Which one is a purine base?

In the sugar that differentiates $RNA$ from $DNA$,the presence or absence of the $-OH$ group occurs at which carbon atom?

$A$ gene is a segment of a molecule of

The structure of $DNA$ is

$A$ segment of $DNA$ molecule which codes or specifies for one polypeptide chain is called:

Which of the following correctly represents hydrogen bonded pairs in $DNA$?