As a corollary to the “central dogma of molecular biology” that genetic information carried in DNA is utilized to produce proteins which determine the phenotype, concepts of “junk” or “selfish” DNA were advanced to explain the “C-value paradox”, leading to ignorance of parts of genomes that were not involved in protein synthesis. However, the everincreasing numbers of studies during the past 10-15 years have confirmed that bulk of the nuclear DNA is indeed transcribed and that the non-coding transcripts actually provide a complex multi-layered regulatory network essential for the self-organized state. Commensurate with the evolutionary increase in biological complexity, the “non-coding” RNAs (ncRNAs) have also diversified. This brief review highlights the various classes of ncRNAs in eukaryotes taking examples of actions of some of the earliest known long ncRNAs like the Xist and roX, implicated in dosage compensation in mammals and Drosophila, respectively, and the hsrω long ncRNAs of Drosophila. Among the 7 transcripts produced by the Drosophila hsrω gene, the long nuclear transcripts that contain >5kb of tandem repeat sequences organize the omega speckles, which act as nucleoplasmic stores of a variety of RNA-binding and some other proteins to regulate their availability. Such long ncRNAs act as hubs in cellular networks through their interactions with diverse arrays of proteins. In view of the increasing evidence and realization of the importance of non-coding components of human and other genomes in maintaining normal homeostasis and because of their critical involvement in many human disorders, it is necessary to proactively explore their diversity and functions in different organisms.
