Kleckner too, thinks mechanical forces are likely to be important during meiosis. In a June 2008 Cell paper, Kleckner presented evidence that chromosomal movement within the nucleus during the intermediate stages of meiosis is in part due to chromosomes' connections with actin—molecules that form connective cables in the cytoskeleton of a cell.

“Obviously there has to be some movement,” says Kleckner. “The question is how much of it is directed by the cytoskeleton.”

		Meiosis in C. elegans cells Abby Dernburg's microscopy reveals the steps of meiosis. Photos: Dernburg lab

In both worms and yeast, and humans too, when a chromosome finally snuggles against its match, the two use each other as templates to fix their breaks, and they exchange other parts of DNA. What results are chromosomes that ideally (for evolutionary benefit) don't look exactly like either parent's chromosome, but have some genes from each.

After this exchange, the first division of meiosis occurs—sectioning off pairs of chromosomes into different daughter cells. Later, a second division—much less mysterious, as it resembles the well-researched sole division in mitosis—splits the sister chromatids apart. Four sets of distinct genetic material are sealed in four separate cells. The dance is winding down.

As a dance, though, meiosis is full of missteps. In fact, says HHMI investigator Angelika Amon, that's what makes it so fascinating to study. “We're looking at evolution still in progress,” she says.

In humans, almost one-third of the eggs produced by meiosis have the wrong number of chromosomes. In yeast, only one in ten thousand products of meiosis has the wrong number of chromosomes; in flies it is less than one in a thousand, and in mice one in a hundred.

In humans—females particularly—this error rate grows dramatically with age, and Amon, a cell biologist at the Massachusetts Institute of Technology, wants to know why. Her lab has recently shown that old yeast cells don't undergo meiosis very well either, and she's just starting to find the molecules responsible.

“Once a cell decides to undergo meiosis, certain genes are turned on to control the process,” says Amon. One yeast gene, called IME1, plays a notably large role. “What we've found is that old yeast cells don't enter the meiotic program any more, and they don't express IME1 any more.”

In human females, meiosis doesn't happen all at once, and researchers think that may lead to the problems with age. Eggs go through the first half of meiosis while a female embryo is developing; then they rest in the ovary in an inactive state. After a girl reaches puberty, one of these eggs is released each month during ovulation. The egg completes meiosis fully only if it's fertilized by a sperm.

“So when you're close to menopause, these eggs that you're ovulating have been sitting in this inactive state for 50 years,” says Amon. “It's no wonder they're getting worn out!”

Amon's broader research focuses on how the cell coordinates all the stages of meiosis. How does it know when to move from one stage to the next? What signals are important?

Her findings, and those of other modern meiosis researchers, have made the original terminology for the phases of meiosis largely irrelevant. Today, researchers talk about synapsis, pairing, and recombination more often than leptotene, zygotene, and pachytene—after all, they now know the molecular detail behind “thin threads,” “paired threads,” and “thick threads.” No matter what the steps are called, the same chromosomal dance seen a century ago still plays out in the cells of organisms large and small, with scientists still learning the moves.

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