Patterning a Simple System

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Patterning a Simple System
determined = cells cannot alter their anterior/posterior identity early in
development (though they can alter proximal distal identity)
HOX genes - determine mammalian anterior/posterior fate
located on chromosome in same order as they are expressed
homeotic genes - determine Drosophila anterior/posterior fate
similar to HOX genes
Unfertilized Drosophila egg: MATERNAL expression
bicoid - mRNA expressed at head
nanos - mRNA expressed at tail
**both code for MORPHOGENS: substances present in a gradient
specify different cell fates at different concentrations**
**both are TF's**
Early Drosophila embryo is a SYNCITIUM
before cellularization (when cells reform), gradients form
bicoid expression leads to HUNCHBACK protein txn/expression
a threshold of bicoid is needed to activate Hb txn
nanos codes for a transcriptional REPRESSOR
no nanos --> Hb is expressed throughout ENTIRE embryo
**bicoid and nanos create a gradient of Hb (which is itself a TF)**
Hunchback regulates expression of other TF's (GAP genes)
kruppel (hunchback activates)
knirps (hunchback represses)
giant (hunchback represses)
high hunchback expression --> all repressed
60% of egg length --> kruppel activated, other repressed
45% --> kruppel inactive, knirps active, giant repressed
30% --> kruppel inactive, knirps REPRESSED, giant active
0% --> kruppel inactive, knirps repressed, giant REPRESSED
what represses knirps and giant repression in absence of Hb??
embryonic TAILLESS
HOX
homeotic
bicoid
nanos
morphogen
syncitium
Hb
gap genes:
kr
kni
gt
tailless
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Patterning a Simple System
Tailless is expressed in response to Torso-Like (made by maternal
anterior and posterior follicle cells)
Torso-like activates Torso (RTK) – both are maternal
Even though Torso is found all over oocyte, only the anterior
and posterior ends are activated due to follicle cell localization
no torso like --> no head and tail in embryo once fertilized
too much TL = ubiquitous torso activation --> midsection
abnormalities
DORSAL/VENTRAL PATTERNING
due to asymmetric nuclear localization of DORSAL in blastocyst
ventral: nuclear-localized
activates twist and snail
represses decapentaplegic, tolloid, zerknult
dorsal: cytoplasmic
does not act as a TF
decapentaplegic, tolloid, zerknult are expressed
Pre-Fertilization: Gurken causes a differentiation of follicle cells
Post-Fertilization: ventral follicle cells secrete Spatzle
Spatzle binds Toll (on ventral side)
cascade activated that results in dorsal migration
***TorsoLike, Gurken, and Spatzle are involved in an EXTRACELLULAR
GRADIENT that causes an intracellular gradient***
torso-like
torso
dorsal
gurken
spatzle
toll
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