Amazing DNA Folding

By Nader

Whenever I carry a wire earphone in my pocket, the next time I want to use it I find it tangled so badly that I have to spend 5 min detangling it! There are a lot of similar examples around us. Every string has a tendency to get tangled. Look at the wires behind your computer or TV. It is a mess. Now, think of the DNA chain with 3 billion base pairs and total length of 2 meter. This long and thin (2.5nm) chain is packed inside the incredibly small nucleus with diameter of 0.01mm (about 1/10 of the size of dot at the end of this sentence.). This is approximately 1000 times denser than the density of common silicon memory on computers today. The amazing thing is that the packing is dynamically done in such a way that the DNA chain never gets tangled. DNA is also packed in each organ differently (called fractal globule) so that the appropriate part of DNA which is used for making the related protein for that organ, i.e. active DNA, is available to the cell readily while the inactive part is hidden!

This spatial organization of genome inside the nucleus has been under study for quite a while and last week some cool results where published on the 3D imaging of this process.

What is even more fascinating, is to think about the evolutionary path that the eukaryotes took to get to such an efficient and elegant folding mechanism. What were the deriving forces and advantages of developing such complex mechanisms from point of view of evolution? There is no universal theory about the original of nucleus. Some scientists believe that nucleus was an independent bacteria which started a symbiotic relationship with some other archaebacteria and they gradually formed eukaryotes. There have probably been other forms of symbiotic relationships among early bacteria as well but it is not clear exactly why the one that has such a complex DNA folding mechanism thrived. Bacteria and archaea have also dense supercoiled DNA structure called nucleoid but their density is not close to the density of DNA inside the nucleus of eukaryotes. Why the bisexual organisms did not develop from a prokaryote-like chromosome? What is the advantage of having such a dense nucleus?