Take a look at these four molecules. They’re all fairly similar, right? These are all types of monosaccharide furanose sugar. Sugars which are chemically based on a single pentagon shaped ring. At a glance, you probably can’t see much difference between them. In theory, as it happens, neither can life.
Click the image for a labelled version with the names of the sugars, if you like. The significance of these sugars is that two of them are ribose (upper left) and deoxyribose (lower right). The sugars that build the backbone for RNA (ribonucleic acid) and DNA (deoxyribonucleic acid)!★ These sugars team up with phosphorus and oxygen to make the sugar phosphate backbone that holds DNA together. Without them, I wouldn’t be here to write this, and you wouldn’t be there to read it.
But why these two sugars and not the others? In theory, any sugar should be able to make a sugar phosphate backbone, forming what’s been called xeno nucleic acids, or XNA. As as Greg Fish explains over at World of Weird Things, the X in XNA could be pretty much anything. Greg mentions that hexose would make HNA, glycol (technically not even a sugar) would make GNA, and cyclohexane (most definitely not a sugar!) would make CaNA. Which leads you to wonder what else you could sandwich between two phosphate groups to use as a nucleic acid. Following the molecular template that Earth life uses, as long as it’s a polymer which can bind to a set of nucleobases, it should work. Why exactly life chose ribose is a mystery. It probably comes down to a simple matter of thermodynamics and exactly which sugars formed most abundantly in whatever proverbial primordial soup life arose from.
But actually, there’s even more than that. Apparently, even the sugar phosphate is optional. This is peptide nucleic acid:
In the image, the B shows where the bases would be. Peptide nucleic acid (PNA) has been suggested as the original genetic molecule. A more primitive form of nucleic acid from the dim and distant past, not much more sophisticated than a protein, before sugar phosphate backbones had even developed. This does seem to make sense. Amino acids which form proteins have been seen to form readily, and are found in meteorites. What’s more, given the right conditions, amino acids will polymerise spontaneously. Could PNA be where life started? And if conditions had been different, could we have ended up with some other XNA instead of the DNA in our cells?♣ Perhaps there isn’t anything special about DNA at all. Perhaps DNA just happened to arise out of a collection of molecules which happened to be in the right place at the right time!
★ As for the other two, fructose is the sugar which makes fruit taste sweet, and galactose is what your body makes when it digests lactose, the sugar found in milk. And chocolate. Fructose, incidentally, is the sweetest of all sugars.
♣ Even those nucleobases used to encode our genes (adenine, guanine, cytosine, and thiamine in the case of DNA) may be variable. Any number of others may exist, including some which use metal ions. But that’s really a post for another day…