Purines of $DNA$ are represented by

Nucleotide pairs present in one turn of $DNA$ helix

Identify $X$ and $Y$

Assertion : In a $DNA$ molecule, $A-T$ rich parts melt before $G-C$ rich parts.

Reason : In between $A$ and $T$ there are three $H-$ bond, whereas in between $G$ and $C$ there are two $H-$ bonds.

The $A/U$ and $G/U$ ratio in $RNA$ is