DNA Sequences That Might Act as Enhancers (Regulatory Elements) Can

DNA sequences that might act as enhancers (regulatory elements) can be attached to the gene for green fluorescent protein (GFP) and the combined DNA sequence can be reintroduced into an organism. If the DNA sequence attached to GFP actually can act as an enhancer, glowing green pigment will be observed in particular regions of the organism. Gene A is expressed in developing nerve cells, muscle cells, and intestinal cells in the fruit fly embryo. The region of DNA around gene A is depicted below. The pieces of DNA indicated with lines 1-5 were attached to GFP and tested for the ability to activate gene expression in the fly embryo. In piece 5, the "X" indicates a deletion of a single base pair. $$\begin{array} { | l | l | } \hline \text { DNA } & { \text { GFP Expression } } \\\hline \text { Piece 1 } & \text { Muscle cells } \\\hline \text { Piece 2 } & \text { Nerve cells, intestinal cells } \\\hline \text { Piece 3 } & \text { No expression } \\\hline \text { Piece 4 } & \text { Nerve cells, intestinal cells } \\\hline \text { Piece 5 } & \text { Intestinal cells } \\\hline\end{array}$$ a. Based on the pattern of GFP expression when attached to either piece 1 or piece 2, propose a model for how gene A is activated in the different cells of the embryo.
b. Propose an explanation for the difference in GFP expression caused by piece 3 and piece 4.
c. What has the deletion "X" in piece 5 likely affected to cause the change of GFP expression compared to piece 2? How does this result refine the model proposed in (a)?