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System determined

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University of Guelph
BIOL 3010
Scott Schau

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 HOX located on chromosome in same order as they are expressed homeotic genes - determine Drosophila anterior/posterior fate homeotic similar to HOX genes Unfertilized Drosophila egg: MATERNAL expression bicoid - mRNA expressed at head bicoid nanos - mRNA expressed at tail nanos **both code for MORPHOGENS: substances present in a gradient morphogen specify different cell fates at different concentrations** **both are TF's** Early Drosophila embryo is a SYNCITIUM syncitium before cellularization (when cells reform), gradients form bicoid expression leads to HUNCHBACK protein txn/expression Hb 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) gap genes: kruppel (hunchback activates) kr knirps (hunchback represses) kni giant (hunchback represses) gt 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 tailless Patterning a Simple System Tailless is expressed in response to Torso-Like (made by maternal torso-like anterior and posterior follicle cells) Torso-like activates Torso (RTK) – both are maternal torso 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 dorsal 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 gurken Post-Fertilization: ventral follicle cells secrete Spatzle spatzle Spatzle binds Toll (on ventral si
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