Why is DNA wrapped
The organization of DNA
The two strands of DNA on a typical human chromosome are each up to 4 cm long - if they were pulled apart completely. A metaphase chromosome is only 1/100 millimeter long; so it is 8000 times shorter than the two DNA threads it contains! The reason for this is quite simple: the long DNA threads have to fit into the rather small nucleus of the eukaryotic cell.
How does this strong shortening (condensation) come about? Very easily! Imagine a string ten feet long. Now wind up this string into a ball. This ball then has a diameter of maybe 4 or 5 centimeters.
The DNA threads of a human cell are condensed (compressed) in a similar way. However, this condensation is not quite as easy as that of the string. If we condense a 10 meter long string into a ball 5 cm in diameter, we have a condensation factor of 1000: 5 or 200: 1. The length of the string after condensation is only 1/200 of the original length. However, one chromosome, as can be seen in the light microscope during the metaphase of mitosis, is condensed to 1/8000. A simple "winding up" or "crumpling up" is not enough here. The "winding up" of a DNA thread takes place in several stages.
Stage 1: nucleosomes
On the picture "Organization of a nucleosome" you can see how the DNA wraps itself around a complex of eight small proteins (approx. 1.6 turns with 146 to 147 base pairs). This complex is formed from the histones H2A, H2B, H3 and H4; each protein appears twice in the complex. Histones are slightly positively charged basic proteins that electrically attract negatively charged DNA. The histone H1 plays a special role in linking the individual nucleosomes to the next higher structure.
By "wrapping" the DNA around the histone complex, a condensation factor of approx. 5 to 10 is achieved. That's not very much when you think of the actual factor of 8,000. But we are far from finished with condensing ...
Level 2: 10nm fiber
Between the individual nucleosomes there is a kind of "naked" DNA, about 50 to 70 base pairs long.
A quick graph of the 10 nm fiber
This picture builds on the picture by Darekk2 (see Fig. 1).
Therefore it is also subject to the "Creative Commons Attribution-Share Alike 3.0 Unported" license.
Imagine a pearl necklace around which someone has wrapped fine silver wire, then you get a rough idea of the structure of the so-called 10 nm fiber, the next stage in the organization of DNA.
Level 3: 30 nm fiber
Under certain conditions (high ion concentration) this 10 nm fiber in turn forms a spiral with a diameter of 30 nm.
A rough sketch of the 30 nm fiber
This structure is then referred to as a 30 nm fiber or solenoid structure. However, there are also other structures, depending on the salt content and pH value of the environment. One turn of this spiral consists of six nucleosomes. The condensation factor of DNA is now around 50. That is still not very much.
Level 4: 300 nm fiber
When the 30 nm fiber is "wound up" into a structure ten times as thick, the histones no longer play a role; now other proteins come into play. According to more recent findings, these proteins form the basic structure of a chromosome, into which the 30 nm fibers are then "hooked". Under certain conditions, fibers with a diameter of 300 nm can be seen as the next higher structure in the electron microscope ...
Level 5: 700 nm fiber
... which then organize again into fibers with a diameter of approx. 700 nm. These fibers are then "wound up" to form the actual chromosome.
Stage 6: chromosome
That brings us to the end of the DNA condensation.
Here is another nice picture that summarizes the current state of knowledge about the organization of DNA. The original picture is quite large, you can get to the original by clicking on the picture above.
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