Epigenetic code. Illustration of the different packaging levels of the structure of DNA (deoxyribonucleic acid), showing the chemical tags that control the epigenetic code. This is a mechanism that controls gene activity. One sort of chemical tag (purple diamonds, upper left) marks the DNA helix. Another chemical tag (purple triangles, lower left) marks the tails (light blue strands) of histone proteins. These markings determine whether genes will be transcribed by RNA polymerase. Genes hidden from RNA polymerase are not expressed. One form of DNA packaging, a chromosome, is at lower right. See C026/0513 for a labelled version of this diagram.