The approximate shape of the chromatin subunit called the nucleosome is now known, but its internal architecture is not well understood. Recent studies reveal details of the organisation of DNA within the nucleosome, and show that the arginine-rich histones are essential to DNA folding. Nucleosomes or structures related to them seem to be present at points of DNA replication and transcription; interactions within and between nucleosomes are likely to play a critical part in these processes.