Let's look at a fancier example of how
environmental factors control the regulatory elements of DNA. Suppose that
Gene 4037 (not its real nameit has one, but I'll spare you the jargon),
when left to its own devices, is transcriptionally active, generating its
protein. However, as long as a particular messenger binds to a regulatory
element that comes just before 4037 in the DNA string, Gene 4037
Far from being autonomous sources of information, then, genes must obey
other factors that regulate when and how they function. Very often, those
factors are environmental. For example, suppose something stressful happens
to a primate. A drought, say, forces to it forage miles each day for food.
As a result, the animal secretes a stress hormone, cortisol, from its adrenals.
Cortisol molecules enter fat cells and bind to cortisol receptors. These
hormone- receptor complexes find their way to the DNA and bind to a regulatory
stretch of DNA. Whereupon a gene downstream is activated, which produces
a protein, which indirectly inhibits that fat cell from storing fat. It's
a logical thing to dowhile starving and walking the grasslands in search
of a meal, the primate needs fat to fuel muscles, not to laze around in
fat cells.
In effect, regulatory elements introduce the possibility of environmentally modulated if- then clauses. If the environment is tough and you're working hard to find food, then make use of your genes to divert energy to exercising muscles. The environment, of course, doesn't mean just the weather. The biology is essentially the same if a human refugee travels miles from home with insufficient food because of civil strife. The behavior of one human can change the pattern of gene activity in another.
Let's look at a fancier example of how environmental factors control the regulatory elements of DNA. Suppose that Gene 4037 (not its real nameit has one, but I'll spare you the jargon), when left to its own devices, is transcriptionally active, generating its protein. However, as long as a particular messenger binds to a regulatory element that comes just before 4037 in the DNA string, Gene 4037 shuts down. Fine. Now suppose that inhibitory messenger happens to be very sensitive to temperature. In fact, if the cell gets hot, the messenger goes to pieces and comes floating off the regulatory element. Freed from the inhibitory regulation, Gene 4037 will suddenly become active. Maybe it's a gene that works in the kidney and codes for a protein relevant to water retention. Boringanother metabolic story, this one having to do with how a warm environment triggers metabolic adaptations that stave off dehydration. But suppose, instead, Gene 4037 codes for an array of proteins that have something to do with sexual behavior. What have you just invented? Seasonal mating. Winter is waning, each day gets a little warmer, and in relevant cells in the brain, pituitary, or gonads, genes like 4037 are gradually becoming active. Finally some threshold is passed and, wham, everyone starts rutting and ovulating, snorting and pawing at the ground, and generally carrying on. (Actually, in most seasonal matters, the environmental signal for mating is the amount of daily light exposure, or the days are getting longer, rather than temperature, or the days are getting warmer. But the principle is the same.)
In EACH OF THESE EXAMPLES you can begin to see the logic, an elegance that teams of engineers couldn't do much to improve. And now for the two facts about this regulation of genes that will dramatically change your view of them. First, when it comes to mammals available' more than 95 percent of DNA is noncoding. Ninety-five percent. Sure, a lot of it may have no function, but your average gene comes with a huge instruction manual for how to operate it, and the operator IS very often environmental. With a percentage like that, if you think about genes and behavior, you have to think about how the environment regulates genes and behavior.