Transcript Chapter 16

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)