Question

What is the purpose of histones?

Answer 1

So as you know, DNA is very very long
The purpose of histone is to pack the DNA together, into nucleosome
So your body can have a lot more spaces to fit in more DNA

Answer 2

Cmkc is right: the primary function of histones, according to the textbooks, is to organize DNA into nucleosomes so it doesn't get 'tangled up.'

I am reluctant to add to that answer because I'd enjoy ebaxter's explanation of this so much, but here goes: histones also play a very important (if indirect) role in regulating gene expression which scientists are just starting to understand and appreciate. Each of the eight histones in a nucleosome has a 'tail' which is a long, unfolded region sticking out into solution. These tails are modified by addition of various chemical groups and the status of the tails (i.e., methylated or unmethylated) affects whether the genes in the DNA will be transcriptionally active.

For anyone who reads Nature, there was a wonderful article on the role of histone modification in cancer genomics. "Driver mutations in histone H3.3 and chromatin remodelling genes in paediatric glioblastoma" in volume 482. It's the first time anyone has found a direct role of histone modification in development of a disease and is a pretty seminal paper in it's field.

Answer 3

Blues is right about histones as both structural elements allowing packing of DNA in the nucleus and as part of the trasncriptional regulation machinery. is thought that certain proteins in the transcription factor class can move the histones to allow access to the DNA. So in short most of the DNA is packed away and can't be transcribed but some expressed genes have a more open structure. The histones can be moved or removed from the DNA in active genes - this is studied by exposing the chromatin to nuclease enzymes and then studying the extracted DNA for breaks (this is known as DNAseI - hypersensitive site mapping). Must look that paper up.