Why do genes in Arabidopsis have more C and G nucleotides than intergenic regions?

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Q. A recent study of the mouse-ear plant Arabidopsis thaliana found that 44.9% of the DNA nucleotides within genes are either C or G (cytosine or guanine), but those same two bases represent only 37.6% of the nucleotides in the regions between genes. Clearly explain why chromosomal regions that do not code for proteins might experience natural selection for reducing the number of C and G nucleotides. Then propose a specific, testable evolutionary hypothesis for why regions that do code for proteins might have a greater number of C and G nucleotides.


I don't really understand what my teacher wants me to say here. This is for my freshman biology class. It's strange because I've taken Bio before in high school, but this seems a bit awkward to me. Any help would be greatly appreciated. I'm going to try and work this one out as well as a few others.
 
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Your professor wants you to explain why is it that the G and C nucleotides appear more frequently in coding regions than non-coding regions of the DNA molecule in arabidopsis thaliana and then propose an experiment to test your explanation.
 
Just to get you started

For the gene/coding region, think of the elements that are at play during translation. The phenomenon is also well documented and doing some web search may yield some answer.

For non-coding region, think of the elements that are required for regulation of expression.
 
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