DNA transcription. Illustration of the process of transcription of deoxyribonucleic acid (DNA, blue) to produce a complementary copy of ribonucleic acid (RNA). This is done by the enzyme RNA polymerase (yellow, lower right). Other components of the initiation complex shown here include basal factors and coactivators (orange, lower left), and activator and repressor proteins (red, upper left). The sequences of DNA to which these bind are called enhancers and silencers respectively. The presence of such regulatory proteins in different tissues can explain tissue-specific gene expression. For this artwork with labels, see C023/8878.