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Nancy Kleckner wants to know how matching chromosomes find each other and exchange bits of information. Abby Dernburg uses microscopy to follow meiosis in the roundworm C. elegans.
Most cells in the human body boast two copies of 23 chromosomes—one copy from Mom and the other from Dad. If both parents passed along an entire set of genes, their children would have four copies of each chromosome, and chromosomes are one case where there's no point in being greedy: even one extra chromosome in a cell can kill it. So germ cells undergo meiosis to ensure just one version of each chromosome—and it's not a blueprint of either chromosome found in other cells in the body, but a mix of the two.
The first step in the programmed dance of meiosis is to copy each chromosome pair. The original two—one from Mom plus one from Dad—are considered homologues: they are similar in makeup but have different versions of some genes. The new copies of each are sister chromatids—identical Xeroxes. As each copy is made, it remains tightly bound to its sister by proteins that embrace both chromatids along their length.
But then, the cell does something daring: it breaks the DNA at various points along the paired sister chromatids. “This is a really dangerous game for the cell to be playing, breaking its own DNA,” says Keeney, whose research focuses on how and where these breaks are made. The unexpected action, he says, forces the attached sister chromatids to seek out the only source available to find the missing data—their homologous chromosomes.
In 1997, Keeney made a grand entrance into the meiosis field, while a postdoc in Kleckner's lab, by discovering the protein that makes these breaks in yeast chromosomes. Kleckner was one of the first to study how so-called double-strand breaks in DNA led to crossovers of genetic material between homologous chromosomes. In most cells that her lab group observed, the double-strand breaks seemed to come and go during meiosis. That transience made it hard to detail their collective number and locations. But in one yeast mutant, the breaks just seemed to collect.
“That observation was kind of apocryphal lore actually,” says Keeney. “No one followed up on it, but then I came into the lab and attacked it.” Keeney's background studying proteins that bind to, and cut, DNA during mitosis gave him an idea: perhaps the protein responsible for double-strand breaks in the Kleckner lab mutant was bound to each break, preventing it from being resealed. He—as well as researchers in two other labs working on the problem at the same time—showed that was, in fact, the case. Keeney then isolated the protein, dubbed Spo11. He's still trying to answer one of the most basic questions about its function: where does Spo11 cut?
“Where you exchange bits of DNA is going to affect the evolution of the genome from one generation to the next, so understanding how it works is really a fundamental problem in biology,” he says. Keeney and others took advantage of the fact that Spo11 could be found at each double-strand break in the mutant to map out where those breaks occurred on a whole-genome basis. “But the special resolution of these maps is still fairly low,” explains Keeney. “These methods are like me looking out my window without my glasses on—it's a very different picture from when I do have my glasses on.”
His latest scheme is to work with a different step in the recombination pathway—the point at which Spo11 and a short piece of the DNA where it binds are cut off at a break site. Because Spo11 remains attached to the bit that's released, Keeney can collect and scan hundreds of thousands of the bits to figure out where they came from—and therefore where Spo11 was cutting. The resulting maps are two to three orders of magnitude higher resolution than older versions.
Photos: Kleckner: Matt Kalinowski, Dernburg: Noah Berger/ AP ©HHMI