What type of bond exists between $A$ (Adenine) and $T$ (Thymine)?

$DNA$ differs from $RNA$ in:

In an experiment,$DNA$ is treated with a compound that intercalates between the stacks of nitrogenous base pairs. As a result,the distance between two consecutive base pairs increases from $0.34 \, nm$ to $0.44 \, nm$. Calculate the length of the $DNA$ double helix (which has $2 \times 10^9 \, bp$) in the presence of a saturating amount of this compound. (in $m$)

If the sequence of one strand of $DNA$ is $5^{\prime}-ATGCATCG-3^{\prime}$,find the sequence of the complementary strand in the $5^{\prime} \rightarrow 3^{\prime}$ direction.

Explain the role of the following scientists in the discovery of the structure of $DNA$:
$1.$ Watson and Crick $(1953)$
$2.$ Erwin Chargaff

In a double-stranded $DNA$,the sequence of base pairs in one strand is $AGCTAAGCC$. What is the complementary sequence on the other strand?