Question

why can uracil be used as a \clue\ to identify rna in a strand?
because uracil is found in both dna and rna.
because uracil indicates a damaged dna strand.
because uracil is always paired with thymine.
because uracil is unique to rna.

Explanation:

DNA has thymine (\(T\)) as a nitrogen - containing base, while RNA has uracil (\(U\)) instead. So, if uracil is present in a nucleic acid strand, it is a strong indicator that the strand is RNA.

• Option 1: Uracil is not found in DNA (except in some cases of damaged DNA, but not in normal DNA). So this option is incorrect.
• Option 2: While uracil can be formed from damaged cytosine in DNA (cytosine deamination), this is not the main reason it is used as a clue for RNA. The key is its normal presence in RNA.
• Option 3: In RNA, uracil pairs with adenine (\(A\)), not thymine. Thymine is in DNA.
• Option 4: Uracil is a unique base to RNA (in normal, undamaged nucleic acids).

Answer:

Because uracil is unique to RNA.