1. Chapter 16 The Genetic Basis of Development

2. Determination is a multistep process Pluripotent embryonic cells MesodermEctoderm Nerve cells Skin cells Determination Determination is due to the “permanent” repression of specific genes. This involves:  Chromatin structure modifications  DNA methylation

3. Repress genes by packaging them into inaccessible chromatin. Fig. 2-22 More condensed Less condensed Nucleosome

4. Drosophila embryogenesis 3 h 10 h 22 h Cellular blastoderm Segmented embryo 1st instar larva pole cells (germ line) AnteriorPosterior

5. bicoid (bcd) mRNA Protein A P Translated following fertilization forms gradient of bcd protein by diffusion. Establishing positional information along anterior-posterior axis Bcd protein = TF Maternally transcribed bcd mRNA localized at anterior end of oocyte.

6. Bcd regulates txn of Gap genes (zygotically expressed genes) High levels of Bcd activates txn of hb High levels of Bcd represses txn of Kr Slightly lower levels of Bcd are able to repress txn of kni Hunchback (hb) Kruppel (Kr) Knirps (kni)