Final answer:
According to Chargaff's rules, if an organism's DNA contains 18% adenine, there is an equal amount of thymine, and the remainder of the DNA is split equally between guanine and cytosine, giving us 32% guanine.
Step-by-step explanation:
If an organism's DNA contains 18% adenine nucleotides, according to Chargaff's rules, we can determine the percentage of guanine nucleotides in its DNA. Chargaff's rules state that DNA's structure is such that the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C). Therefore, if there's 18% adenine, there must also be 18% thymine due to base pairing. Because adenine and thymine together account for 36% of the DNA, the remaining 64% must be split equally between guanine and cytosine. Thus, guanine would also account for 32% of the nucleotides in this DNA sample.