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Chapter 16 The Genetic Basis of Development Determination is a multistep process Pluripotent embryonic cells Determination Mesoderm Ectoderm Determination Nerve cells Skin cells Determination is due to the “permanent” repression of specific genes. This involves: Chromatin structure modifications DNA methylation Nucleosome Repress genes by packaging them into inaccessible chromatin. Less condensed More condensed Fig. 2-22 Drosophila embryogenesis 3h Cellular blastoderm Anterior pole cells (germ line) Posterior 10 h Segmented embryo 22 h 1st instar larva Establishing positional information along anterior-posterior axis bicoid (bcd) Maternally transcribed bcd mRNA localized at anterior end of oocyte. mRNA A P Protein Translated following fertilization forms gradient of bcd protein by diffusion. Bcd protein = TF Bcd regulates txn of Gap genes (zygotically expressed genes) High levels of Bcd activates txn of hb Hunchback (hb) High levels of Bcd represses txn of Kr Kruppel (Kr) Slightly lower levels of Bcd are able to repress txn of kni Knirps (kni)