$tRNA$ is composed of about how many nucleotides?

Thymine is also known as . . . . . . .

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.

Which of the following bases is present in $RNA$ instead of $Thymine$ found in $DNA$?

$DNA$ exists in a double-stranded form whereas $RNA$ is mainly a single-stranded molecule. What is the likely reason for $DNA$ being double-stranded?

In $DNA$,the two polynucleotide chains are . . . . . . .