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I know that histones are proteins that package and order the DNA into structural units called nucleosomes, my question is Why DNA needs histones to be packaged and ordered ? Why DNA cannot do that by itself ?

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  • $\begingroup$ Not all organisms have histones. Prokaryotes generally don't. $\endgroup$ – Galen Nov 22 '16 at 21:13
  • $\begingroup$ Structural packing of a lot of DNA into a small space is a useful function. $\endgroup$ – Galen Nov 22 '16 at 21:14
  • $\begingroup$ The packing and ordering also facilitates mitosis/meiosis. $\endgroup$ – Galen Nov 22 '16 at 21:15
  • $\begingroup$ Packaging also controls gene expression under certain conditions. $\endgroup$ – Galen Nov 22 '16 at 21:16
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    $\begingroup$ @Galen You could probably attempt at an answer from all of your comments. $\endgroup$ – Remi.b Nov 22 '16 at 21:17
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Sort of like having one of these:
a garden hose on a reel

instead of this:
a hose lying on the ground

Surely you could simply wrap DNA up in a coil, but to do it in a neat fashion that allows you to package and repackage sections as needed, it's a lot better to have some dedicated machinery.

It is also difficult to organize DNA without a supporting structure because the negative charge of the DNA polymer repels other parts of the molecule, whereas histones are rich in positive charges.

As Galen mentioned in comments, histones both pack DNA into a smaller space (human DNA from one cell would be about the height of an adult if it was all in a straight line) and contribute to gene regulation. Modifications of histones control whether the DNA around that histone can be accessed by transcription machinery and therefore whether those genes will be expressed.

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    $\begingroup$ The wrapping of DNA around each histone is ~1.6 rotations. $\endgroup$ – Galen Nov 22 '16 at 21:15
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    $\begingroup$ Yes, indeed I didn't mean to be literal. $\endgroup$ – Bryan Krause Nov 22 '16 at 21:17
  • $\begingroup$ Great visual analogy! :) $\endgroup$ – Rodrigo Nov 23 '16 at 6:28
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    $\begingroup$ You might like to add the point that DNA is itself negatively charged, so it cannot pack by itself due to repulsion. Histones have positive charge (lysine and arginine rich proteins) so that they can easily hold DNA around them. $\endgroup$ – another 'Homo sapien' Feb 1 '17 at 4:45
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Why DNA needs histones to be packaged and ordered?

Histones counteract retroelements, and this defense against parasitic (viral) DNA is presently considered to be their original physiological role.

see: Madhani, Cell, 2013: "The Frustrated Gene: Origins of Eukaryotic Gene Expression"

Why DNA needs histones to be packaged and ordered ? Why DNA cannot do that by itself ?

Histones are not needed to package and order DNA; Another way to order DNA would be through supercoiling – which can also be done by bacteria (lacking classical histones or histone-like proteins).

see: Griswold, Nature Education, 2008, "Genome Packaging in Prokaryotes"

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