What’s in a loop?

DNAs are different from proteins in many ways. Our genomic DNA molecules are vast, even when compared to the largest proteins we know. DNA is essentially composed of 4 building blocks that are at best modified with a few extra side bits here and there. In proteins we find at least 20 different amino acids and more than one hundred types of posttranslational modifications.The genomic DNAs of eukaryotes live mostly in one confined cell compartment, while proteins lurk in virtually every corner of the cell and many of them whiz about.DNA seems to be usually just able to coil into spirals that coil into bigger spirals (30 nm fibre) that coil into even bigger spirals (200 nm fibre/chromosome), while proteins can take up a plethora of diverse and highly complex shapes, or, for intrinsically disordered proteins, no apparent shapes at all.Most DNA seems to have the capacity to live forever, while probably all proteins have a quite limited lifespan.Despite all of these differences, DNA and proteins have of course a number of things in common. Both are extremely important classes of biomolecules and both are, for example, able to store information. In addition, they share an architectural feature related to complex information processing: substantially sized loop structures.In genomic DNAs, these loops have already been studied for decades and in some detail, but exciting new results are still constantly emerging [1-3].To reveal its information, DNA must be untangled and often distant regions within one molecule or between fellow DNA molecules have to interact. These communicating loops enable promoters, enhancers and other regulatory elements, which are sometimes megabases apart, to come together in space and time in a highly dynamic process which is not entirely understood [4,5]. An early example for this type of long-distance interaction was the finding that the beta-globin enhancer, which is located far upstream of the globin genes, comes into close proximity when the gen