Which structure consists of two polynucleotide chains forming a helical structure?

Provide definitions/explanations for the following terms: $Nucleosome$ and $Euchromatin$.

Study the following statements:
$(I)$ Bacteriophage lambda has $48502$ base pairs $(bp)$
$(II)$ $E. \text{ coli}$ has $4.6 \times 10^6$ $bp$
$(III)$ Haploid content of human $DNA$ is $3.3 \times 10^9$ $bp$
Find the option for the incorrect statement.

$A$ $DNA$ fragment contains $3.2$ kilobases $(kb)$. If the $DNA$ fragment contains $820$ adenine molecules,calculate the number of cytosine molecules.

What is the width (distance) between the two polynucleotide chains of $DNA$?

$A$: In a $DNA$ molecule,the proportion of purine and pyrimidine bases is equal.
$R$: In $DNA$,two purines or two pyrimidines do not pair opposite each other.