The DNA it’s a molecule essential for the lifewhich with its characteristic double helix molecular structure encodes all the information that regulate the functioning of organisms. Despite its importance, it is surprising to discover how much it is long: human DNA can extend for approximately 2 meters! But how can such a long filament fit inside a single cell? Well, all thanks to the wrapping around the nucleosomesreducing its length by approximately 10,000 times. Let’s see in detail how works L’packaging of DNA and what are the main ones models which describe its structure.
Where is DNA found?
The cells animals are rounded, delimited by a cell membrane and contain inside all the cellular organelles necessary for the functioning of the cell itself and of the organism. Inside these cellular units there is a structure with a diameter smaller than a human hair called nucleus. Every cell in the body contains a copy of our DNA inside the nucleus: we are therefore made up of an enormous number of cells and an equally large number of DNA molecules.
But the truly incredible thing is in the dimension of the cells and in the length of this very important filament. In the human species DNA is almost 2 meters long and can fit into a core with a diameter of a few micrometers (μm).
DNA packaging
Being able to make it stay something very big in a small space It’s quite complicated. We know it well: almost certainly each of us has a closet at home where we throw clothes or objects in bulk (admit it!). You will know well, therefore, that the only way to make everything fit is to dedicate some time to doing orderorganize and fold clothes. An organized closet seems more spacious, but it is only an illusion: it is the objects that take up less space because they are more compact. The same thing, if we want, also happens with DNA.
DNA is not just tangled up haphazardly and thrown into the nucleus most of the time, it wouldn’t even be easy to transcribe. It might seem counterintuitive to require thehelp of “rocchetti” but it is only thanks to them that the structure does not tangle, compresses neatly and unwinds when the DNA needs to be interrogated (during transcription or duplication). complexificationfurthermore, allows the very long DNA strand to occupy as little volume as possible.
Over the years, they have been developed different theories on DNA packaging and some of these have given rise to models still considered valid. In any case, we still know too little about it and, believe us, validating or refuting hypotheses on topics of this kind is far from simple.

The solenoid model
The classic model of packaging predicts that DNA is most of the time complexed, associated with specific structural proteins called histonesconstituting what is defined chromatin. It is an indistinct mass of filament that, if necessary, is further compacted.
You have to imagine histones as positively charged spools. They are proteins that aggregate in groups of 8 and allow the DNA strand to wrap yourself around them a specific number of times. In short, DNA makes a little less than one and three quarters turns (about 1.7 turns) around these spools, winding around these structures about 150 pairs of nitrogenous bases (the AT, CG pairs). The fact that DNA is negatively charged (due to the massive presence of phosphate groups along the chain) ensures that this long molecule can easily aggregate with histones due to the mutual attraction between opposite charges.
If we add an additional histone to block the complex we have obtained a nucleosome, the basic subunit of chromatin.
At this point the nucleosome, of the size of 11nm (0.000000011 m, or 11 billionths of a meter) can be further compacted thanks to a short stretch of filament that separates it from adjacent nucleosomes, called Connective DNAA sort of is formed Pearl necklace which is further aggregated by joining “pearls” consecutive.
When cells duplicate, a further level of compaction can be reached: the DNA is replicated so that it can divide equally and be transferred to the two daughter cells and it is precisely at this moment that the duplicated chromatin takes on the shape of a rod, the chromosome.

The zigzag pattern
Compared to the solenoid model, characterized by interactions between consecutive nucleosomes and therefore “pearls” in a row, the zigzag model predicts interactions between alternating nucleosomesThe two models also differ in the length, trajectory, and degree of curvature of the DNA connecting two nucleosomes (linker DNA).
The triple origami model
DNA is usually found in the form of double helixformed by two complementary filaments joined together to form a sort of zipper. In addition to the so-called Watson-Crick interaction that holds the two halves together, recent research hypothesizes the presence of a further type of interaction called Hoogsteen interaction.
There would be points along the filament where a triple helix structure which folds and compacts DNA generating two- and three-dimensional complexes. The authors suggest that the triplex origami, generated in the laboratory, may play a fundamental role in the natural packaging of DNA in addition to gene therapywhere it is already in use.

DNA in numbers
Based on the data in our possession, we try to make a simplified calculation to better understand the units of measurement.
Considering that a haploid human genome contains approximately 3 billion base pairs aggregated in 23 chromosomes and that, excluding reproductive cells, the vast majority of our cells contain 46 chromosomes (I am therefore diploid), each cell contains approximately 6 billion base pairs. Since each base pair is approximately 1.5 billion base pairs long, 0.34 nanometerswith a simple multiplication we can obtain the length of human DNA, a measurement that is close to 2 meters.
Packaging into nucleosomes allows for compact DNA by about 10,000 timesincredibly going from centimeters to micrometers (cm → μm)!
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