Final answer:
In the context of DNA methylation, the nucleotide that is predominantly methylated is cytosine. If 30% of the bases are adenine, then thymine would also be at 30%, as they are complementary bases that pair together. The statement that adenine binds to guanine is incorrect; adenine binds to thymine.
Step-by-step explanation:
DNA Methylation and Base Pairing
DNA is methylated on certain nucleotides in the genome. Specifically, methylation occurs predominantly on the cytosine nucleotide within the context of a cytosine-guanine (CpG) dinucleotide in eukaryotic DNA. This methylation plays a critical role in gene regulation and the organization of chromatin within the cell. In contrast, adenine may also be methylated in some organisms at the N6 position, but this is less common in mammals.
Complementary Base Pairing Rules
According to the rules of complementary base pairing in DNA, if 30% of the bases within a DNA molecule are adenine (A), then the percentage of thymine (T) will also be 30%, as adenine and thymine pair together. Therefore, the correct answer to the student's question would be C. 30%. As for the base pairing rule that is incorrect, the statement 'Adenine binds to guanine' is false because adenine always pairs with thymine, while guanine pairs with cytosine, following the principle that purines pair with pyrimidines. Thus, the correct base pairing pattern upholds the stability and integrity of the DNA double helix.