Chapter 2 part 2 Flashcards
What activates zygotic transcription of Hunchback?
Bicoid, therefor zygotic hunchback will be localized in the anterior portion of the embryo
Why is zygotic hunchback referred to as a master gap gene?
Because hunchback will collaborate with maternal genes to specify the expression of other gap genes.
Why are gap genes called gap genes?
Because their absence will leave fairly large gaps in the embryo. They specify wide regions.
It is said that the domain of gap gene expression overlaps with the domain of phenotypic expression. What does this mean?
It means that the area expressing the gene, is getting the phenotype from the gene. It is a direct cause and effect relationship.
What are denticles?
What are denticles used for?
Bands which are used as landmarks on the ventral surface the embryo. You can tell which regions disappear based on the disappearance of denticles.
Concentration of bicoid dictates the domain of hunchback expression. There therefor must be threshold. Given that bicoid is an activation of transcription of hunchback, do we see hunchback expression at concentrations above or below the concentration of the threshold.
We see expression of hunchback above the threshold. Hunchback will be seen above because the protein bicoid must either be a transcriptional activator of hunchback or activate a chain of events which activates hunchback.
How could you test to confirm that hunchback is indeed being set by bicoid concentration?
You could double bicoid amounts. You would expect and extension of the expression domain of hunchback. I could make a bicoidless mutant, I would expect no zygotic formation of hunchback, which I could test for with a beta galactosidase gene. However this would be controversial, as it is possible that bicoid expression is so important that the embryo would fail to develop to the point of hunchback expression. This however I doubt would completely override the validity of this study.
Kruppel forms a band within the center of the embryo, this band is specified by hunchback, which has a gradient from anterior to posterior. How can it specify a single band?
Hunchback is both an inhibitor and activator of kruppel. Hunchback inhibitory site must have a lower affinity then it’s activating site. It’s inhibitory site must also override thee activator. Therefore we would see that at high concentrations the weaker inhibitor site is bound too. As we go lower the poor site cannot adequately inhibit the translation of kruppel, as concentration drops further even the higher affinity binding site of the activator can no longer be activated at significant levels by it’s higher affinity binding site, and thus expression disappears. Walla! A band of kruppel expression set by a single gradient of hunchback (zygotic).
How could we check and confirm that hunchback is acting a both an activator and inhibitor of kruppel?
I could increase hunchback expression across the whole cell, which would result in the loss of kruppel. I could remove zygotic hunchback and expect to also see the loss of kruppel expression. I could create a environment were hunchback is expressed over a larger region at the level between its too regulatory sites, and I would expect to see activation of kruppel across that entire zone. I do not believe however that this proves that hunchback is not activating another pathway which is doing the regulation. But that’s ok I suppose. It would prove that kruppel expression is being solely regulated by hunchback expression.
Hunchback is a…
Transcription factor!
The embryo is divided into parasegments, how do these parasegments relate to the actual segments.
Parasegments are spaced exactly one half segment out of phase from the actual segments which will form. Parasegments are aligning with pair in rule expression.
Eve (even-skipped) and Ftz (fushi tarazu) are pair in rule genes. They form segments which can be seen 2.7 hrs after fertilization. The same segments will have sharpened by 3.5 hours after fertilization. What allows for this sharpening of expression of eve and ftz?
Eve expression activates eve (positive feedback), amplifying it’s own expression, eve also inhibits ftz expression.
Ftz expression meanwhile is activating Ftz, positive feedback, and inhibiting expression of even-skipped. So we have a war between pair in rule genes which sharpens their band formation
How is expression of different stripes of Eve or Ftz achieved on the level of gene expression?
Each stripe is activated individually, they each have their own regulatory activating gene (identified by insertion of beta-galactosidase expression in the gene. Each one will have it’s own unique combination of activators and repressor to trigger expression.
Eve stripe 2 is a classic example of how this is achieved. Explain the regulation on eve stripe two.
Bicoid and hunchback are activators (so it is an anterior located stripe), Kruppel is an inhibitor, therefor stripe two cannot appear in the middle. Giant is also an inhibitor, and it is present anterior to eve 2 stripe. Therefor we see a single stripe, located posterior to giant, anterior to kruppel and where we see both bicoid and hunchback expression.
How could we experimentally show that each part of even stripe two regulation works as supposed?
Knock out of individual pieces. For example, if I knocked out giant, I would expect stripe two to try to extend further anterior, as its inhibitor is gone. This would however mean that its expression domain would overlap with ftz, a known inhibitor, so it is debatable whether expression would be seen as expected.
Where are segmentation genes expressed?
They are expressed in segments of the parasegments created by pair in rule genes. Further dividing them, and showing were we are at in the parasegment.
Syncytial define:
Multinucleated single cell.
Segmentation genes are not expressed in a syncytial manner, what does that mean?
It means that the expression occurs on an individual cell basis. Specification does not occur during the early embryo, but later in development based off of cell to cell communication.
What is included within a single parasegment in terms of expression domains of pair in rule genes.
A single parasegment would start at the cells expressing even skipped and end right before the cells expressing ftz. The next parasegment would start at the cells expressing ftz and end right before the cells expressing eve.
