The DNA in the 23 pairs of chromosomes in each of the billions of cells of the human body is so tightly packed that it would measure six feet in length if stretched end to end. A genome of this size can squeeze into a cell's tiny nucleus because it is compressed into highly condensed chromatin fibers by proteins called histones.
All chromatin in the cell nucleus represents a massive condensation of the genetic material. But a portion of it might well be called super-condensed; it forms a kind of chromatin called heterochromatin. The genes contained within these portions of the genome are effectively "silenced" because they cannot be accessed by the cell's DNA-activating machinery. These "hidden" parts of the genome also include highly repetitive, gene-poor, regions. Some of these, if unpacked, would set loose DNA sequences that act like parasites able to jump around to other areas, sometimes randomly, unleashing genetic chaos.
To assemble heterochromatin, numerous molecules participate in an elaborate series of maneuvers that have gradually come to light. "But scientists have been a little hazy on the initial steps and requirements that get this process going," says Professor Leemor Joshua-Tor, Ph.D., of Cold Spring Harbor Laboratory (CSHL). She and her research team have now brought this process into sharper focus by identifying a critical requirement for heterochromatin to be established in the nucleus.
In a report that appears online on April 9th in the journal Molecular Cell, they show that the assembly of heterochromatin depends on the strength with which a protein called Chp1 binds to a specific target site located on a histone protein that has attached to the double helix.
RNAi's role in heterochromatin formation
In a typical chromosome -- roughly the shape of a bow-tie heterochromatin is concentrated at (and supports the structure of) the centromere, the bow's central "knot." Since cen
|Contact: Hema Bashyam|
Cold Spring Harbor Laboratory