On a more theoretical level

On a more theoretical level - distributed as a gradient...

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On a more theoretical level, morphogens have been hypothesized to operate in a threshold -like manner in more localized examples of pattern formation such as the generation of additional bristles in Drosophila or specific patterns of striping in mammals (see figs. below). Specific molecules causing "prepattern" such as one sees in zebras have yet to be identified, in contrast to the major advances made in Drosophila, but mating zebras is a major undertaking. There is solid support for such ideas in Drosophila development. The RNA encoded by the bicoid gene is localized in the anterior portion of the embryo. The protein translated from this localized RNA is
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Unformatted text preview: distributed as a gradient from anterior to posterior across the embryo. The bicoid protein affects the distribution of the RNA of another gene, hunchback . This RNA ( hunchback ) is not distributed as a gradient but in a discrete way: present in the anterior, absent in the posterior. Thus there appears to be positional information in the concentration of bicoid which is read by hunchback as a threshold . One could imagine that a mutation that affected the localization of one morphogen could alter the localization of important thresholds of different morphogens, which would in turn lead to the development of new morphologies....
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