Histone

Histones are small proteins, rich in the positively charged amino acids lysine and arginine, that package DNA in the nuclei of eukaryotic cells. Their positive charge attracts the negatively charged phosphate backbone of DNA. Two copies each of four core histones, H2A, H2B, H3 and H4, form an eight-protein core, and about 150 base pairs of DNA wrap almost twice around it to make a nucleosome. Under the electron microscope, a chain of nucleosomes looks like beads on a string. A fifth histone, H1, binds the linker DNA between nucleosomes and helps coil the chain into denser chromatin.

This packing lets roughly two meters of DNA fit inside each human nucleus, but it also controls which genes can be read. Enzymes add and remove chemical tags, such as acetyl, methyl and phosphate groups, on the flexible tails of histones. Acetylation lowers the positive charge, loosening the grip on DNA and generally allowing transcription, while certain methyl marks keep chromatin tightly closed and genes silent. These changes are a central part of epigenetics, and drugs that block the enzymes removing acetyl groups are used against some lymphomas.

Histones are among the most conserved proteins known: histone H4 is almost identical in peas and cows.