(a) The bicoid and nanos mRNAs are localized to the anterior and posterior poles, respectively, whereas the hunchback and caudal mRNAs are found throughout the syncytial embryo.
(b) Diffusion of the Bicoid and Nanos proteins leads to the formation of concentration gradients. Bicoid activates hunchback transcription and represses Caudal translation; Nanos represses hunchback translation, leading to graded distributions of all four proteins. (c) The hunchback gene contains several binding sites for the Bicoid protein upstream of the promoter. Occupancy of these sites leads to activation of the hunchback gene in the anterior half of the embryo.
Source: Modified from Gilbert S. Developmental biology, 5th edn. Sunderland: Sinauer Associates, 1997.
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