DNA microarrays have become commonplace in the last two decades, but the synthesis of other
nucleic acids biochips, most importantly
RNA, has only recently been developed to a similar extent.
RNA microarrays can be seen as organized surfaces displaying a potentially very large number of unique sequences and are of invaluable help in understanding the complexity of
RNA structure and function as they allow the probing and treatment of each of the many different sequences simultaneously. Three approaches have emerged for the fabrication of
RNA microarrays. The earliest examples used a direct, manual or mechanical, deposition of pre-synthesized, purified
RNA oligonucleotides onto the surface in a process called
spotting. In a second approach, pre-spotted or in situ-synthesized
DNA microarrays are employed as templates for the transcription of
RNA, subsequently or immediately captured on the surface. Finally, a third approach attempts to mirror the
phosphoramidite-based protocols for in situ synthesis of high-density
DNA arrays in order to produce in situ synthesized
RNA microarrays. In this mini-review, we describe the chemistry and the engineering behind the fabrications methods, underlining the advantages and shortcomings of each, and illustrate how versatile these platforms can be by presenting some of their applications.