nucleosomes and histones
/ NOO-klee-oh-sohm and HISS-tohn /
To keep two meters of thread from becoming a hopeless tangle, you could wind it around a series of small spools spaced along its length. The cell does exactly this. The spools are clusters of proteins called histones, and the bit of DNA wound around one spool plus the spool itself is called a nucleosome. Under a microscope a stretched-out chromatin fiber looks like beads on a string — each bead is a nucleosome.
More precisely, a nucleosome is a core of eight histone proteins with about 147 base-pairs of DNA wrapped almost twice around it. Short linker stretches of DNA connect one nucleosome to the next, giving the 'beads on a string' look. This is the first and most basic level of DNA packing, and it already compacts the DNA several-fold. Histones are unusual proteins: they carry a positive charge, which lets them grip the negatively charged DNA backbone tightly.
Nucleosomes are not just passive packing material — they are also a control system. The histones have tails that stick out and can be decorated with chemical tags; these tags loosen or tighten the wrap and signal whether nearby genes should be read. A gene wrapped tightly on a nucleosome is hard to access and tends to be off; loosen the wrap and it can switch on. This makes the humble nucleosome a central player in gene regulation and epigenetics.
Histones are among the most conserved proteins in all of life — a histone from a pea plant and one from a cow differ by only a couple of amino acids, a sign of how essential their packing job is.
So vital that evolution barely changed them in a billion years.
Bacteria generally lack histones and don't form classic nucleosomes; nucleosomal packing is a defining feature of eukaryotes (and of some archaea), which is one reason eukaryotic gene control is so much more layered.