Which of the following bases is a purine derivative?

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

For a double strand $DNA$,one strand is given below:
$5'-A-G-T-C-A-C-G-T-A-A-G-T-C-3'$
The amount of energy required to split the double strand $DNA$ into two single strands is $.... \ kcal \ mol^{-1}$.
[Given: Average energy per $H$-bond for $A-T$ base pair $= 1.0 \ kcal \ mol^{-1}$,$G-C$ base pair $= 1.5 \ kcal \ mol^{-1}$. Ignore electrostatic repulsion between the phosphate groups.]

In nucleic acids,the nucleotides are joined together by

How many nucleotides in a sequence form a codon for an amino acid in messenger $RNA$?

$DNA$ contains a base that is not present in $RNA$. Identify it.