[Chromatin transcription]

Eukaryotic genome is organized in DNA--histone complexes called chromatin. The nucleosome core (approximately 150 bp DNA tightly wrapped around the histone octamer in two superhelical coils) is a minimal unit of chromatin organization. Chromatin is designed to allow both tight DNA packaging and proper functioning and regulation of enzymes such as DNA- and RNA-polymerases. The mechanisms allowing DNA to be accessible to enzymes of DNA metabolism in regulated manner are beginning to emerge. Thus the histone octamer has a remarkable ability to survive transcription by stepping around the transcribing polymerase without ever going free in solution. Moreover, nucleosomes can present a high barrier for transcribing polymerases that dramatically decreases the rate of transcription and probably is regulated in vivo. The mechanism of transcription through nucleosomal barrier will be discussed in detail below.