First time accepted submitter mcswell
writes "Daniël Noordermeer and Denis Duboule, two researchers at Ecole Polytechnique Fédérale de Lausanne and the University of Geneva claim to have discovered how vertebrae get built in sequence in embryos (and by extension, how ribs, arms and so forth wind up in the right place). The story is that the DNA strands contain a linear series of HOX genes, and that the strands slowly unwind over a period of two days, successively exposing each HOX gene, thereby allowing it to be transcribed to form the segments of the vertebra. Snakes, it seems, have a defect that causes the system not to shut down; eventually it 'runs out of steam.' The same process is said to apply in many invertebrates, including worms (presumably segmented worms) and insects."