Final answer:
The nucleic acid with 20% guanine and 27% thymine is most likely double-stranded DNA due to the rules of complementary base pairing. The presence of thymine indicates that the molecule is not RNA, which uses uracil instead. Option A is the correct answer.
Step-by-step explanation:
If a nucleic acid is found to contain 20% G (guanine) and 27% T (thymine), the molecule is most likely double-stranded DNA. In the structure of nucleic acids, the rule of complementary base pairing suggests that in DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).
Since guanine's percentage is typically equal to cytosine's and adenine's is to thymine's in a double-stranded DNA molecule, having 20% guanine suggests that there should also be 20% cytosine. Given that there is 27% thymine, this would mean there should be 27% adenine as well. The total of G+C and A+T should add up to 100%, meaning the remaining percentage must be for adenine and cytosine, which would be 53% combined, divided equally due to base pairing rules.
However, RNA differs from DNA in a few key ways: RNA is single-stranded, contains uracil (U) instead of thymine (T), and includes ribose sugar rather than deoxyribose. It does not follow the strict complementary base pairing rule of double-stranded DNA. As a result, the presence of thymine indicates that the molecule in question is not RNA.
In conclusion, based on the given percentages of nucleotide bases, the correct option for a nucleic acid with 20% G and 27% T is most likely to be double-stranded DNA.