Like all DNA-based processes, eukaryotic transcription takes place in the context of chromatin. The wrapping of 147 base pairs of DNA around an octamer of histone pro -teins (two copies each of H2A, H2B, H3 and H4) to form nucleosomes, the repeat-ing unit of chromatin, can have dramatic effects on both transcription initiation and elongation. At promoters, nucleosomes interfere with the binding of sequence-specific transcription factors that may acti -vate or repress their target genes.