Scientists at EMBL have captured how human chromosomes fold into their signature rod form throughout cell division, utilizing a groundbreaking technique referred to as LoopTrace.
By observing overlapping DNA loops forming in excessive decision, they revealed that enormous loops kind first, adopted by nested smaller loops, all repelling one another into compact buildings. This new perception not solely reshapes our understanding of chromosome mechanics however may additionally assist clarify errors that result in most cancers and genetic problems.
The Thriller of Chromosome Division
One of the vital exceptional skills of dwelling cells is their capability to divide, permitting organisms to develop, heal, and renew themselves. To do that, a cell should first make an actual copy of its DNA, its genome, and guarantee every daughter cell receives a whole set.
In people, which means fastidiously packaging 46 chromosomes and distributing them equally. Earlier than division, every chromosome transforms right into a compact, X-shaped construction fabricated from two an identical, rod-like copies. However precisely how cells handle to reshape and manage their DNA for this course of has remained a thriller.
Now, for the primary time, scientists at EMBL have straight visualized this course of in excessive decision utilizing a brand new chromatin tracing approach. Their examine reveals that in cell division, the lengthy strands of DNA kind a collection of overlapping loops that push away from each other. This repulsion causes the loops to stack, in the end giving every chromosome its attribute rod-like form.
Looping DNA to Form Chromosomes
Scientists have lengthy hypothesized the significance of DNA loops in constructing and sustaining chromosomal construction. First recognized within the Nineteen Nineties, condensins are giant protein complexes that bind DNA throughout cell division and extrude it to create loops of various sizes. Earlier research from EMBL have make clear the structural mechanics of this course of and their important position in packing chromosomes into kinds that may be simply moved between cells.
Actually, mutations in condensin construction can lead to extreme chromosome segregation defects and result in cell loss of life, most cancers formation, or uncommon developmental problems referred to as ‘condensinopathies’.
Fixing the DNA Imaging Drawback
“Nonetheless, observing how this looping course of happens on the mobile scale and contributes to chromosome construction is difficult,” mentioned Andreas Brunner, postdoc in EMBL Heidelberg’s Ellenberg Group and a lead creator of the brand new paper. “It’s because strategies for visualizing DNA with excessive decision are normally chemically harsh and require excessive temperatures, which collectively disrupt the native construction of DNA.”
Kai Beckwith, a former postdoc within the Ellenberg Group and presently an affiliate professor on the Norwegian College of Science and Expertise (NTNU), got down to resolve this drawback. Beckwith and colleagues used a technique to softly take away one strand of DNA in cells at numerous phases of cell division, retaining the chromosome construction intact. They might then use focused units of DNA-binding labels to watch the nanoscale group of this uncovered DNA strand. This system, referred to as LoopTrace, helped the researchers straight observe DNA in dividing cells because it progressively fashioned loops and folds.
“Andreas and I had been now in a position to visualize the construction of chromosomes as they began to alter form,” mentioned Beckwith. “This was essential for understanding how the DNA was folded by the condensin complexes.”
Nested Loops and DNA Compaction
From their information, the scientists realized that in cell division, DNA kinds loops in two phases. First, it kinds steady giant loops, which then subdivide into smaller, short-lived nested loops, rising the compaction at every stage. Two varieties of condensin protein complexes allow this course of.
To grasp how this looping ultimately provides rise to rod-shaped chromosomes, the researchers constructed a computational mannequin primarily based on two easy assumptions. First, as noticed, DNA kinds overlapping loops – first giant after which small – throughout its size with the assistance of Condensins. Second, these loops repel one another resulting from their construction and the chemistry of DNA. When the scientists fed these two assumptions into their mannequin, they discovered that this was ample to present rise to a rod-shaped chromosome construction.
Overlapping Loops Are Key
“We realized that these condensin-driven loops are a lot bigger than beforehand thought, and that it was crucial that the big loops overlap to a big extent,” mentioned Beckwith. “Solely these options allowed us to recapitulate the native construction of mitotic chromosomes in our mannequin and perceive how they are often segregated throughout cell division.”
Sooner or later, the researchers plan to review this course of in additional element, particularly to grasp how further elements, resembling molecular regulators, have an effect on this compaction course of. In 2024, Jan Ellenberg and his staff obtained funding of €3.1 million (~$3.4 million) as an ERC Superior Grant, to review the folding ideas of chromosomes throughout and following cell division.
A Milestone for Chromosome Biology
“Our latest paper revealed within the scientific journal Cell marks a milestone in our understanding of how the cell is ready to pack chromosomes for his or her correct segregation into daughter cells,” mentioned Jan Ellenberg, Senior Scientist at EMBL Heidelberg. “Will probably be the idea to grasp the molecular mechanism of rescaling the genome for devoted inheritance and thus rationally predict how errors on this course of that underlie human illness might be prevented sooner or later.”
Within the meantime, a second examine from the Ellenberg Staff, led by Andreas Brunner and not too long ago revealed within the Journal of Cell Biology, reveals that the nested loop mechanism is prime to the biology of cells, and continues in the course of the cell’s progress section with one other household of DNA loop forming protein complexes, referred to as cohesins.
Looping Mechanisms Throughout Cell Phases
“We had been stunned to search out that the identical core precept of sequential and hierarchical DNA loop formation is used to both tightly pack chromosomes throughout division into safely movable entities, or to unpack them afterward to learn out the data they include,” mentioned Ellenberg. “In the long run, small, however key mechanistic variations, such because the non-overlapping nature of cohesin-driven loops in comparison with the strongly overlapping condensin-driven loops could be ample to clarify the huge variations that we see within the form the genome takes in interphase and mitosis below the microscope.”
References:
Reference: “Nanoscale DNA tracing reveals the self-organization mechanism of mitotic chromosomes” by Kai Sandvold Beckwith, Andreas Brunner, Natalia Rosalia Morero, Ralf Jungmann and Jan Ellenberg, 24 March 2025, Cell.
DOI: 10.1016/j.cell.2025.02.028
“Quantitative imaging of loop extruders rebuilding interphase genome structure after mitosis” by Andreas Brunner, Natalia Rosalía Morero, Wanlu Zhang, M. Julius Hossain, Marko Lampe, Hannah Pflaumer, Aliaksandr Halavatyi, Jan-Michael Peters, Kai S. Beckwith and Jan Ellenberg, 9 January 2025, Journal of Cell Biology.
DOI: 10.1083/jcb.202405169