The RNA world

So far this book has focused on proteins and the genes that encode them. The human genome encodes some 21000 different proteins and the vast majority of them are important. On the other hand, there is a whole range of RNAs transcribed from the human genome that do not code for proteins, but have other functions. We refer to these RNAs as non-coding RNAs (ncRNAs). In fact, a major portion of the human genome is transcribed, although only about 1.5% of it corresponds to coding regions. We still do not know the function of many of these RNAs, but there are a large number of ncRNA families that have been characterized. Classic examples are tRNAs and ribosomal RNAs, which are part of the translation machinery. A set of U RNAs are involved in splicing (Chapter 16) and there are catalytically important RNA molecules of the RNA-processing enzymes RNases P and MRP. A vital and highly populated class of ncRNA is the RNAs involved in gene silencing as described in Chapter 3.