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 ?
Answer
Sort of like having one of these:
instead of this:
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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