Mitotic chromosomes, the tightly packed structures that carry genetic information during cell division, may be assembled from pre-existing chromatin "building blocks" rather than being created from scratch, according to a recent review in Trends in Genetics. Inside each human cell, about 2 meters of DNA must be precisely copied and equally divided between two new cells. To do this, the DNA is condensed into compact structures called mitotic chromosomes, but the exact process of how this condensation happens has remained a central mystery in cell biology. In the 1970s, researchers including Ulrich K. Laemmli discovered that even when proteins called histones were removed from mitotic chromosomes, a structural framework—resembling the chromosome's shape—still remained. This structure was named the "chromosome scaffold," and it suggested that non-histone proteins play a critical role in shaping chromosomes. Later studies identified specific proteins, such as condensins and topoisomerase IIα, as key components of this scaffold. These proteins are now known to be essential for organizing and shaping mitotic chromosomes during cell division. Recent research has shown that mitotic chromosomes are not simply built from regular, uniform structures of DNA. Instead, they are dynamic and irregular, shaped by a combination of molecular and physical processes. Kazuhiro Maeshima, one of the researchers involved in the review, noted that while proteins like condensins and topoisomerase IIα are important, they don’t explain the full picture. The review aims to bridge the gap between the role of these proteins and the physical properties of chromatin—the complex of DNA and proteins that makes up chromosomes. One of the key ideas proposed in the review is that mitotic chromosomes may be assembled from pre-existing chromatin structures that exist in the cell before division. These chromatin domains, which are condensed during the non-dividing phase of the cell cycle, could act as "building blocks" that are reorganized during mitosis. Masa A. Shimazoe explained that this view connects the chromatin in the non-dividing state with the mitotic chromosomes, suggesting that cells may reorganize these existing structures rather than starting from scratch. This perspective offers a more integrated understanding of how chromosomes are formed, combining older models like the chromosome scaffold with modern insights on proteins and physical forces.