help is needed

Oren Nahum orenna at zahav.net.il
Sun Jun 9 15:16:58 EST 2002


Hi all,

I have been given an exercise and I need some help. The following are two
question on which I have to answer. If someone can help with that, or give
me some leading point that would be great.

Thanks
Oren.

1) Sonic hedgehog appears to provide the molecular basis for the morphogenic
role of the polarizing region (or zone of polarizing activity in Lodish’s
term) . But we know that the precise location of the fingers or toes of the
chick limb, along the anterior/posterior axis, occurs at specific thresholds
of the polarizing activity (recall Tickle’s experiment with the addition of
graded numbers of polarizing cells). How can a gradient of a morphogenetic
signal produce different responses at various threshold concentrations?
Consider the case of hedgehog as a morphogen.


2) Various morphogenetic signals can be restricted to a local region by
lipid modifications or by binding to the extracellular matrix. As a specific
example, describe how the diffusion of hedgehog is restricted.






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