Molecular - gap genes etc Flashcards
(26 cards)
Timeline
Nuclei divide and distribute around cell, but still one cell. 1.25 hrs = 128 nuclei, 2hrs 1500, 2.5 = 5000 - Cellular blastoderm, then cellularization/division.
Between fertilization and cellularization what is happening?
2.5hrs for maternal effect gradient to be read, requires cascade of transcription factors.
Maternal effect genes
Nanos at post., bicoid at ant (concentration gradients). Hunchback and caudal throughout, but hunchback repressed by nanos and caudal repressed by bicoid.
What helps anchoring?
exuperantia for bicoid, oskar tudor and vasa for nanos
Third maternal genes set
Terminal group including torso develops structures at both extremes (antennae, anus)
Second set: Gap genes
Some of first embryonic genes to be expressed. They are TFs. Mutations result in gaps in segmentation. More complicated gradients.
Examples of gap genes
Tailless, huckebein, buttonhead, empty spiracles, kruppel
How are gap genes turned on?
Respond to bicoid, caudal, hunchback, torso, and others, which are binding to the promotor regions of the gap genes.
Gap gene regulation
Affected by enhancers and enhancer synergy. Ex. 2 enhancers are important in the expression of each of hunchback, knirps and kruppel
Pair-rule genes
Found in alternating parasegments, post half of one future segment and anterior half of next (ie offset). This is post-cellular blastoderm expression. They respond to the gap genes.
Examples of pair-rule genes
Even-skipped and fushi-tarazu
How is even-skipped set up?
Coding region with enhancer elements up (3, 2+7) and downstream (4+6, 1 and 5). Each affects a stripe or two
Even-skipped expression experiment
Formed constructs with response elements in front of lacZ which causes blue precipitate so we can see where that response element is activated normally. Different elements are individually responsible for one to two stripes.
Even-skipped and gap genes
Activated by hunchback, inhibited by kruppel, giant and knirps. Eve is expressed where [inhibitors] is lowest
Even-skipped 2nd stripe
Hunchback binds on top, kruppel and giant on bottom of enhancer. Each stripe is a sum of the competition of the activators and repressors for the response elements.
Promotors of pair-rule genes
Complex - different activators or repressors bind to the enhancers around the promotor, so very regulated. Determines which stripe is expressed where
Penultimate set - segment polarity genes
Set up the polarity of the parasegment. Ant end will have high hedgehog and the post end with have high wingless. Positive feedback loop b/w these two gives post. polarity to segment
Engrailed
Segment polarity gene. Engrailed is activated by high levels of even-skip or fushi tarazu; it is a TF and induces hedgehog.
Hedgehog
Induced by engrailed. Activates cells anterior to it (which are in fact the post end of the next parasegment) and bind to patched there.
Wingless
wingless is never expressed if neither eve or ftz at high []. Wingless is secreted, binds frizzled, which starts engrailed expression and induces hedgehog.
Final set - homeotic genes
Determine differentiation pattern. Identified by phenotype from a mutation in these. Code for TF which activate cascades of genes
How do we identify homeotic genes?
Look at homeobox (60 aa), highly conserved
Homeotic complexes in drosophila
Two clusters of genes on same chromosomes: Antennapedia (lab, pb, Dfd, scr, Antp) and Bithorax (Ubx, abdA, AbdB)
Homeotic gene expressions
Position in the genome matches expression in embryo, spatially and temporally. Results from reading the concentrations of other genes.
