The missing link

GluRBRGpremRNA4Genes get a good press. The DNA double helix is a iconic, and we are all know that genes are how we inherit our mother’s blue eyes or our father’s curly hair. Proteins also have a good press. Food is sold as being “high in protein” — we know we need protein as part of a balanced diet. But the third key biological polymer, RNA, gets much less publicity. This is a shame, as our cells have a lots of it (more than DNA), and it is essential to all life on Earth.

RNA forms the link between genes and proteins in the sense that an RNA molecule is made from a gene and then the protein is made from that RNA molecule, the RNA forms the intermediate between DNA and the protein. Their properties are also, roughly speaking, intermediate between those of DNA and proteins. DNA is highly stable but boring. This is just what you want in an information storage medium — which is what DNA in your cells is. Many proteins by contrast are often rather fragile things, always falling apart and being recycled, but they catalyse many chemical reactions. I have just eaten an apple, as I type proteins are chopping the molecules of the apple into smaller molecules that will then be absorbed by my body.

RNA can also catalyse chemical reactions but it is not as versatile as proteins. RNA can also form structures like the double helix most of your DNA is in. Part of one is shown above where the RNA loops back on itself and forms about two turns of a helix (follow the strand in gold). It can also store genetic information. The genes of some viruses, HIV is notable example, are made of RNA not DNA. HIV has nine genes but zero DNA.

Maybe RNA is rather neglected as it is jack of all trades. But the fact that it can do everything, both store information and drive the chemistry of life, means it is our best candidate for the origin of life on Earth. Life may have begun as replicating RNA molecules, only later evolving to use the more stable DNA to store genetic information, and the more chemically versatile proteins, to allow a better more sophisticated metabolism.



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