chromatin
/ KROH-muh-tin /
When DNA is just sitting in the nucleus doing its everyday work — being read, copied, repaired — it isn't a bare thread and it isn't yet a tidy X-shaped chromosome. It exists as a fluffy, tangled mix of DNA wound around proteins, a bit like thread loosely wrapped around hundreds of tiny spools. That DNA-plus-protein material is called chromatin. The name comes from the fact that it readily soaks up dyes, making it visible under a microscope.
Chromatin is the working form of the genome inside a non-dividing cell. It is built from DNA wrapped around protein cores (histones) to form repeating beads called nucleosomes, which then fold and coil to different degrees. Where the chromatin is loosely packed, genes are accessible and can be switched on; where it is tightly wound, the DNA is buried and effectively silenced. A chromosome, then, is chromatin condensed to its maximum — the same material, just packed for transport during cell division.
This is a profound idea: by changing how tightly chromatin is packed, a cell controls which parts of its identical genome it actually uses. That is how a liver cell and a nerve cell, carrying exactly the same DNA, become so different. Chemical tags added to the DNA and to histones loosen or tighten the packing, which is the heart of epigenetics — heritable changes in gene activity that don't alter the DNA sequence itself.
Stain a non-dividing cell's nucleus and you see chromatin as patchy dark and light regions — the dark, dense patches are tightly packed silent DNA, the lighter areas are loosely packed active DNA.
Dark patches vs light: the same DNA, packed differently.
Chromatin and chromosome are not different substances — they are the same DNA-protein material at different levels of compaction, like the same yarn either loosely fluffed out or wound tight into a ball.