Free Microcosm by Carl Zimmer Page B

Some give their offspring a lot of permeases on their membranes and a lot of lactose molecules floating through their interiors. Others give their offspring neither.
    Combine this peculiar switch with
E. coli
’s unpredictable bursts and you have a recipe for individuality. If a colony of
E. coli
encounters some lactose, some of the bacteria will respond with a huge burst of proteins from their
lac
operon. They will push themselves over the threshold from reluctant to eager, and they will stay that way even if the lactose drops. Other
E. coli
will respond to the lactose with no proteins at all. They will remain reluctant. These clones take on different personalities thanks to chance alone.
    E. coli
also gets some of its personality from an extra layer of heredity. Some of its DNA is covered with caps made of hydrogen and carbon atoms. These caps, known as methyl groups, change the response of
E. coli’
s genes to incoming signals. They can, in effect, shut a gene down for a microbe’s entire life without harming the gene itself. When
E. coli
divides in two, it bequeaths its pattern of methyl groups to its offspring. But under certain conditions,
E. coli
will pull methyl groups off its DNA and put new groups on—for reasons scientists don’t yet understand.
    Some of the factors that spin the wheel for
E. coli
spin it for us as well. To smell, for example, we depend on hundreds of different receptors on the nerve endings in our noses. Each neuron makes only one type of receptor. Which receptor it makes seems to be a matter of chance, determined by the unpredictable bursts of proteins within each neuron. Our DNA carries methyl groups as well, and over our lifetime their pattern can change. Pure chance may be responsible for some changes; nutrients and toxins may trigger others. Identical twins may have 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 after all.
    These different patterns are also one reason why clones of humans and animals can never be perfect replicas. In 2002, scientists in Texas reported that they had used DNA from a calico cat named Rainbow to create the first cloned kitten, which they named Cc. But Cc is not a carbon copy of Rainbow. Rainbow is white with splotches of brown, tan, and gold. Cc has gray stripes. Rainbow is shy. Cc is outgoing. Rainbow is heavy, and Cc is sleek. New methylation patterns probably account for some of those differences. Clones may also get hit by a unique series of protein bursts. The very molecules that make them up turn them into individuals in their own right.
    At the very least,
E. coli’
s individuality should be a warning to those who would put human nature down to any sort of simple genetic determinism. Living things are more than just programs run by genetic software. Even in minuscule microbes, the same genes and the same genetic network can lead to different fates.

Four
    THE
E. COLI
WATCHER’S FIELD GUIDE
    A HUMAN KRAKATAU
              ON AUGUST 26, 1883, a little world was born. An island volcano called Krakatau, located between Java and Sumatra in the Sunda Strait, hurled a column of ash twenty miles into the air. Rock turned to vapor and roared across the strait at 300 miles an hour. The eruption left a submerged pit where the cone of the volcano had been, along with a few lifeless islands. Nine months later, a naturalist who visited the scene reported that the only living thing he could find was a single small spider.
    The new islands of Krakatau lay twenty-seven miles from the nearest land. It took years for life to make its way across the water and take hold again. A film of blue-green algae grew over the ash. Ferns and mosses sprouted. By the 1890s a savanna had emerged.