exam 2- morphogens and gradients Flashcards
a morphogen is any compound or diffusible molecule that influences the ___ and ___ of cells during ___ through the formation of ___
movement
organization
development
a concentration gradient
morphogens are important in the establishment of ___ in embryos
polarity
can morphogens be effective at very low concentrations?
yes (10^-9 to 10^-11 M)
only ___ in morphogen level is required to change gene expression and cell differentitation
small (2-3 fold) change
however, many morphogen gradients can have a large range (25-50 fold)
one morphogen can result in several…
whats needed?
different cell types or differentiations
- morphogen gradient
- multiple thresholds of response to that morphogen
in transplant experiments, tissue does what?
tissue maintains its identity based on origin
in transplant experiments, reciprocal transplants develop according to…
but tissue differentiates according to…
final positions in “donor”
its new positional instructions (new morphogen levels)
Frog to newt transplant experiments indicate that a ___ can drive expression, BUT only expression of genes that ___
morphogen gradient
the tissue contains (or of its origin)
for a diffusible molecule to be considered a morphogen, it must be demonstrated that: (2 things)
1- cells respond directly to that molecule
2- the differentiation of those cells depends on the concentration of that molecule
how does the protein nodal function as a simple morphogen?
nodal (morphogen) regulates the expression of other genes over a gradient like the french flag
nodal levels / other gene regulation:
- high = goosecoid
- medium = float head
- low = no-tail
varying nodal level alters ____
the gradient (alters other gene expression)
simple threshold model: bicoid is a ____ and regulates ___ (gene expression)
morphogen
Hunchback
Hunchback (hb) responds to the dose of ___ protein
bicoid
what concentration activates hunchback
concentration above threshold level activates expression of hb
- the more bicoid transcripts, the further back hb expression occurs in the embryo
concentrations of hunchback must be ___ to be activated, but ___ to not be repressed
above a level
below another level
kruppel gene expression needs threshold levels where?
needs threshold levels to be in the “just right” range
bicoid + hunchback –>
eve (giant & kruppel)
name the 6 mechanisms used to interpret or regulate a morphogen gradient
1- differential binding site affinity
2- combinatorial input
3- feed forward loop
4- positive feedback
5- cross repression
6- reciprocal repressor gradient
differential binding site affinity
morphogen ___ in gene promoters with diff binding affinity (low or high) can cause ________
binding sites
gene activation or repression by a morphogen
combinatorial input
additional factors, such as enhancers and repressors are needed to act in combination for gene transcription
- more than just the morphogen is required to activate
feed forward loop
a simple regulatory circuit, where the morphogen X activates gene Y (transcription factor) … in turn, the product Y functions in a combinatory input mechanism with the morphogen to activate another target gene
positive feedback is like a feed forward loop except…
the signal is not needed once its started
cross repression is repression of one ___ by ___, allowing the establishment of ___
morphogen gradient gene by a different one
differential expression (gap, pair rule)
two types of cross repression:
1- asymmetric: one gene specific represses another gene
2- symmetric: two genes, that repress each others expression
