who discovered it?
Friedrich Miescher
Key discoveries in RNA biology
Research on RNA has led to many important biological discoveries and numerous Nobel Prizes. Nucleic acids were discovered in 1868 by Friedrich Miescher, 
rna consists of how many chains of subunits Small RNAs mainly include 5.8S ribosomal RNA (rRNA), 5S rRNA, transfer RNA (tRNA), microRNA (miRNA), small interfering RNA (siRNA), small nucleolar RNA (snoRNAs), Piwi-interacting RNA (piRNA), tRNA-derived small RNA (tsRNA) and small rDNA-derived RNA (srRNA) are they also double-stranded? Yes, Double-stranded RNA is RNA with two complementary strands how is it different from dna? Analysis of mRNA, tRNA, rRNA, snRNAs, and other non-coding RNAs  has revealed that self-complementary sequences that allow parts of the RNA to fold and pair with itself to form double helices and they are highly structured. Unlike DNA, their structures do not consist of long double helices, but rather collections of short helices packed together into structures akin to proteins what is the function of the non-coding ones? Silencing of blocks of chromatin via recruitment of Polycomb complex so that messenger RNA could not be transcribed from them
UNANSWERABLE Jeannie T. Lee. Robert W. Holley "et al."
RNA Non-coding RNA
Long non-coding RNAs History and discovery
Their roles, at first mysterious, were shown by Jeannie T. Lee and others to be the silencing of blocks of chromatin via recruitment of Polycomb complex so that messenger RNA could not be transcribed from them.  Additional lncRNAs The first non-coding RNA to be characterised was an alanine tRNA found in baker's yeast, its structure was published in 1965. To produce a purified alanine tRNA sample, Robert W. Holley "et al." 
wiki:25651101
RNA [SEP] Key discoveries in RNA biology
Research on RNA has led to many important biological discoveries and numerous Nobel Prizes. Nucleic acids were discovered in 1868 by Friedrich Miescher, who called the material 'nuclein' since it was found in the nucleus. It was later discovered that prokaryotic cells, which do not have a nucleus, also contain nucleic acids. The role of RNA in protein synthesis was suspected already in 1939. Severo Ochoa won the 1959 Nobel Prize in Medicine (shared with Arthur Kornberg) after he discovered an enzyme that can synthesize RNA in the laboratory. However, the enzyme discovered by Ochoa (polynucleotide phosphorylase) was later shown to be responsible for RNA degradation, not RNA synthesis.