Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [15]
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 before its chromosomes have moved away, it will cut them into pieces. If it splits itself too far toward either end, one of its offspring will have a pair of chromosomes and the other will have none. These disasters almost never take place. E. coli nearly always divides itself almost precisely at its midpoint, and almost always after its two chromosomes are safely tucked away at either end.
A few types of proteins work together to create this precise dance. When E. coli is ready to divide, a protein called FtsZ begins to form a ring along the interior wall of the microbe at midcell. It attracts other proteins, which then begin to close the ring. Some proteins act like winches, helping to drag the chromosomes away from the closing ring. Others add extra membrane molecules to seal the ends of the two new microbes.
FtsZ proteins form their ring without consulting a map of the microbe, without measuring it with a ruler. Instead, it appears that FtsZ is forced by other proteins to form the ring at midcell. Another protein, called MinD, forms into spirals that grow along the inside wall of the microbe. The MinD spiral can scrape off any FtsZ it encounters attached to the wall. But the MinD spiral itself is fleeting. Another protein attaches to the back end of the spiral and pulls the MinD proteins off the wall one at a time.
A pattern emerges: the MinD spiral grows from one end toward the middle but falls apart before it gets there. The dislodged MinD proteins float around the cell and begin to form a new spiral at the other end. But as the MinD spiral grows toward the middle again, its back end gets destroyed