To Save Space, Genomic DNA Is Packaged Into Chromatin

UNDER WRAPS

If stretched out and laid end-to-end, the DNA in a human cell's 46 chromosomes would span about 6 feet. Yet it fits inside of a nucleus only micrometers in diameter. Somehow, the DNA gets compacted by a factor of about 10,000.

Decades ago, researchers got their first peek at the cell's packing secret: Each chromosome is strung with thousands upon thousands of protein complexes called nucleosomes, like beads on a string. There are millions of nucleosomes in the nucleus of a human cell. Made of eight histone proteins each, these complexes provide the means for shrinking DNA down.

In an initial stage of compaction, the DNA wraps twice around each of these spoollike histone complexes. The resulting DNA-protein nucleosomes then fold in upon one another, forming a 30-nm-wide chromatin fiber. This fiber then buckles and coils, and buckles and coils again. With a few additional molecular players, the result is the familiar X- and Y-shaped chromosomes.

First named chromatin in 1882, this magnificent macromolecular structure has become the focus of many researchers who are convinced it is the central stage whose very structure helps determine which genes in a particular cell are active or silent. In recent years, scientists have been uncovering a plethora of gene-controlling chemical modifications that occur at specific locations in the vast chromatin landscape. "It is an amazing choreography," says C. David Allis, a chromatin researcher at Rockefeller University. The research field devoted to studying all of this chromatin activity, called epigenetics, is on a meteoric rise.