GREAT BARRINGTON, MA—A new study from Bard College at Simon's Rock examining how chromosome size and shape vary between species finds that, on average, chromosomes with more DNA are both longer and wider, with chromosome width increasing like the square root of chromosome length.
The research demonstrates a novel scaling which appears to be the same for both plants and animals. The findings appear in the May issue of Frontiers in Cell and Developmental Biology, a leading open-access journal which publishes rigorously peer-reviewed research on the fundamental processes of life.
Dr. Eric Kramer, faculty in physics at Simon’s Rock, and students Pailyn Tayjasanant and Bethan Cordone, set out to observe how different species’ mitotic chromosomes—X-shaped collections of DNA and proteins—change as DNA weight changes among different species
The team surveyed more than 50 years’ worth of published papers on vertebrate chromosomes and built a database of length, width, and DNA content for more than 200 animal species. They also compiled a second database for flowering plants, which show a range of chromosome sizes just as diverse as vertebrates. Next, they plotted chromosome width as a function of chromosome length to demonstrate that the amount of DNA in a chromosome correlates to its width and length, and that width grows like the square root of the length. In other words, a species with chromosomes four times as long has chromosomes about twice as wide.
When a cell is about to divide, it manufactures proteins that bind the strands of DNA in the nucleus and temporarily bundle them into dense packages that can be moved by the cell with ease. Once the chromosomes are bundled, they split down the middle and each half is rapidly transported to a new location that becomes the nucleus of a daughter cell. Meanwhile, the dividing cell monitors the zone the chromosomes are exiting. If parts of the chromosomes loiter for too long, cell division stalls. Thus, it’s critical that the chromosomes move as a tightly bound unit.
“We realized that a square root power law is exactly the scaling one might expect if chromosomes evolved to stay compact during their move into a new cell,” noted Dr. Kramer. “Viscous drag inside a cell is high, and if longer chromosomes aren't proportionately thicker, then trying to move a chromosome would be like trying to move a loosely coiled spring by pulling on one end. The chromosome would mostly just stretch, which would be bad news for the dividing cell.”
The team’s findings affirm that DNA content per unit volume is approximately constant, and that the cross-sectional area increases proportionately with chromosome length.