New Criticals

Ok so that doesn’t tell you a whole lot about lipids or carbs, but hopefully it should remind you that you are made up of a lot of fat and sugar and that even though you’re perhaps accustomed to thinking that you get too much of these things you also depend on them for survival and they constitute part of your being.

The last two macromolecule types constitute the central informational knot of life: proteins and DNA. Both are hetero-polymers: they are made up of different repeating units. DNA and RNA are both made up of four kinds of chemical units which we abbreviate A, T, C, and G (In RNA the T is a slightly different chemical abbreviated U and there are minor structural changes but the differences between DNA and RNA aren’t important for this discussion yet). Proteins are also heteropolymers made up of 20 different naturally occurring amino acids. Being a heteropolymer, unlike starch, means that proteins and nucleic acids are capable of carrying information. So here is the first, necessary but insufficient criteria for biochemical information: A given symbol set must be capable of being permuted in alternative arrangements.

Proteins and nucleic acids both meet this criteria because there are no physical or chemical constraints on them that determine, in advance, how the monomer units should be ordered when they form a polymer chain. DNA forms polymers in such a manner that it really makes no physical or chemical difference at all what the identity of the next nucleotide in the chain will be. Proteins contain a few more self-ordering rules due to the kinds of shapes that they make, so not all possible sequences of amino acids will polymerize into a protein as readily as will any possible sequence of nucleotides turn into a strand of DNA. Language is more like protein in this regard. For example, in English, when we find the letter ‘Q’ in a word it is highly probable that the next letter will be ‘U.’ Certain amino acids, likewise, would rather be next to one another or far away from one another in a sequence for energetic reasons. This seems like a footnote but it will be important when we contrast the metabolism-first view of emergence with the RNA-first view.