Final answer:
Using Chargaff's rules, if a DNA molecule is 30% cytosine, the same will be true for guanine due to base pairing. This leaves 40% of the DNA to be equally divided between adenine and thymine, resulting in 20% adenine.
Step-by-step explanation:
If a double-stranded DNA molecule is 30% cytosine (C), we must use Chargaff's rules to determine the percentage of the other nucleotides. Chargaff's rules state that DNA is made up of nucleotide bases that pair up in a specific way: adenine (A) pairs up with thymine (T), and guanine (G) pairs up with cytosine (C). Given that the DNA is double-stranded, the percentages of the bases on one strand will be the same on the complementary strand due to base pairing.
As the question informs us that cytosine count is 30%, it implies that guanine will also be 30% as they pair together. This accounts for 60% of the DNA (30% C + 30% G). The remaining 40% of the DNA will be made up of adenine and thymine, which also pair together. Therefore, since the proportions of adenine and thymine must be equal, adenine will be 20% and thymine will also be 20%, each accounting for half of the remaining DNA bases.
The correct answer is that the percentage of adenine (A) would be 20%, which corresponds to option e.