It’s said that the legendary Finn MacCool created the Giant’s Causeway—thousands of steplike stones on Ireland’s northeast coast—in order to reach Scotland and thrash an upstart rival. As a child living in Dublin, I was fascinated by this myth. As an adult, I am intrigued by the true origin of this geologic formation: black basaltic rock that cracked into uniform columns after a volcanic eruption, 60 million years ago. And as a biomechanist, I am delighted that the same physical phenomena that led to the Giant’s Causeway can also explain information exchange at the opposite end of the size spectrum, across strands of DNA.
DNA does not remain quietly coiled in the nucleus of a cell. Instead, it goes through cycles of unraveling and compaction, most dynamically during the process of meiosis. Meiosis is a kind of cell division that produces gametes—sperm cells and egg cells—from germ-line cells; no topic brings on cold sweats in biology students quite like it.
Human nuclear DNA is organized into twenty-three pairs of chromosomes, one member of each pair from an egg cell of the mother and the other member from a sperm cell of the father. The genetic information in a pair of chromosomes is not identical, but (with the sole exception of the two sex chromosomes in the male) the genes on both chromosomes are homologous, being arrayed in the same order. For example, a gene for eye color occurs on both members of one of the pairs of chromosomes, but one copy might code for blue eyes and the other for brown eyes.
When a germ-line cell undergoes meiosis, the forty-six individual chromosomes perform a complicated dance in which the DNA is first replicated and, ultimately, divided up into four gamete cells—each with just twenty-three chromosomes. The part of the process immediately following replication, a part that is key to genetic diversity, is what makes the heart of the biomechanist beat faster.
After replication, chromosomal DNA and the proteins closely associated with it “condense,” or compress, and the new and old copies of each chromosome form a tightly wound X-shaped package (not to be confused with the X chromosome). Each X shape then finds its genetically similar partner. Together, the twenty-three pairs of Xs line up along a central axis of the cell.
The next step is “crossing over,” one of the hallmarks of meiosis: the pairs of Xs become entangled. A region on one chromosome (on an arm of one of the Xs) appears to cross over, and stick to, the homologous region on one of the chromosomes that make up the other X shape in the pair [see illustration below]. Wherever crossover occurs, DNA is exchanged between the two chromosomes.
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Once the genetic exchanges have taken place, the four chromosomes that make up a pair of Xs separate from one another. Thus, the chromosomes that wind up in the nucleus of a new gamete are distinct from the chromosomes in the parent cell; this outcome makes crossing over a major source of genetic variation among offspring. Later, if an egg and sperm happen to combine, the cell that results, called a zygote, contains the original complement of twenty-three pairs of chromosomes—half of every pair coming from the egg and the other half from the sperm.
Each human chromosome, on average, has about two crossover sites. Geneticists thought the location of those sites was determined by so-called chaperone molecules, or perhaps simply by random encounters. But Nancy Kleckner, a molecular biologist at Harvard University, and her colleagues have supplied convincing evidence that crossovers are also the by-products of mechanical processes.
At least one crossover is needed for proper assortment of chromosome pairs, yet the location of the crossover site is not the same in every cell that undergoes meiosis. In other words, there are no preset sites for crossovers in any given chromosome. Another odd fact of crossover distribution leads back to the Giant’s Causeway: it turns out that every crossover lowers the chances that a second crossover site occurs nearby. Something about a crossover seems to inhibit the formation of neighboring crossovers.
The Giant’s Causeway started off as a thick layer of volcanic ejecta. As the lava solidified and cooled, it contracted, but the surface layers, exposed to the air, cooled faster than the deeper layers. Hence the surface layers also contracted faster, setting up a tensile stress that could be relieved only by cracking along the surface.
Once a crack formed, though, it relieved tension in the surrounding area, preventing other cracks from forming immediately nearby. At some distance from the first crack, however, the tension would still be high enough to cause another crack to form. The differential cooling led to the hexagonal pattern of cracking visible today. Stress relief provided by a crack explains the appearance of myriad structures, from the crackling of old paint to the crevices that plague Vermont roadways.
But what generates stresses in a chromosome analogous to the stresses in cooling lava? Kleckner proposes several factors that could be acting in concert. First, during the meiotic cycle, the chromosomes condense into more tightly looped skeins of DNA. That compression could set up stresses simply by confining the DNA to a smaller space—possibly causing the DNA to stiffen as well. Second, because DNA is also replicating during meiosis, there is simply more DNA trying to fit into the constant volume of the cell nucleus. Chromosomes might be pushing against the walls of the nucleus—or against each other—so hard that they buckle under the stress.
Kleckner and her colleagues tested a theoretical model, which predicted where such mechanical stresses would cause “cracks” along a chromosome, against maps of observed crossover points—and found the model to hold true. Furthermore, micrographs of chromosomes show clear evidence of stress buildup. Some are twisted like phone cords; others have sharp flexures from buckling.
If there were not enough stress built up in a chromosome to ensure at least one crossover event, chromosome pairs would not separate properly. That possibility alone is a powerful selective force for a stressful environment. In this case it is important to crack, as they say, under pressure.