The chromatin landscape
So why are we poor, forked creatures instead of tumbling beachballs of undifferentiated cells?
An embryo begins by dividing into identical cells, but within hours these cells begin to make genetic decisions, turning off some genes and turning on others. So the ball of cells acquires a front end and a back end, a top and a bottom, nerve cells and muscles cells all still carrying the same DNA, but DNA now packaged in such a way that some genes are shrink wrapped and silent but others are spreadeagled for easy access and active.
We've been taught that DNA is everything, but you could equally well say packaging is everything.
And in eukaryotes (higher organisms like plants and animals), DNA comes in a particularly elaborate package.
In human beings, nucleus of a cell, which is only a few micrometers in diameter, contains two meters of DNA! Trying to get in all that DNA in the nucleus is like packing a thread 10 kilometers long into a pea.
To keep things manageable, the DNA is wound around tiny beads of proteins called histones and these "beads on a string" are then coiled and folded, looped, and then coiled and folded again to "condense" the DNA into a chromosome.
The combination of DNA, histones and other proteins that make up chromosomes is called "chromatin."
The epigenetic code that determines whether genes are silenced or expressed consists of chemical modifications to the DNA, to "tails" that hang off the histones, or other packaging proteins.
"ENCODE and modENCODE are much more complicated projects than the Human Genome Project," Elgin says, "because the DNA sequence is pretty much the same in every cell type, whereas the chromatin structure is different in every cell type. In fact we believe it is the chromatin structure that differentiates one cell type from another.
"That means we can't just do one genome for the organism. We hav
|Contact: Diana Lutz|
Washington University in St. Louis