Which of the following is not found in $DNA$?

$E. coli$ bacterium possesses a single large circular $DNA$ as its genetic material. This strand codes for:

In $DNA$,how are the nucleotides linked together?

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.

What is the ratio of purine and pyrimidine bases in each molecule of $DNA$?

The link between generations is provided by: