"The X chromosome (bottom left) is clearly special," says Elgin. "It's got a lot of the green state, which we think is part of dosage compensation. Sex is determined differently in flies than in people. If a fly has one X chromosome, it is male. If it has two it is female. The Y chromosome exists but is not involved in sex determination.
"Because male and female flies have different numbers of copies of the X chromosome genes," Elgin says, "something must be done to adjust gene expression based on the fly's sex. That's what we think the special mark for X is all about."
Elgin is particularly interested in chromosome 4 (bottom right). This tiny chromosome is almost entirely silenced (the dark blue state), and yet, she says, there are red marks (active transcription start sites) sprinkled throughout the sea of blue. "These are 80 active genes, genes that must have some kind of mechanism for elbowing aside silencing marks at the start of transcription," she says. "We don't know how that happens, but it's one of the things we'd really like to get at. "
In bacteria, Elgin says, most of the DNA consists of genes; it actually codes for proteins. But in our genome, only a small percent of the DNA actually codes for protein. Most of it as much as 95 percent is silenced to some degree, expressed at a low level if at all. Much of this DNA appears to be derived from retroviruses, devious viruses able to convert their RNA to DNA and insert it in our genome, or from DNA transposons, small bits of DNA that can move around in the genome.
"We know that silencing this DNA is essential for maintaining the stability of the genome," Elgin says. When bits of it lose their packaging and move about they can cause devastating diseases, such as some forms of muscular dystrophy. Conversely if the silencing mechanism is tricked into making a mistake and shuts down an important gene, the results can be equally detrimental; this is apparently
|Contact: Diana Lutz|
Washington University in St. Louis