Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

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5 The oscillatory clock that drives somite forma- tion in vertebrates involves three essential components Her7 (an unstable repressor of its own synthesis), Delta (a transmembrane signaling molecule), and Notch (a trans- membrane receptor for Delta). Notch is bound by Delta on neighboring cells, activating the Notch signaling pathway, which then activates Her7 transcription. Normally, this system works flawlessly to create sharply defined somites (Figure Q21-2A). In the absence of Delta, however, only the first five somites form normally, and the rest are poorly defined (Figure Q21-2B). If a pulse of Delta is supplied later, somite formation returns to normal in the regions where Delta was present (Figure Q21-2C). A diagram of the connections between the components of the clock and how they interact in adjacent cells is shown in Figure Q21-2D. In the absence of Delta, why do the cells become unsynchronized? What is it about the presence of Delta that keeps adjacent cells oscillating in synchrony?