What is RNA?

RNA (ribose nucleic acid) is a complex molecule that plays a big part in living cells. It is similar in structure to DNA, which carries the genes that give rise to all living things. Both molecules comprise a sugar-phosphate backbone, although in RNA the sugar is ribose while in DNA it is deoxyribose. Both also carry information encoded in four nitrogenous bases: in RNA these are adenine, cytosine, guanine and uracil, whereas DNA uses a base called thymine instead of uracil. The most apparent difference, however, is that DNA forms a double-stranded helix, while RNA is almost always single stranded.

While DNA has one function – to carry the information needed to create life – RNA plays a variety of roles within the cell. When information from a section of DNA is decoded to make a protein, the first step entails making an RNA copy. This so-called messenger RNA (mRNA) carries a code in its bases that reflects the DNA sequence. Another type of RNA, called transfer RNA (tRNA), then binds to the mRNA bringing with it the amino acids specified by the code. These building blocks of protein are assembled by cellular organelles called ribosomes, comprised of ribosomal RNA (rRNA) and proteins.

In recent years, other functions for RNA have been discovered. Animals, plants and some viruses contain microRNAs (miRNA), which can silence genes and regulate their expression. RNA also helps regulate various cellular processes from cell division and growth to ageing and death. And RNA can even act as an enzyme, speeding up various biochemical reactions within the cell.

RNA, being simpler than DNA, probably evolved first. Life emerged some 3.5 billion years ago, and one popular hypothesis, called the “RNA world”, holds that RNA was the spark that generated it. With its coded information and enzymatic action, RNA could, in principle, self-assemble and reproduce by making copies of itself. However, biochemists struggled to do this in the lab, and many now question the RNA world hypothesis. Instead of RNA coming first and somehow creating the other essential aspects of life, there is growing support for the idea that life emerged fully formed, as a crude prototype, in a sort of biological big bang.