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:

Uracil is one of the four nitrogen - containing bases in RNA. DNA contains thymine instead of uracil. So, if uracil is present in a nucleic acid strand, it can be used as a clue to identify that strand as RNA.

• Option 1: Uracil is not found in DNA (except in some cases of DNA damage where it can be a result of cytosine deamination, but not as a normal base). So this option is incorrect.
• Option 2: While uracil can be present in a damaged DNA strand (due to cytosine deamination), this is not the main reason it is used as a clue for RNA. RNA has uracil as a normal base. So this option is not the best answer.
• Option 3: Uracil pairs with adenine in RNA (similar to thymine pairing with adenine in DNA). There is no pairing of uracil with thymine. So this option is incorrect.
• Option 4: Since uracil is a unique base to RNA (DNA has thymine), it can be used as a clue to identify RNA. This is the correct reason.

Answer:

Because uracil is unique to RNA.