Which of the following nitrogen bases of nucleic acids is derived from purine?

Which of the following is not present in a nucleotide?

What is the difference between a nucleoside and a nucleotide?

Which from the following nitrogen bases of nucleic acids is derived from purine?

Which base is present in $RNA$ but not in $DNA$?

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.]