Parallel-stranded DNA and RNA duplexes - structural features and potential applications.

Nowadays, decades after the discovery of the right-handed B form of DNA, it is well known that nucleic acids have great conformational flexibility, exhibiting a large degree of variation in their structure. In nature, DNA and RNA exist in an antiparallel orientation, stabilized by Watson-Crick base pairs. However, in some cases, nucleic acid fragments with specific nucleotide sequences can adopt a parallel orientation involving non-canonical base pairing. Interestingly, parallel-stranded duplexes have been found in specific chromosome regions. Furthermore, parallel oriented regions have also been found in bacterial (Escherichia coli, Listeria innocua) and insect genomes (Drosophila melanogaster). These unusual structures could have a remarkable evolutionary role, as well as significant impact on biological processes. For example, parallel stretches were shown to be involved in processing the 3' ends of mRNAs and in specific gene silencing. Moreover, certain types of parallel-stranded duplexes may be useful tools, with several practical applications. They can constitute excellent templates for the formation of other structures and for the development of antigene and antisense approaches.