This membrane, so vanishingly thin, looms large... for bacteria use it for generating their energy. - Nick Lane

I would define complexity, not really as genetic complexity because if you take it purely as genetic complexity, E. coli... a single cell may have 4,000 genes but the metagenome, the pool of genes in E. coli around the place may be on the order to 30,000 or more... [T]hat's the level of complexity equivalent to the human genome, or even more complex than the human genome, but it's organized and structured in a different way. ...You might say that it's structured in a similar way to an ... but I think an ant colony has taken that level of Eusocial behavior a long way beyond anything you would see in E. coli. So I would define it as morphologically complex, meaning cells are larger and have a lot of stuff in them.

[T]he '' contends that ageing and many of... [its associated] diseases... are caused by... free radicals leaking from mitochondria during normal . ...As they burn up food using oxygen, the free-radical sparks escape to damage adjacent structures ...Many cruel inherited conditions... are linked with mutations caused by free radicals attacking mitochondrial genes.

The sequence of letters in a specifies the sequence of s in a protein. If the sequence of letters is changed—a ''—this may change the structure of the protein (...not always, there is some redundancy... technically degeneracy..—several combinations... can code for the same amino acid.)