Noncoding DNA is not "junk" but a necessity for origin and evolution of biological complexity

Abstract

All eukaryotic genomes contain, besides the coding infor-mation for amino acids in different proteins, a significant amount of noncoding sequences, which may or may not be transcribed. In general, the more evolved or biologically complex the organisms are, greater is the proportion of the noncoding component in their genomes. The popularity and success of "central dogma of molecular biology" during the last quarter of the 20^th century relegated the noncoding DNA sequences to a mortifying status of "junk" or "selfish", even though during the pre- "molecular biology" days there were good indications that such regions of the genome may function in as yet unknown ways. A resurgence of studies on the noncoding sequences in various genomes during the past several years makes it clear that the complex biological organization demands much more than a rich proteome. Although the more popularly known noncoding RNAs are the small microRNAs and other similar species, other types of larger noncoding RNAs with critical functions in regulating gene activity at various levels are being increasingly identified and characterized. Many noncoding RNAs are involved in epigenetic modifications, including imprinting of genes. A comprehensive understanding of the significance of noncoding DNA sequences in eukaryotic genomes is essential for understanding the origin and sustenance of complex biological organization of multicellular organisms.