery of their cell-specific expression in several organisms. In recent years, studies have shown that circular RNAs are long noncoding, endogenous, single-stranded molecules that are enclosed in a covalent circle and are not polyadenylated. In eukaryotic organisms, circular RNA can be divided into 3 groups according to their structure and formation. These are exonic, intronic and exon-intron circRNAs. The most researched way of their formation is the process of backsplicing, which is regulated by cis and trans acting elements. Reverse complementary sequences in the surrounding introns, which bind to each other and stimulate circRNA formation, have a great influence on circRNA expression. CircRNAs affect cell proliferation, cell cycle, cell pluripotency and differentiation, aging, and the development of certain diseases. Their best-known mode of action is regulation of protein-encoding genes expression through binding to miRNAs. Their circular structure may be the reason for the late discovery of circRNAs, as most detection methods focus on the detection of polyadenylated RNAs. The most common methods for biochemical characterization of circRNA are quantitative RT-PCR and Northern transfer in combination with the enzymes RNAase R and RNase H. However, the development of methods based on bioinformatics and statistics that predict and detect circRNA from RNA-seq datasets is on the rise.
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