Structural Biology: Researchers Get First Glimpse Of How An Animal Genome Is Packaged

The human cell nucleus is home to a phenomenal feat of packaging: Two meters of genomic DNA are wound neatly around protein spools called nucleosomes and packed into an overall parcel called chromatin.

The first peek at the 11-Å structure of a well-studied chromatin fiber comes courtesy of cryoelectron microscropy (cryoEM) and a research team led by Ping Zhu and Guohong Li at the Chinese Academy of Sciences in Beijing (Science 2014, DOI: 10.1126/science.1251413). Scientists have long argued about how DNA is packed into chromatin. This is in part because so many forms of chromatin exist, from DNA packed for long-term storage to gene sequences that must be regularly accessed by cellular transcription machinery, comments Andrew Travers at the MRC Laboratory of Molecular Biology in Cambridge, England.

The Chinese team solved the structure of a chromatin form found in certain chicken and mammalian cells that no longer divide or differentiate. They found that DNA (green in schematic at left) winds around each nucleosome (colored cylinders) twice and that three sets of four nucleosomes (blue, orange, purple) stack tightly together in the cryoEM structure (right). The structure could better explain how epigenetic marks such as methylation and acetylation modify chromatin structure and thus gene expression, Travers adds.