Genome sequencing has revealed that protein-coding DNA accounts for only 1.5% of the human genome and a similarly small percentage of the genomes of many other multi-cellular eukaryotes. A very small fraction of the non-protein-coding DNA consists of genes for RNAs such as ribosomal RNA and transfer RNA. Until recently, scientists assumed that most of the remaining DNA was not transcribed, thinking that since it didn’t specify proteins or the few known types of RNA, such DNA didn’t contain meaningful genetic information—in fact, it was called “junk DNA.” However, a flood of recent data has contradicted this idea. For example, a massive study of the entire human genome showed that roughly 75% of the genome is transcribed at some point in any given cell. Introns account for only a fraction of this transcribed, nontranslated RNA. These and other results suggest that a significant amount of the genome may be transcribed into non-protein-coding RNAs—also called noncoding RNAs, or ncRNAs—including a variety of small RNAs. Researchers are uncovering more evidence of the biological roles of these ncRNAs every day.
Biologists are excited about these discoveries, which have revealed a large and diverse population of RNA molecules in the cell that play crucial roles in regulating gene expression— but have gone largely unnoticed until fairly recently. Our long-standing view that because mRNAs code for proteins, they are the most important RNAs functioning in the cell demands revision. This represents a major shift in the thinking of biologists, one that you are witnessing as students entering this field of study.
Urry, Lisa A.. Campbell Biology (p. 377). Pearson Education. Kindle Edition.