What are the structures called that give an appearance as 'beads-on-string' in the chromosomes when viewed under an electron microscope?

The length of one full turn of a $DNA$ helix is:

Histones are rich in

The pitch of the $B-DNA$ is

Purines found both in $DNA$ and $RNA$ are

Identify the correct matching for Euchromatin and Heterochromatin from the given statements:
$I -$ Loosely packed
$II -$ Densely packed
$III -$ Darkly stained region
$IV -$ Lightly stained region
$V -$ Transcriptionally active chromatin
$VI -$ Transcriptionally inactive chromatin
Euchromatin $\quad\quad$ Heterochromatin