Final answer:
In double-stranded DNA, if adenine makes up 20 percent of the bases, then uracil would not be present because DNA does not contain uracil. The percentage of uracil would therefore be 0 percent in DNA. However, in RNA, where uracil replaces thymine, uracil would make up 20 percent if adenine does.
Step-by-step explanation:
If adenine makes up 20 percent of the bases in a sample of double-stranded DNA, then according to Chargaff's rules, thymine would also constitute 20 percent of the bases because adenine (A) pairs with thymine (T) in DNA strands. Since uracil (U) replaces thymine in RNA, not DNA, the percentage of uracil bases in DNA would be 0 percent. However, if the question refers to RNA and the pattern of base pairing holds (where adenine pairs with uracil in RNA), then uracil would also make up 20 percent of the bases in an RNA sample.