The relationship between chromatin structure and transcriptional activity in mammalian genomes

In cells, chromatin is folded into a 30 nm fibre. Recent genome-wide studies have shown that DNaseI-sensitive sites are present in both transcribed and non-transcribed genes and are enriched in the gene-dense regions of the human genome. The distribution of open chromatin has also been shown to correlate with gene density rather than transcription. In this review it is suggested that open chromatin corresponds to a 30 nm fibre interspersed with discontinuities, and that blocks of open chromatin might facilitate gene transcription, but are neither necessary nor sufficient. The nature of these discontinuities is not known but could correspond to alterations in chromatin fibre structure caused by irregular nucleosome positioning, nucleosome remodelling activities, variant histones or the binding of specific transcription factors.