by Denyse O'Leary
From "From whales to earthworms, the mechanism that gives shape to life" (News Mediacom, 14.10.11), we learn,
During the development of an embryo, everything happens at a specific moment. In about 48 hours, it will grow from the top to the bottom, one slice at a time â€“ scientists call this the embryo's segmentation. "We're made up of thirty-odd horizontal slices," explains Denis Duboule, a professor at EPFL and Unige. "These slices correspond more or less to the number of vertebrae we have."
Every hour and a half, a new segment is built. The genes corresponding to the cervical vertebrae, the thoracic vertebrae, the lumbar vertebrae and the tailbone become activated at exactly the right moment one after another."
The process is astonishingly simple. In the embryo's first moments, the Hox genes are dormant, packaged like a spool of wound yarn on the DNA. When the time is right, the strand begins to unwind. When the embryo begins to form the upper levels, the genes encoding the formation of cervical vertebrae come off the spool and become activated. Then it is the thoracic vertebrae's turn, and so on down to the tailbone. The DNA strand acts a bit like an old-fashioned computer punchcard, delivering specific instructions as it progressively goes through the machine.
The punch line:
"A new gene comes out of the spool every ninety minutes, which corresponds to the time needed for a new layer of the embryo to be built," explains Duboule. "It takes two days for the strand to completely unwind; this is the same time that's needed for all the layers of the embryo to be completed." This system is the first "mechanical" clock ever discovered in genetics. And it explains why the system is so remarkably precise.
The Hox clock is a demonstration of the extraordinary complexity of evolution.Or of something.
Happily, these guys don't offer a hoked-up "evolutionary" explanation.
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