Final answer:
Histone proteins organize DNA into nucleosomes, which are the fundamental repeating units of chromatin. Nucleosomes act as beads on a string, which are the initial steps towards DNA compaction within the nucleus.
Step-by-step explanation:
Histone proteins pack DNA into a repeating array of DNA-protein particles called nucleosomes. These structures are fundamental to the compaction of DNA within the eukaryotic nucleus. DNA wraps around an octamer of histone proteins forming a structure that resembles "beads on a string". This histone-DNA complex can further fold into higher order structures, with nucleosomes acting as the first and most basic level of chromatin organization.
Heterochromatin and euchromatin are two distinct forms of chromatin. Heterochromatin is highly condensed and typically transcriptionally inactive, while euchromatin is less condensed and usually contains genes that are being transcribed.
The nucleolus is the part of the nucleus where ribosomal RNA is synthesized and ribosomal subunits are assembled, and it is not directly involved in the packing of DNA.