euchromatin and heterochromatin
/ YOO-kroh-muh-tin and HET-er-oh-KROH-muh-tin /
Think of a library where some books are out on open tables, easy to grab and read, while others are locked in storage where nobody can get to them. Inside the nucleus, the genome is sorted the same way. Loosely packed, readable DNA is called euchromatin; tightly packed, shut-away DNA is called heterochromatin. The cell is constantly deciding which genes belong on the open table and which go into storage.
Euchromatin is chromatin in a relaxed, open state — its genes are accessible to the machinery that reads DNA, so this is where most active genes live. Heterochromatin is densely compacted and generally silent; its DNA is wound up so tightly that the reading machinery can't reach in. Some heterochromatin is permanently locked (for example, repetitive DNA near centromeres), while other regions can switch between open and closed depending on the cell's needs. Chemical marks on the DNA and histones are what tip the balance one way or the other.
This open-versus-closed sorting is one of the main ways a cell controls its identity. Because every cell carries the same genome, what distinguishes a muscle cell from a skin cell is largely which regions are kept as open euchromatin and which are buried as heterochromatin. The pattern is even heritable through cell division, which is a cornerstone of epigenetics — and disruptions to it appear in aging and cancer.
One of the two X chromosomes in each female cell is packed into a compact heterochromatin clump called a Barr body — visibly switched off — so that females don't make a double dose of X-linked gene products.
A whole switched-off chromosome, visible as a dense clump.
Heterochromatin being 'silent' is a tendency, not an absolute rule — and the labels describe packing state, not DNA sequence; the same stretch of DNA can be euchromatic in one cell type and heterochromatic in another.