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> Goes back to Archaea:
> 
> https://phys.org/news/2022-05-mcm-molecules-impede-formation-dna.html
> 
> "'In this current study, they have now jointly identified the minichromosome maintenance (MCM) complex as a new class of barriers in the formation of DNA loops. In the process of DNA loop formation, also called loop extrusion, three proteins or protein complexes are mainly involved: first cohesin, second the zinc finger protein CTCF, and third the MCM complex. Cohesin binds to DNA and initiates the formation of a loop. Cohesin coils the DNA and this results in the progressive growth of a loop.
> 
> "'Loop extrusion stops when cohesin encounters the DNA-bound protein CTCF, which was the only known loop extrusion barrier prior to this work. "Although the details differ, one can imagine that it is a little like placing a ring on a ribbon and threading the ribbon through the ring, which remains at the base of the loop," says Matthias Scherr, co-first author of the study. Kikuë Tachibana explains: "Unlike CTCF, which is a vertebrate-specific barrier, MCMs are conserved across eukaryotes and archaea. It is therefore fascinating to consider that this might be an ancestral barrier to loop extrusion." The work raises the possibility that the encounters of loop extruding cohesin complexes with MCM complexes are part of a fundamental mechanism that organizes the genome folding of a wide variety of organisms."
> 
> Comment: folding six feet of DNA into the tiny nucleus must have carefully controlled folding. Logically the development of very long DNA molecules in evolution had to accompanied by the folding mechanism. They had to be desi gned together. How could it have happened stepwise?