Prometheus 6

The key to understanding E. coli’s fingerprints is to recognize that the bacteria are not simple machines. Unlike wires and transistors, E. coli’s molecules are floppy, twitchy and unpredictable. In an electronic device, like a computer or a radio, electrons stream in a steady flow through the machine’s circuits, but the molecules in E. coli jostle and wander. When E. coli begins using a gene to make a protein, it does not produce a smoothly increasing supply. It spurts out the proteins in fits and starts. One clone may produce half a dozen copies of a protein in an hour, while a clone right next to it produces none....

Identical genes can also behave differently in our cells because some of our DNA is capped by carbon and hydrogen atoms called methyl groups. Methyl groups can control whether genes make proteins or remain silent. In humans (as well as in other organisms like E. coli), methyl groups sometimes fall off of DNA or become attached to new spots. Pure chance may be responsible for changing some methyl groups; nutrients and toxins may change others.

Identical twins may have nearly identical genes, but their methyl groups are distinctive by the time they are born and become increasingly different as the years pass. As the patterns change, people become more or less vulnerable to cancer or other diseases. This experience may be the reason why identical twins often die many years apart. They are not identical at all.

Expressing Our Individuality, the Way E. Coli Do
By CARL ZIMMER

We humans differ from one another in too many ways to count. We are shy and bold, freckled and pale, truckers and hairdressers, Buddhists and Presbyterians. We get cancers in the third grade and live for a century. We have fingerprints.

Scientists have only a rough understanding of how this diversity arises. Some of it stems from the different experiences we have, from our time in the womb on through childhood and into our mature years. These molding influences include things like the books we read and the air we breathe. Our diversity also stems from our genes — the millions of typographical differences between one genome and another.

We put a far bigger premium on nature than nurture when it comes to our individuality. That’s one reason why reproductive cloning inspires so much horror. If genes equal identity, then a person carrying someone else’s DNA has no distinct self.

But there’s a deep flaw in this way of thinking, one that blinds us to how biology — human or otherwise — really works. A good counterexample is E. coli, a species of bacteria that lives harmlessly in every person’s gut by the billions. A typical E. coli contains about 4,000 genes (we have about 20,000). Feeding on sugar, the microbe grows till it is ready to split in two. It makes two copies of its genome, almost always managing to produce perfect copies of the original. The single microbe splits in two, and each new E. coli receives one of the identical genomes. These two bacteria are, in other words, clones.

Surely, then, E. coli must be all nature and no nurture. A colony descended from a single E. coli ancestor is just a billion identical cousins, all responding to the world with the same set of genes.

Yet as plausible as this sounds, it’s far from the truth. A colony of genetically identical E. coli is, in fact, a mob of individuals. Under identical conditions, they will behave in different ways. They have fingerprints of their own.