The double-helical structure of $DNA$ is due to the action of . . . . . . .

The function of $DNA$ in an organism is

Assertion $(A)$: Adenine and guanine are present in $RNA$ as purine bases.
Reason $(R)$: The base uracil is present in $RNA$.
The correct option among the following is:

. . . . . . nucleotide is not present in $RNA$.

The presence or absence of a hydroxyl group on which carbon atom of the sugar differentiates $RNA$ and $DNA$?

When $RNA$ is hydrolysed,there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of $RNA$?