This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Almost every cell in our body contains around two meters (6.6 feet) of DNA, packed into a tiny nucleus. Yet for the cell to function, this enormous amount of genetic material must remain organized and accessible.
Researchers at the University of Basel have now discovered three distinct types of large chromosomal domains that help organize DNA in nuclei. It is hard to imagine how two meters (6.6 feet) of DNA can be squeezed into a tiny cell nucleus—analogous to packing a fishing line stretching across the length of Switzerland into an Emmentaler cheese wheel. And all this without becoming hopelessly tangled.
Yet almost every cell in our body accomplishes this feat of packaging. But DNA cannot simply be crammed into the nucleus. Genes must remain accessible when needed, and the genetic material must be copied and repaired.
"We know that chromosomes are not randomly arranged within the nucleus," explains professor Susan Mango. "But for a long time, there was a gap between what we could see under the microscope and what molecular studies told us about how DNA is organized." While microscopy revealed distinct structures resembling small neighborhoods, molecular studies identified two major organizational principles: topologically associating domains (TADs), in which neighboring stretches of DNA frequently interact, and larger compartments in which TADs are grouped together. The researchers investigated how chromosomes are organized during early embryonic development in the roundworm C. elegans.
The key was looking at individual DNA molecules one by one. Previous studies had suggested that, unlike in vertebrates, TADs were largely absent from C. elegans. However, these studies relied on measurements averaged across thousands of DNA molecules, potentially obscuring individual structures.
First author Dania Camila Pulido-Barrera therefore took a different approach. Using a microscopy-based technique called chromatin tracing, she examined individual DNA molecules and grouped them according to their three-dimensional folding patterns. "When you average thousands of DNA molecules together, important differences can disappear," explains Pulido-Barrera.
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