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order.
    Many of those keys were first discovered in
E. coli. E. coli
must grapple with several organizational nightmares in order to survive, but none so big as keeping its DNA in order. Its chromosome is a thousand times longer than the microbe itself. If it were packed carelessly into the microbe’s interior, its double helix structure would coil in on itself like twisted string, creating an awful snarl. It would be impossible for the microbe’s gene-reading enzymes to make head or tail of such a molecule.
    There’s another reason why
E. coli
must take special care of its DNA: the molecule is exquisitely vulnerable to attack. As the microbe turns food into energy, its waste includes charged atoms, which can crash into DNA, creating nicks in the strands. Water molecules are attracted to nicks, where they rip the bonds between the two DNA strands, pulling the chromosome apart like a zipper.
    Only in the past few years have scientists begun to see how
E. coli
organizes its DNA. Their experiments suggest that it folds its chromosome into hundreds of loops, held in place by tweezerlike proteins. Each loop twists in on itself, but the tweezers prevent the coiling from spreading to the rest of the chromosome. When
E. coli
needs to read a particular gene, a cluster of proteins moves to the loop where the gene resides. It pulls the two strands of DNA apart, allowing other proteins to slide along one of the strands and produce an RNA copy of the gene. Still other proteins keep the strands apart so that they won’t snarl and tangle during the copying. Once the RNA molecule has been built, the proteins close the strands of the DNA again.
E. coli’
s tweezers also make the damage from unzipping DNA easier to manage. When a nick appears in the DNA, only a single loop will come undone because the tweezers keep the damage from spreading farther.
E. coli
can then use repair enzymes to stitch up the wounded loop.
    E. coli
faces a far bigger challenge to its order when it reproduces. To reproduce, it must create a copy of its DNA, pull those chromosomes to either end of its interior, and slice itself in half. Yet
E. coli
can do all of that with almost perfect accuracy in as little as twenty minutes.
    The first step in building a new
E. coli
—copying more than a million base pairs of DNA—begins when two dozen different kinds of enzymes swoop down on a single spot along
E. coli’
s chromosome. Some of them pull the two strands of DNA apart while others grip the strands to prevent them from twisting away or collapsing back on each other. Two squadrons of enzymes begin marching down each strand, grabbing loose molecules to build it a partner. The squadrons can add a thousand new bases to a DNA strand every second. They manage this speed despite running into heavy traffic along the way. Sometimes they encounter the sticky tweezers that keep DNA in order; scientists suspect that the tweezers must open to let the replication squadrons pass through, then close again. The squadrons also end up stuck behind other proteins that are slowly copying genes into RNA and must wait patiently until they finish up and fall away before racing off again. Despite these obstacles, the DNA-building squadrons are not just fast but awesomely accurate. In every 10 billion bases they add, they may leave just a single error behind.
    As these enzymes race around
E. coli’
s DNA, two new chromosomes form and move to either end of the microbe. Although scientists have learned a great deal about how
E. coli
copies its DNA, they still debate how exactly the chromosomes move. Perhaps they are pulled, perhaps they are pushed. However they move, they remain tethered like two links in a chain. A special enzyme handles the final step of snipping them apart and sealing each back together. Once liberated, the chromosomes finish moving apart, and
E. coli
can begin to divide itself in two.
    The microbe must slice itself precisely, in both space and time. If it starts dividing