Nuclei were isolated from brain, liver, and muscle. The nuclei were then incubated with $\alpha -\left[{}^{32}\mathrm{P}\right]\mathrm{UTP}$ under conditions that allow RNA synthesis, except that an inhibitor of RNA initiation was present. The radioactive RNA was isolated and annealed to various DNA sequences that had been attached to a gene chip. In the adjoining graphs, the intensity of the shading indicates roughly how much mRNA was attached to each RNA sequence.(a) Why does the intensity of hybridization differ between genes?(b) What is the significance of the fact that some of the RNA molecules display different hybridization patterns in different tissues?(c) Some genes are expressed in all three tissues. What would you guess is the nature of these genes?(d) Suggest a reason why an initiation inhibitor was included in the reaction mixture.

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