Which nitrogenous base is not present in $RNA$?

Assertion: In a $DNA$ molecule,$A-T$ rich parts melt before $G-C$ rich parts.
Reason: In between $A$ and $T$ there are three $H-$ bonds,whereas in between $G$ and $C$ there are two $H-$ bonds.

Genetic information is carried by long-chain molecules made up of

When was $DNA$ first discovered?

The heterochromatin part of chromosome is . . . . . . times more rich in $DNA$ than euchromatin.

What are the full forms of $mRNA$ and $tRNA$?