How are DNAs woven into chromosomes?

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Science  03 Nov 2017:
Vol. 358, Issue 6363, pp. 589-590
DOI: 10.1126/science.aap8729

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It was not recognized that chromosomes contain one immensely long DNA molecule until Kleinschmidt et al. published electron micrographs of a lysed bacterial cell with DNA spilling out that contained no apparent breaks and, indeed, not even any free ends (1). We now know that before replication, each of our chromosomes contains a single DNA molecule of immense length. If the DNA of an average human chromatid were a wire with a diameter of 2 mm, then this wire would be 50 km long. The vast length of chromosomal DNAs poses a number of fundamental problems. For example, how are they packaged during cell division into cylindrical threadlike chromatids? On page 672 of this issue, Terakawa et al. (2) describe an activity associated with a protein complex called condensin that has the potential to answer this. Remarkably, it might also explain chromosome segregation in bacteria as well as the mechanism that regulates enhancer-promoter interactions during development.