BODY WATCH : Telling Time Is a Genetic Thing

Just about all plants and animals live by the clock. But how do they know what time it is?

The answer is that they have an innate cellular clock that precisely marks the passage of time.

Not surprisingly, the period of time that innate, or endogenous, clocks mark is close to 24 hours. The clock marks the cycle even in the absence of light and darkness. Three recent papers in the journal Science suggest how this may happen. Researchers at Rockefeller University, the University of Pennsylvania and Harvard Medical School studied the internal clock of the fruit fly. A gene called “per”--discovered several years ago--was found to be crucial to 24-hour rhythms in the fly. The scientists, however, theorized there was probably at least one other gene involved.

Many genes regulate their own activity by what’s known as a “feedback loop.” A gene turns on and directs the production of a protein. As that protein builds up, it turns off (or turns down) the gene, so less protein is made. As the level of protein falls, there is less suppression of the gene. The gene then becomes more active, more protein is made, and another cycle begins. Such loops eventually create a “steady state” system of protein production. But they do not oscillate enough to be clocks.

The researchers found a second gene, called “timeless,” whose protein product appears to interact with per’s protein. When the genes are active, their products slowly accumulate inside the cell. They seem to bind to each other, but only weakly. Eventually, the paired proteins reach sufficient concentration that some make their way back into the cell nucleus where they turn off the per gene, and perhaps timeless. When there are not enough to keep the genes suppressed, the genes “wake up” and the cycle begins anew. The researchers believe the long time it takes for per and timeless proteins to accumulate and enter the nucleus creates an oscillation of sufficient length and magnitude to function as a clock.