Figure 1.

Chromatin components and their effect on gene expression. (A) The basic unit of chromatin is the nucleosome, which consists of 147 bp DNA wrapped around histones H2A, H2B, H3 and H4. The N terminus of histones is subject to different posttranslational modifications; (B) The levels and types of histone marks are dynamically regulated by antagonistic histone modifying enzymes (writers and erasers). Histone marks are recognized and interpreted by specific nuclear proteins (readers), resulting in the recruitment of either repressor or activator transcription regulatory complexes onto the target promoters. Some examples of repressive and activating chromatin modifications are listed in the boxed text.