Eukaryotic DNA is much longer than the nucleus that contains it, so it must be compacted while remaining accessible to proteins that copy and transcribe it. The DNA-protein material that accomplishes this is chromatin.

The basic repeating unit of chromatin is the nucleosome. A segment of double-stranded DNA wraps around a core of histone proteins, forming a compact DNA-protein particle. Linker DNA connects neighboring nucleosomes.

A useful schematic is

DNA → nucleosomes → folded chromatin → highly condensed chromosome

This hierarchy is dynamic rather than a permanent rigid packing state.

Packaging and accessibility are linked

DNA wrapped in nucleosomes is physically associated with histones and is not equally accessible at every position. Cells can reposition or alter nucleosomes so that particular DNA regions become more or less available to polymerases and regulatory proteins.

Chromatin can therefore be relatively open, with DNA more accessible, or relatively compact, with access reduced. These are local and changeable properties rather than two perfectly discrete material states.

Chromatin is not the same as a chromosome

A chromosome is a particular long DNA molecule with its associated material. Chromatin describes the DNA-protein complex from which eukaryotic chromosomes are organized. Chromosomes can become especially condensed during cell division, but DNA remains organized as chromatin even when it is less compact.

Packaging therefore solves two problems at once: it compacts the genome and provides a controllable layer of DNA accessibility. Later regulation can exploit this accessibility without changing the underlying nucleotide sequence.