What if a mutation caused a change in the code so the message read GAG instead of GAC? How would this affect the protein produced?

There will be no effect on the protein produced

Explanation:

According to the genetic code, the triplet codon GAG code for the amino acid, glutamate, while GAC code for aspartate.

Since GAG and GAC code for amino acids with similar properties (aspartate and glutamate are both acidic amino acids due to the extra carboxyl group present in their side chain), the change in nucleotide is still similar to the original, so will result in a different amino acid sequence, but will not alter the function of the protein at all. Thus, this kind of mutation is called silent mutation.

a- all cells come from pre-existing cells