You do an experiment to study the sonic hedgehog locus in the zebrafish (as a model organism that should provide some insight into how shh works in humans as well). In your experiment, you isolate two tissues from developing zebrafish embryos: a structure called the notochord and the dorsal part of the midbrain. You extract chromatin from each tissue and incubate it with DNase I. You find that the chromatin from notochord is more sensitive to DNase I activity than the dorsal midbrain tissue. What might explain this difference?

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