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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Forming Complex Pattern: Utilizing Positional Information to Establish Cell Fates Figure 16-26. A depiction of the hierarchical cascade that activates the elements forming the AP segmentation pattern in Drosophila. The maternally derived bcd and nos mRNAs are located at the anterior and posterior poles, respectively. Early in embryogenesis, these mRNAs are translated to produce a steep anteriorposterior gradient of BCD transcription factor. The posterioranterior gradient of NOS inhibits translation of hb-m mRNA, thereby creating a shallow anteriorposterior gradient of HB-M transcription factor (shown as an arrow). The gap genes, which are the AP cardinal genes, are activated in different parts of the embryo in response to the anteriorposterior gradients of the two factors BCD and HB-M. (The posterior band of HB-Z expression gives rise to certain internal organs, not to segments.) The correct number of segments is determined by activation of the pair-rule genes in a zebra-stripe pattern in response to the gap geneencoded transcription factors. The segment-polarity genes are then activated in response to the activities of the several pair-rule proteins, leading to further refinement of the organization within each segment. The correct identities of the segments are determined by expression of the homeotic genes due to direct regulation by the transcription factors encoded by the gap genes. (Modified from J. D. Watson, M. Gilman, J. Witkowski, and M. Zoller, Recombinant DNA, 2d ed. Copyright © 1992 by James D. Watson, Michael Gilman, Jan Witkowski, and Mark Zoller.)

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